Rich Felker
9565a349f2
it's essential to decrement the stack pointer before writing to new stack space, rather than afterwards. otherwise there is a race condition during which asynchronous code (signals) could clobber the data being stored. it may be possible to optimize the code further using stwu, but I wanted to avoid making any changes to the actual stack layout in this commit. further improvements can be made separately if desired.
84 lines
1.7 KiB
ArmAsm
84 lines
1.7 KiB
ArmAsm
.text
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.global __clone
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.type __clone, %function
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__clone:
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# int clone(fn, stack, flags, arg, ptid, tls, ctid)
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# a b c d e f g
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# 3 4 5 6 7 8 9
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# pseudo C code:
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# tid = syscall(SYS_clone,c,b,e,f,g);
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# if (!tid) syscall(SYS_exit, a(d));
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# return tid;
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# SYS_clone = 120
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# SYS_exit = 1
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# in order that the child can find the start func and its arg, we need to store it into
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# non-volative regs. to do so, we have to store those 2 regs into our stackframe, so
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# we can restore them later.
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subi 1, 1, 16
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stw 30, 12(1)
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stw 31, 8(1)
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# save r3 (func) into r30, and r6(arg) into r31
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mr 30, 3
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mr 31, 6
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#move c into first arg
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mr 3, 5
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#mr 4, 4
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mr 5, 7
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mr 6, 8
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mr 7, 9
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# move syscall number into r0
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li 0, 120
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sc
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# check for syscall error
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#this code should be more efficient, but it borks
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#bns+ 