as reported by Tavian Barnes, a dup2 file action for the internal pipe
fd used by posix_spawn could cause it to remain open after execve and
allow the child to write an artificial error into it, confusing the
parent. POSIX allows internal use of file descriptors by the
implementation, with undefined behavior for poking at them, so this is
not a conformance problem, but it seems preferable to diagnose and
prevent the error when we can do so easily.
catch attempts to apply a dup2 action to the internal pipe fd and
emulate EBADF for it instead.
commit c8b49b2fbc introduced code that
checked bestsym to determine whether a matching symbol was found, but
bestsym is uninitialized if not. instead use best, consistent with use
in the rest of the function.
simplified from bug report and patch by Cheng Liu.
this was apparently copied from x86_64; it's not part of the kernel
API for riscv64. this change eliminates the need for a
riscv64-specific bits header and lets it use the generic one.
the linux syscall treats this argument as having type int, so passing
extremely long buffer sizes would be misinterpreted by the kernel.
since "short reads" are always acceptable, just cap it down.
patch based on report and suggested change by Florian Weimer.
after commit a48ccc159a removed the use
of _Noreturn on the stage3_func type (which only worked due to it
being defined to the "GNU C" attribute in C99 mode), GCC could no
longer assume that the ends of __dls2 and __dls2b are unreachable, and
produced a warning that a function marked _Noreturn returns.
also, since commit 4390383b32, the
_Noreturn declaration for __libc_start_main in crt1/rcrt1 has been not
only inconsistent with the definition, but wrong. formally,
__libc_start_main does return, via a (hopefully) tail call to a helper
function after the barrier. incorrect usage of _Noreturn in the
declaration was probably formal UB.
the _Noreturn specifiers were not useful in any of these places, so
remove them all. now, the only remaining usage of _Noreturn is in
public interfaces where _Noreturn is part of their contract.
previously, POSIX erroneously required this to fail with EINVAL
despite the traditional glibc implementation, on which the POSIX
interface was based, allowing it. the resolution of Austin Group issue
818 removes the requirement to fail.
this reverts commit f552c792c7, which
exposed the sysmacros.h macros (device major/minor calculations) for
BSD and GNU profiles to mimic an unintentional glibc behavior some
code depended on. glibc has deprecated and since removed them as the
resolution to bug #19239, so it makes no sense for us to keep this
behavior. affected code should all have been fixed by now, and if it's
not yet fixed it needs to be for use with modern glibc anyway.
Author: Alex Suykov <[email protected]>
Author: Aric Belsito <[email protected]>
Author: Drew DeVault <[email protected]>
Author: Michael Clark <[email protected]>
Author: Michael Forney <[email protected]>
Author: Stefan O'Rear <[email protected]>
This port has involved the work of many people over several years. I
have tried to ensure that everyone with substantial contributions has
been credited above; if any omissions are found they will be noted
later in an update to the authors/contributors list in the COPYRIGHT
file.
The version committed here comes from the riscv/riscv-musl repo's
commit 3fe7e2c75df78eef42dcdc352a55757729f451e2, with minor changes by
me for issues found during final review:
- a_ll/a_sc atomics are removed (according to the ISA spec, lr/sc
are not safe to use in separate inline asm fragments)
- a_cas[_p] is fixed to be a memory barrier
- the call from the _start assembly into the C part of crt1/ldso is
changed to allow for the possibility that the linker does not place
them nearby each other.
- DTP_OFFSET is defined correctly so that local-dynamic TLS works
- reloc.h LDSO_ARCH logic is simplified and made explicit.
- unused, non-functional crti/n asm files are removed.
- an empty .sdata section is added to crt1 so that the
__global_pointer reference is resolvable.
- indentation style errors in some asm files are fixed.
with the glibc generation counter model for reusing dynamic tls slots
after dlclose, it's really not possible to get away with fewer than 4
working registers. for us however it's always been possible, but
tricky, and only became apparent after the switch to installing new
dynamic tls at dlopen time. by merging the negated thread pointer into
the addend early, the register holding the thread pointer can
immediately be reused, bringing the working register count down to
three. this allows saving/restoring via a single stp/ldp pair, since
the return register x0 does not need to be saved.
net reduction of 3 instructions, 2 of which were push/pop.
between v2 and v3 of the powerpc64 port patch, the change was made
from a 32x4 array of 32-bit unsigned ints for vrregs[] to a 32-element
array of __int128. this mismatches the API applications working with
mcontext_t expect from glibc, and seems to have been motivated by a
misinterpretation of a comment on how aarch64 did things as a
suggestion to do the same on powerpc64.
the mistaken layout seems to have been adapted from 32-bit powerpc,
where vscr and vrsave are packed into the same 128-bit slot in a way
that looks like it relies on non-overlapping-ness of the value bits in
big endian.
the powerpc64 port accounted for the fact that the 64-bit ABI puts
each in its own 128-bit slot, but ordered them incorrectly (matching
the bit order used on the 32-bit ABI), and failed to account for vscr
being padded according to endianness so that it can be accessed via
vector moves.
in addition to ABI layout, our definition used different logical
member layout/naming from glibc, where vscr is a structure to
facilitate access as a 32-bit word or a 128-bit vector. the
inconsistency here was unintentional, so fix it.
commit 648c3b4e18 omitted this change,
which is needed to be able to use uid/gid values greater than INT_MAX
with these interfaces. it fixes alpine linux bug #10460.
we have to avoid using ebx unconditionally in asm constraints for
i386, because gcc 3 and 4 and possibly other simplistic compilers
(pcc?) implement PIC via making ebx a fixed-use register, and disallow
its use for anything else. rather than hard-coding knowledge of which
compilers work (at least gcc 5+ and clang), perform a configure test;
this should give us the good codegen on any new compilers we don't yet
know about.
swapping ebx and edx is kept for 1- and 2-arg syscalls because it
avoids having any spills/stack-frame at all in small functions. for
6-arg, if ebx is directly usable, the complex shuffling introduced in
commit c8798ef974 can be avoided, and
ebp can be loaded the same way ebx is in 5-arg syscalls for compilers
that don't support direct use of ebx.
commit 22e5bbd0de inlined the i386
syscall mechanism, but wrongly assumed memory operands to the 5- and
6-argument syscall asm would be esp-based. however, nothing in the
constraints prevented them from being ebx- or ebp-based, and in those
cases, ebx and ebp could be clobbered before use of the memory operand
was complete. in the 6-argument case, this prevented restoration of
the original register values before the end of the asm block, breaking
the asm contract since ebx and ebp are not marked as clobbered. (they
can't be, because lots of compilers don't accept these registers in
constraints or clobbers if PIC or frame pointer is enabled).
doing this right is complicated by the fact that, after a single push,
no operands which might be memory operands are usable. if they are
esp-based, the value of esp has changed, rendering them invalid.
introduce some new dances to load the registers. for the 5-arg case,
push the operand that may be a memory operand first, and after that,
it doesn't matter if the operand is invalid, since we'll just use the
newly pushed value. for the 6-arg case, we need to put both operands
in memory to begin with, like the old non-inline code prior to commit
22e5bbd0de accepted, so that there's
only one potentially memory-based operand to the asm. this can then be
saved with a single push, and after that the values can be read off
into the registers they're needed in.
there's some size overhead, but still a lot less execution overhead
than the old out-of-line code. doing it better depends on a modern
compiler that lets you use ebx and ebp in asm constraints without
restriction. the failure modes on compilers where this doesn't work
are inconsistent and dangerous (on at least some gcc versions 4.x and
earlier, wrong codegen!), so this is a delicate matter. it can be
addressed later if needed.
this is a requirement in POSIX that's omitted, and seemed potentially
non-conforming, in the C standard. as such it was omitted here.
however, as part of Austin Group issue #1170, the discrepancy was
raised with WG14 and determined to be unintended; future versions of
the C standard will require the error indicator to be set, as POSIX
does.
commit 788d5e24ca exposed the breakage
at build time by removing support for 7-argument syscalls; however,
the external __syscall function provided for mips before did not pass
a 7th argument from the stack, so the behavior was just silently
broken.
commit 788d5e24ca noted that we could
add this if needed, and in fact it is needed, but not for one of the
archs documented as having a 7th syscall arg register. rather, it's
needed for mips (o32), where all but the first 4 arguments are passed
on the stack, and the stack can accommodate a 7th.
this has been wrong since the beginning of the microblaze port: the
syscall ABI for microblaze does not align 64-bit arguments on even
register boundaries. commit 788d5e24ca
exposed the problem by introducing references to a nonexistent
__syscall7. the ABI is not documented well anywhere, but I was able to
confirm against both strace source and glibc source that microblaze is
not using the alignment.
per the syscall(2) man page, posix_fadvise, ftruncate, pread, pwrite,
readahead, sync_file_range, and truncate were all affected and either
did not work at all, or only worked by chance, e.g. when the affected
argument slots were all zero.
analogous to commit efda534b21 for
powerpc. commit 587f5a53bc moved the
definition of SO_PEERSEC to bits/socket.h for archs where the SO_*
macros differ.
commit b50d315fd2 introduced
fp_force_eval implemented by default with a dead store to a volatile
variable. unfortunately introduces warnings with -Wunused-variable and
breaks the ability to use -Werror with the default warning options set
by configure when warnings are enabled.
we could just call fp_barrier instead, but that results in a spurious
load after the store due to volatile semantics.
the fix committed here avoids the load. it will still produce warnings
without -Wno-unused-but-set-variable, but that's part of our default
warning profile, and there are already other locations in the source
where an unused variable warning will occur without it.
the weak version of __syscall_cp_c was using a tail call to __syscall
to avoid duplicating the 6-argument syscall code inline in small
static-linked programs, but now that __syscall no longer exists, the
inline expansion is no longer duplication.
the syscall.h machinery suppported up to 7 syscall arguments, only via
an external __syscall function, but we presently have no syscall call
points that actually make use of that many, and the kernel only
defines 7-argument calling conventions for arm, powerpc (32-bit), and
sh. if it turns out we need them in the future, they can easily be
added.
n32 and n64 ABIs add new argument registers vs o32, so that passing on
the stack is not necessary, so it's not clear why the 5- and
6-argument versions were special-cased to begin with; it seems to have
been pattern-copying from arch/mips (o32).
i've treated the new argument registers like the first 4 in terms of
clobber status (non-clobbered). hopefully this is correct.
the OABI passes these on the stack, using the convention that their
position on the stack is as if the first four arguments (in registers)
also had stack slots. originally this was deemed too awkward to do
inline, falling back to external __syscall, but it's not that bad and
now that external __syscall is being removed, it's necessary.
the inline syscall code is copied directly from powerpc64. the extent
of register clobber specifiers may be excessive on both; if that turns
out to be the case it can be fixed later.
it was never demonstrated to me that this workaround was needed, and
seems likely that, if there ever was any clang version for which it
was needed, it's old enough to be unusably buggy in other ways. if it
turns out some compilers actually can't do the register allocation
right, we'll need to replace this with inline shuffling code, since
the external __syscall dependency is being removed.
this is the first part of a series of patches intended to make
__syscall fully self-contained in the object file produced using
syscall.h, which will make it possible for crt1 code to perform
syscalls.
the (confusingly named) i386 __vsyscall mechanism, which this commit
removes, was introduced before the presence of a valid thread pointer
was mandatory; back then the thread pointer was setup lazily only if
threads were used. the intent was to be able to perform syscalls using
the kernel's fast entry point in the VDSO, which can use the sysenter
(Intel) or syscall (AMD) instruction instead of int $128, but without
inlining an access to the __syscall global at the point of each
syscall, which would incur a significant size cost from PIC setup
everywhere. the mechanism also shuffled registers/calling convention
around to avoid spills of call-saved registers, and to avoid
allocating ebx or ebp via asm constraints, since there are plenty of
broken-but-supported compiler versions which are incapable of
allocating ebx with -fPIC or ebp with -fno-omit-frame-pointer.
the new mechanism preserves the properties of avoiding spills and
avoiding allocation of ebx/ebp in constraints, but does it inline,
using some fairly simple register shuffling, and uses a field of the
thread structure rather than global data for the vdso-provided syscall
code address.
for now, the external __syscall function is refactored not to use the
old __vsyscall so it can be kept, but the intent is to remove it too.
this is a workaround to avoid a crashing regression on qemu-user when
dynamic TLS is installed at dlopen time. the sigaction syscall should
not be able to fail, but it does fail for implementation-internal
signals under qemu user-level emulation if the host libc qemu is
running under reserves the same signals for implementation-internal
use, since qemu makes no provision to redirect/emulate them. after
sigaction fails, the subsequent tkill would terminate the process
abnormally as the default action.
no provision to account for membarrier failing is made in the dynamic
linker code that installs new TLS. at the formal level, the missing
barrier in this case is incorrect, and perhaps we should fail the
dlopen operation, but in practice all the archs we support (and
probably all real-world archs except alpha, which isn't yet supported)
should give the right behavior with no barrier at all as a consequence
of consume-order properties.
in the long term, this workaround should be supplemented or replaced
by something better -- a different fallback approach to ensuring
memory consistency, or dynamic allocation of implementation-internal
signals. the latter is appealing in that it would allow cancellation
to work under qemu-user too, and would even allow many levels of
nested emulation.
commit 54ca677983 inadvertently
introduced bitwise and where logical and was intended. since the
right-hand operand is always 0 or -1 whenever the left-hand operand is
nonzero, the behavior happened to be equivalent.
priority inheritance is a feature to mitigate priority inversion
situations, where a execution of a medium-priority thread can
unboundedly block forward progress of a high-priority thread when a
lock it needs is held by a low-priority thread.
the natural way to do priority inheritance would be with a simple
futex flag to donate the calling thread's priority to a target thread
while it waits on the futex. unfortunately, linux does not offer such
an interface, but instead insists on implementing the whole locking
protocol in kernelspace with special futex commands that exist solely
for the purpose of doing PI mutexes. this would require the entire
"trylock" logic to be duplicated in the timedlock code path for PI
mutexes, since, once the previous lock holder releases the lock and
the futex call returns, the lock is already held by the caller.
obviously such code duplication is undesirable.
instead, I've made the PI timedlock success path set the mutex lock
count to -1, which can be thought of as "not yet complete", since a
lock count of 0 is "locked, with no recursive references". a simple
branch in a non-hot path of pthread_mutex_trylock can then see and act
on this state, skipping past the code that would check and take the
lock to the same code path that runs after the lock is obtained for a
non-PI mutex.
because we're forced to let the kernel perform the actual lock and
unlock operations whenever the mutex is contended, we have to patch
things up when it does the wrong thing:
1. the lock operation is not aware of whether the mutex is
error-checking, so it will always fail with EDEADLK rather than
deadlocking.
2. the lock operation is not aware of whether the mutex is robust, so
it will successfully obtain mutexes in the owner-died state even if
they're non-robust, whereas this operation should deadlock.
3. the unlock operation always sets the lock value to zero, whereas
for robust mutexes, we want to set it to a special value indicating
that the mutex obtained after its owner died was unlocked without
marking it consistent, so that future operations all fail with
ENOTRECOVERABLE.
the first of these is easy to solve, just by performing a futex wait
on a dummy futex address to simulate deadlock or ETIMEDOUT as
appropriate. but problems 2 and 3 interact in a nasty way. to solve
problem 2, we need to back out the spurious success. but if waiters
are present -- which we can't just ignore, because even if we don't
want to wake them, the calling thread is incorrectly inheriting their
priorities -- this requires using the kernel's unlock operation, which
will zero the lock value, thereby losing the "owner died with lock
held" state.
to solve these problems, we overload the mutex's waiters field, which
is unused for PI mutexes since they don't call the normal futex wait
functions, as an indicator that the PI mutex is permanently
non-lockable. originally I wanted to use the count field, but there is
one code path that needs to access this flag without synchronization:
trylock's CAS failure path needs to be able to decide whether to fail
with EBUSY or ENOTRECOVERABLE, the waiters field is already treated as
a relaxed-order atomic in our memory model, so this works out nicely.
there was no point in masking off the pshared bit when first loading
the type, since every subsequent access involves a mask anyway. not
masking it may avoid a subsequent load to check the pshared flag, and
it's just simpler.
commit 84d061d5a3 wrongly moved the
access to the global next_key outside of the scope of the lock. the
error manifested as spurious failure to find an available key slot
under concurrent calls to pthread_key_create, since the stopping
condition could be met after only a small number of slots were
examined.
commit d6c855caa8 caused this
"regression", though the behavior was undefined before, overlooking
that f->shend=0 was being used as a sentinel for "EOF" status (actual
EOF or hitting the scanf field width) of the stream helper (shgetc)
functions.
obviously the shgetc macro could be adjusted to check for a null
pointer in addition to the != comparison, but it's the hot path, and
adding extra code/branches to it begins to defeat the purpose.
so instead of setting shend to a null pointer to block further reads,
which no longer works, set it to the current position (rpos). this
makes the shgetc macro work with no change, but it breaks shunget,
which can no longer look at the value of shend to determine whether to
back up. Szabolcs Nagy suggested a solution which I'm using here:
setting shlim to a negative value is inexpensive to test at shunget
time, and automatically re-trips the cnt>=shlim stop condition in
__shgetc no matter what the original limit was.
commit 2de29bc994 left behind one
reference to pthread_mutex_trylock. fixing this also improves code
generation due to the namespace-safe version being hidde.
as noted in Austin Group issue #1236, the XSI shading for TSVTX is
misplaced in the html version of the standard; it was only supposed to
be on the description text. the intent was that the definition always
be visible, which is reflected in the pdf version of the standard.
this reverts commits d93c0740d8 and
729fef0a93.
C11 removed the requirement that FILE be a complete type, which was
deemed erroneous, as part of the changes introduced by N1439 regarding
completeness of types (see footnote 6 for specific mention of FILE).
however the current version of POSIX is still based on C99 and
incorporates the old requirement that FILE be a complete type.
expose an arbitrary, useless complete type definition because the
actual object used to represent FILE streams cannot be public/ABI.
thanks to commit 13d1afa46f, we now have
a framework for suppressing the public complete-type definition of FILE
when stdio.h is included internally, so that a different internal
definition can be provided. this is perfectly well-defined, since the
same struct tag can refer to different types in different translation
units. it would be a problem if the implementation were accessing the
application's FILE objects or vice versa, but either would be
undefined behavior.
this affected the error path where dlopen successfully found and
loaded the requested dso and all its dependencies, but failed to
resolve one or more relocations, causing the operation to fail after
storage for the ctor queue was allocated.
commit 188759bbee wrongly put the free
for the ctor_queue array in the error path inside a loop over each
loaded dso that needed to be backed-out, rather than just doing it
once. in addition, the exit path also observed the ctor_queue pointer
still being nonzero, and would attempt to call ctors on the backed-out
dsos unless the double-free crashed the process first.
historically, and likely accidentally, sigaltstack was specified to
fail with EINVAL if any flag bit other than SS_DISABLE was set. the
resolution of Austin Group issue 1187 fixes this so that the
requirement is only to fail for SS_ONSTACK (which cannot be set) or
"invalid" flags.
Linux fails on the kernel side for invalid flags, but historically
accepts SS_ONSTACK as a no-op, so it needs to be rejected in userspace
still.
with this change, the Linux-specific SS_AUTODISARM, provided since
commit 9680e1d03a but unusable due to
rejection at runtime, is now usable.
together with the previous two commits, this completes restoration of
the property that dynamic-linked apps with no external deps and no tls
have no failure paths before entry.
neither has or can have any dependencies, but since commit
4035556907, gratuitous zero-length deps
arrays were being allocated for them. use a dummy array instead.
traditionally, we've provided a guarantee that dynamic-linked
applications with no external dependencies (nothing but libc) and no
thread-local storage have no failure paths before the entry point.
normally, thanks to reclaim_gaps, such a malloc will not require a
syscall anyway, but if segment alignment is unlucky, it might. use a
builtin array for this common special case.
in the case where malloc is being replaced, it's not valid to call
malloc between final relocations and main app's crt1 entry point; on
fdpic archs the main app's entry point will not yet have performed the
self-fixups necessary to call its code.
to fix, reorder queue_ctors before final relocations. an alternative
solution would be doing the allocation from __libc_start_init, after
the entry point but before any ctors run. this is less desirable,
since it would leave a call to malloc that might be provided by the
application happening at startup when doing so can be easily avoided.
previously, going way back, there was simply no synchronization here.
a call to exit concurrent with ctor execution from dlopen could cause
a dtor to execute concurrently with its corresponding ctor, or could
cause dtors for newly-constructed libraries to be skipped.
introduce a shutting_down state that blocks further ctor execution,
producing the quiescence the dtor execution loop needs to ensure any
kind of consistency, and that blocks further calls to dlopen so that a
call into dlopen from a dtor cannot deadlock.
better approaches to some of this may be possible, but the changes
here at least make things safe.
previously, shared library constructors at program start and dlopen
time were executed in reverse load order. some libraries, however,
rely on a depth-first dependency order, which most other dynamic
linker implementations provide. this is a much more reasonable, less
arbitrary order, and it turns out to have much better properties with
regard to how slow-running ctors affect multi-threaded programs, and
how recursive dlopen behaves.
this commit builds on previous work tracking direct dependencies of
each dso (commit 4035556907), and
performs a topological sort on the dependency graph at load time while
the main ldso lock is held and before success is committed, producing
a queue of constructors needed by the newly-loaded dso (or main
application). in the case of circular dependencies, the dependency
chain is simply broken at points where it becomes circular.
when the ctor queue is run, the init_fini_lock is held only for
iteration purposes; it's released during execution of each ctor, so
that arbitrarily-long-running application code no longer runs with a
lock held in the caller. this prevents a dlopen with slow ctors in one
thread from arbitrarily delaying other threads that call dlopen.
fully-independent ctors can run concurrently; when multiple threads
call dlopen with a shared dependency, one will end up executing the
ctor while the other waits on a condvar for it to finish.
another corner case improved by these changes is recursive dlopen
(call from a ctor). previously, recursive calls to dlopen could cause
a ctor for a library to be executed before the ctor for its
dependency, even when there was no relation between the calling
library and the library it was loading, simply due to the naive
reverse-load-order traversal. now, we can guarantee that recursive
dlopen in non-circular-dependency usage preserves the desired ctor
execution order properties, and that even in circular usage, at worst
the libraries whose ctors call dlopen will fail to have completed
construction when ctors that depend on them run.
init_fini_lock is changed to a normal, non-recursive mutex, since it
is no longer held while calling back into application code.
this makes calling dlsym on the main app more consistent with the
global symbol table (load order), and is a prerequisite for
dependency-order ctor execution to work correctly with LD_PRELOAD.
commit 4035556907 introduced runtime
realloc of an array that may have been allocated before symbols were
resolved outside of libc, which is invalid if the allocator has been
replaced. track this condition and manually copy if needed.
dlsym with an explicit handle is specified to use "dependency order",
a breadth-first search rooted at the argument. this has always been
implemented by iterating a flattened dependency list built at dlopen
time. however, the logic for building this list was completely wrong
except in trivial cases; it simply used the list of libraries loaded
since a given library, and their direct dependencies, as that
library's dependencies, which could result in misordering, wrongful
omission of deep dependencies from the search, and wrongful inclusion
of unrelated libraries in the search.
further, libraries did not have any recorded list of resolved
dependencies until they were explicitly dlopened, meaning that
DT_NEEDED entries had to be resolved again whenever a library
participated as a dependency of more than one dlopened library.
with this overhaul, the resolved direct dependency list of each
library is always recorded when it is first loaded, and can be
extended to a full flattened breadth-first search list if dlopen is
called on the library. the extension is performed using the direct
dependency list as a queue and appending copies of the direct
dependency list of each dependency in the queue, excluding duplicates,
until the end of the queue is reached. the direct deps remain
available for future use as the initial subarray of the full deps
array.
first-load logic in dlopen is updated to match these changes, and
clarified.
code introduced in commit 9d44b6460a
wrongly attempted to read past the end of the currently-installed dtv
to determine if a dso provides new, not-already-installed tls. this
logic was probably leftover from an earlier draft of the code that
wrongly installed the new dtv before populating it.
it would work if we instead queried the new, not-yet-installed dtv,
but instead, replace the incorrect check with a simple range check
against old_cnt. this also catches modules that have no tls at all
with a single condition.
code introduced in commit 9d44b6460a
wrongly assumed the dso list tail was the right place to find new dtv
storage. however, this is only true if the last-loaded dependency has
tls. the correct place to get it is the dso corresponding to the tls
module list tail. introduce a container_of macro to get it, and use
it.
ultimately, dynamic tls allocation should be refactored so that this
is not an issue. there is no reason to be allocating new dtv space at
each load_library; instead it could happen after all new libraries
have been loaded but before they are committed. such changes may be
made later, but this commit fixes the present regression.
the motivation for this change is twofold. first, it gets the fallback
logic out of the dynamic linker, improving code readability and
organization. second, it provides application code that wants to use
the membarrier syscall, which depends on preregistration of intent
before the process becomes multithreaded unless unbounded latency is
acceptable, with a symbol that, when linked, ensures that this
registration happens.
this is a prerequisite for factoring the membarrier fallback code into
a function that can be called from a context with the thread list
already locked or independently.
commit 9d44b6460a inadvertently
contained leftover logic from a previous approach to the fallback
signaling loop. it had no adverse effect, since j was always nonzero
if the loop body was reachable, but it makes no sense to be there with
the current approach to avoid signaling self.
addressing &out[k].sa was arguably undefined, despite &out[k] being
defined the slot one past the end of an array, since the member access
.sa is intervening between the [] operator and the & operator.
the backindex stored by getaddrinfo to allow freeaddrinfo to perform
partial-free wrongly used the address result index, rather than the
output slot index, and thus was only valid when they were equal
(nservs==1).
patch based on report with proposed fix by Markus Wichmann.
previously, dynamic loading of new libraries with thread-local storage
allocated the storage needed for all existing threads at load-time,
precluding late failure that can't be handled, but left installation
in existing threads to take place lazily on first access. this imposed
an additional memory access and branch on every dynamic tls access,
and imposed a requirement, which was not actually met, that the
dynamic tlsdesc asm functions preserve all call-clobbered registers
before calling C code to to install new dynamic tls on first access.
the x86[_64] versions of this code wrongly omitted saving and
restoring of fpu/vector registers, assuming the compiler would not
generate anything using them in the called C code. the arm and aarch64
versions saved known existing registers, but failed to be future-proof
against expansion of the register file.
now that we track live threads in a list, it's possible to install the
new dynamic tls for each thread at dlopen time. for the most part,
synchronization is not needed, because if a thread has not
synchronized with completion of the dlopen, there is no way it can
meaningfully request access to a slot past the end of the old dtv,
which remains valid for accessing slots which already existed.
however, it is necessary to ensure that, if a thread sees its new dtv
pointer, it sees correct pointers in each of the slots that existed
prior to the dlopen. my understanding is that, on most real-world
coherency architectures including all the ones we presently support, a
built-in consume order guarantees this; however, don't rely on that.
instead, the SYS_membarrier syscall is used to ensure that all threads
see the stores to the slots of their new dtv prior to the installation
of the new dtv. if it is not supported, the same is implemented in
userspace via signals, using the same mechanism as __synccall.
the __tls_get_addr function, variants, and dynamic tlsdesc asm
functions are all updated to remove the fallback paths for claiming
new dynamic tls, and are now all branch-free.
access to clear the entry in each thread's tsd array for the key being
deleted was not synchronized with __pthread_tsd_run_dtors. I probably
made this mistake from a mistaken belief that the thread list lock was
held during the latter, which of course is not possible since it
executes application code in a still-live-thread context.
while we're at it, expand the interval during which signals are
blocked to cover taking the write lock on key_lock, so that a signal
at an inopportune time doesn't block forward progress of readers.
commit 84d061d5a3 inadvertently
introduced namespace violations by using the pthread-namespace rwlock
functions in pthread_key_create, which is in turn used for C11 tss.
fix that and possible future uses of rwlocks elsewhere.
with the availability of the thread list, there is no need to mark tsd
key slots dirty and clean them up only when a free slot can't be
found. instead, directly iterate threads and clear any value
associated with the key being deleted.
no synchronization is necessary for the clearing, since there is no
way the slot can be accessed without having synchronized with the
creation of a new key occupying the same slot, which is already
sequenced after and synchronized with the deletion of the old key.
the __synccall mechanism provides stop-the-world synchronous execution
of a callback in all threads of the process. it is used to implement
multi-threaded setuid/setgid operations, since Linux lacks them at the
kernel level, and for some other less-critical purposes.
this change eliminates dependency on /proc/self/task to determine the
set of live threads, which in addition to being an unwanted dependency
and a potential point of resource-exhaustion failure, turned out to be
inaccurate. test cases provided by Alexey Izbyshev showed that it
could fail to reflect newly created threads. due to how the
presignaling phase worked, this usually yielded a deadlock if hit, but
in the worst case it could also result in threads being silently
missed (allowed to continue running without executing the callback).
the hard problem here is unlinking threads from a list when they exit
without creating a window of inconsistency where the kernel task for a
thread still exists and is still executing instructions in userspace,
but is not reflected in the list. the magic solution here is getting
rid of per-thread exit futex addresses (set_tid_address), and instead
using the exit futex to unlock the global thread list.
since pthread_join can no longer see the thread enter a detach_state
of EXITED (which depended on the exit futex address pointing to the
detach_state), it must now observe the unlocking of the thread list
lock before it can unmap the joined thread and return. it doesn't
actually have to take the lock. for this, a __tl_sync primitive is
offered, with a signature that will allow it to be enhanced for quick
return even under contention on the lock, if needed. for now, the
exiting thread always performs a futex wake on its detach_state. a
future change could optimize this out except when there is already a
joiner waiting.
initial/dynamic variants of detached state no longer need to be
tracked separately, since the futex address is always set to the
global list lock, not a thread-local address that could become invalid
on detached thread exit. all detached threads, however, must perform a
second sigprocmask syscall to block implementation-internal signals,
since locking the thread list with them already blocked is not
permissible.
the arch-independent C version of __unmapself no longer needs to take
a lock or setup its own futex address to release the lock, since it
must necessarily be called with the thread list lock already held,
guaranteeing exclusive access to the temporary stack.
changes to libc.threads_minus_1 no longer need to be atomic, since
they are guarded by the thread list lock. it is largely vestigial at
this point, and can be replaced with a cheaper boolean indicating
whether the process is multithreaded at some point in the future.
whether signals need to be blocked at thread start, and whether
unblocking is necessary in the entry point function, has historically
depended on intricacies of the cancellation design and on whether
there are scheduling operations to perform on the new thread before
its successful creation can be committed. future changes to track an
AS-safe list of live threads will require signals to be blocked
whenever changes are made to the list, so ...
prior to commits b8742f3260 and
40bae2d32f, a signal mask for the entry
function to restore was part of the pthread structure. it was removed
to trim down the size of the structure, which both saved a small
amount of stack space and improved code generation on archs where
small immediate displacements are less costly than arbitrary ones, by
limiting the range of offsets between the base of the thread
structure, its members, and the thread pointer. these commits moved
the saved mask to a special structure used only when special
scheduling was needed, in which case the pthread_create caller and new
thread had to synchronize with each other and could use this memory to
pass a mask.
this commit partially reverts the above two commits, but instead of
putting the mask back in the pthread structure, it moves all "start
argument" members out of the pthread structure, trimming it down
further, and puts them in a separate structure passed on the new
thread's stack. the code path for explicit scheduling of the new
thread is also changed to synchronize with the calling thread in such
a way to avoid spurious futex wakes.
this eliminates some ugly hacks that were repurposing the start
function and start argument fields in the pthread structure for timer
use, and the need to longjmp out of a signal handler.
__dl_thread_cleanup is called from the context of an exiting thread
that is not in a consistent state valid for calling application code.
since commit c9f415d7ea, it's possible
(and supported usage) for the allocator to have been replaced by the
application, so __dl_thread_cleanup can no longer call free. instead,
reuse the message buffer as a linked-list pointer, and queue it to be
freed the next time any dynamic linker error message is generated.
the way gets was implemented in terms of fgets, it used the location
of the null termination to determine where to find and remove the
newline, if any. an embedded null byte prevented this from working.
this also fixes a one-byte buffer overflow, whereby when gets read an
N-byte line (not counting newline), it would store two null
terminators for a total of N+2 bytes. it's unlikely that anyone would
care that a function whose use is pretty much inherently a buffer
overflow writes too much, but it could break the only possible correct
uses of this function, in conjunction with input of known format from
a trusted/same-privilege-domain source, where the buffer length may
have been selected to exactly match a line length contract.
there seems to be no correct way to implement gets in terms of a
single call to fgets or scanf, and using multiple calls would require
explicit locking, so we might as well just write the logic out
explicitly character-at-a-time. this isn't fast, but nobody cares if a
catastrophically unsafe function that's so bad it was removed from the
C language is fast.
in order to implement ENOTRECOVERABLE, the implementation has
traditionally used a bit of the mutex type field to indicate that it's
recovered after EOWNERDEAD and will go into ENOTRECOVERABLE state if
pthread_mutex_consistent is not called before unlocking. while it's
only the thread that holds the lock that needs access to this
information (except possibly for the sake of pthread_mutex_consistent
choosing between EINVAL and EPERM for erroneous calls), the change to
the type field is formally a data race with all other threads that
perform any operation on the mutex. no individual bits race, and no
write races are possible, so things are "okay" in some sense, but it's
still not good.
this patch moves the recovery/consistency state to the mutex
owner/lock field which is rightfully mutable. bit 30, the same bit the
kernel uses with a zero owner to indicate that the previous owner died
holding the lock, is now used with a nonzero owner to indicate that
the mutex is held but has not yet been marked consistent. note that
the kernel ABI also reserves bit 29 not to appear in any tid, so the
sentinel value we use for ENOTRECOVERABLE, 0x7fffffff, does not clash
with any tid plus bit 30.
fdopendir is specified to fail with EBADF if the file descriptor
passed is not open for reading. while O_PATH is an extension and
arguably exempt from this requirement, it's used, albeit incompletely,
to implement O_SEARCH, and fdopendir should fail when passed an
O_SEARCH file descriptor.
the new check is performed after fstat so that we don't have to
consider the possibility that the fd is invalid.
an alternate solution would be attempting to pre-fill the buffer using
getdents, which would fail with EBADF for us, but that seems more
complex and error-prone and involves either code duplication or
refactoring, so the simple fix with an additional inexpensive syscall
is what I've made for now.
commit 84d061d5a3 attempted to do this
already, but omitted from pthread_key_create.c the weak definition of
__pthread_key_delete_synccall, so that the definition provided by
pthread_key_delete.c was always pulled in.
based on patch by Markus Wichmann, but with a weak alias rather than
weak reference for consistency/policy about dependence on tooling
features.
fallback to /etc/shadow should happen only when the entry is not found
in the TCB shadow. otherwise transient errors or permission errors can
cause inconsistent results.
this reverts commit c0ed5a201b, which
was based on a mistaken reading of POSIX due to inconsistency between
the description (which requires return upon interruption by a signal)
and the errors list (which wrongly lists EINTR as "may fail").
since the previously-introduced behavior was a workaround for an old
kernel bug to ensure safety of correct programs that were not hardened
against the bug, an effort has been made to preserve it for programs
which do not use interrupting signal handlers. the stage for this was
set in commit a63c0104e4, which makes
the futex __timedwait backend suppress EINTR if it's seen when no
interrupting signal handlers have been installed.
based loosely on a patch submitted by Orivej Desh, but with
unnecessary additional changes removed.
the resolution of Austin Group issue #1132 changes the requirement to
fail so that it only applies when the set argument (new mask) is
non-null. this change was made for consistency with the description,
which specified "if set is a null pointer, the value of the argument
how is not significant".
prior to linux 2.6.22, futex wait could fail with EINTR even for
non-interrupting (SA_RESTART) signals. this was no problem provided
the caller simply restarted the wait, but sem_[timed]wait is required
by POSIX to return when interrupted by a signal. commit
a113434cd6 introduced this behavior, and
commit c0ed5a201b reverted it based on a
mistaken belief that it was not required. this belief stems from a bug
in the specification: the description requires the function to return
when interrupted, but the errors section marks EINTR as a "may fail"
condition rather than a "shall fail" one.
since there does seem to be significant value in the change made in
commit c0ed5a201b, making it so that
programs that call sem_wait without checking for EINTR don't silently
make forward progress without obtaining the semaphore or treat it as a
fatal error and abort, add a behind-the-scenes mechanism in the
__timedwait backend to suppress EINTR in programs that have never
installed interrupting signal handlers, and have sigaction track and
report this state. this way the semaphore code is not cluttered by
workarounds and can be updated (to be done in next commit) to reflect
the high-level logic for conforming behavior.
these changes are based loosely on a patch by Markus Wichmann, with
the main changes being atomic update to flag object and moving the
workaround from sem_timedwait to the __timedwait futex backend.
it's not clear whether this is required, but it seems arguable that it
should happen. for example aio_suspend is supposed to return
immediately if any of the operations has "completed", which includes
ending with an error status asynchonously and might also be
interpreted to include doing so synchronously.
the map structures in particular are permanent once created, and thus
a large number of aio function calls with invalid file descriptors
could exhaust memory, whereas, assuming normal resource limits, only a
very small number of entries ever need to be allocated. check validity
of the fd before allocating anything new, so that allocation of large
amounts of memory is only possible when resource limits have been
increased and a large number of files are actually open.
this change also improves error reporting for bad file descriptors to
happen at the time the aio submission call is made, as opposed to
asynchronously.
since commit c9f415d7ea, it has been
possible that the allocator is application-provided code, which cannot
necessarily run safely on io thread stacks, and which should not be
able to see the existence of io threads, since they are an
implementation detail.
instead of having the io thread request and possibly allocate its
queue (and the map structures leading to it), make the submitting
thread responsible for this, and pass the queue pointer into the io
thread via its args structure. this eliminates the only early error
case in io threads, making it no longer necessary to pass an error
status back to the submitting thread via the args structure.
aio threads not using SIGEV_THREAD notification are created with small
stacks and no guard page, which is possible since they only run the
code for the requested io operation, not any application code. the
motivation is not creating a lot of VMAs. however, the io thread needs
to be able to receive a cancellation signal in case aio_cancel
(implemented via pthread_cancel) is called. this requires sufficient
stack space for a signal frame, which PTHREAD_STACK_MIN does not
necessarily include.
in principle MINSIGSTKSZ from signal.h should give us sufficient space
for a signal frame, but the value is incorrect on some existing archs
due to kernel addition of new vector register support without
consideration for impact on ABI. some powerpc models exceed
MINSIGSTKSZ by about 0.5k, and x86[_64] with AVX-512 can exceed it by
up to about 1.5k. so use MINSIGSTKSZ+2048 to allow for the discrepancy
plus some working space.
unfortunately, it's possible that signal frame sizes could continue to
grow, and some archs (aarch64) explicitly specify that they may.
passing of a runtime value for MINSIGSTKSZ via AT_MINSIGSTKSZ in the
aux vector was added to aarch64 linux, and presumably other archs will
use this mechanism to report if they further increase the signal frame
size. when AT_MINSIGSTKSZ is present, assume it's correct, so that we
only need a small amount of working space in addition to it; in this
case just add 512.
commit b9410061e2 inadvertently omitted
optopt from the "dynamic list", causing it to be split into separate
objects that don't share their value if the main program contains a
copy relocation for it (for non-PIE executables that access it, and
some PIE ones, depending on arch and toolchain versions/options).
first, the condition (mem && k < p) is redundant, because mem being
nonzero implies the needle is periodic with period exactly p, in which
case any byte that appears in the needle must appear in the last p
bytes of the needle, bounding the shift (k) by p.
second, the whole point of replacing the shift k by mem (=l-p) is to
prevent shifting by less than mem when discarding the memory on shift,
in which case linear time could not be guaranteed. but as written, the
check also replaced shifts greater than mem by mem, reducing the
benefit of the shift. there is no possible benefit to this reduction of
the shift; since mem is being cleared, the full shift is valid and
more optimal. so only replace the shift by mem when it would be less
than mem.
commit ddc947eda3 fixed the
corresponding bug for exit which was introduced when commit
0b80a7b040 added support for
caller-provided buffers, making it possible for stderr to be a
buffered stream.
fflush(NULL) and __stdio_exit lock individual FILEs while holding the
open file list lock to walk the list. since fclose first locked the
FILE to be closed, then the ofl lock, it could deadlock with these
functions.
also, because fclose removed the FILE to be closed from the open file
list before flushing and closing it, a concurrent fclose or exit could
complete successfully before fclose flushed the FILE it was closing,
resulting in data loss.
reorder the body of fclose to first flush and close the file, then
remove it from the open file list only after unlocking it. this
creates a window where consumers of the open file list can see dead
FILE objects, but in the absence of undefined behavior on the part of
the application, such objects will be in an inactive-buffer state and
processing them will have no side effects.
__unlist_locked_file is also moved so that it's performed only for
non-permanent files. this change is not necessary, but preserves
consistency (and thereby provides safety/hardening) in the case where
an application uses one of the standard streams after closing it while
holding an explicit lock on it. such usage is of course undefined
behavior.
commit aeeac9ca54 introduced fail-safe
invariants that creating a locale_t object for the C locale or C.UTF-8
locale will always succeed. extend the guarantee to also cover the
following:
- newlocale(LC_ALL_MASK, "", 0)
- newlocale(LC_ALL_MASK-LC_CTYPE_MASK, "C", 0)
provided that the LANG/LC_* environment variables have not been
changed by the program. these usages are idiomatic for getting the
default locale, and for getting a locale that behaves as the C locale
except for honoring the default locale's character encoding.
unify the code paths for allocated and non-allocated locale objects,
always using a tmp object. this is necessary to avoid clobbering the
base locale object too soon if we allow for the possibility that
looking up an explicitly requested locale name may fail, and makes the
code simpler and cleaner anyway.
eliminate the complex and fragile logic for checking whether one of
the non-allocated locale objects can be used for the result, and
instead just memcmp against each of them.
commit 63c188ec42 missed making this
change when switching from atomics to locking for modification of the
global locale, leaving access to locale structures unnecessarily
burdened with the restrictions of volatile.
the volatile qualification was originally added in commit
56fbaa3bbe.
introduce a new LOC_MAP_FAILED sentinel for errors, since null
pointers for a category's locale map indicate the C locale. at this
time, __get_locale does not fail, so there should be no functional
change by this commit.
the choice of signed char for lbf was a theoretically space-saving
hack that was not helping, and was unwantedly expensive. while
comparing bytes against a byte-sized member sounds easy, the trick
here was that the byte to be compared was unsigned while the lbf
member was signed, making it possible to set lbf negative to disable
line buffering. however, this imposed a requirement to promote both
operands, zero-extending one and sign-extending the other, in order to
compare them.
to fix this, repurpose the waiters count slot (unused since commit
c21f750727). while we're at it, switch
mode (orientation) from signed char to int as well. this makes no
semantic difference (its only possible values are -1, 0, and 1) but it
might help on archs where byte access is awkward.
to check whether flush due to line buffering is needed, the int-type
character argument must be truncated to unsigned char for comparison.
if the original value is subsequently passed to __overflow, it must be
preserved, adding to register pressure. since it doesn't matter,
truncate all uses so the original value is no longer live.
the internal putc_unlocked macro was wrongly returning a meaningless
boolean result rather than the written character or EOF.
bug was found by reading (very surprising) asm.
Rich Felker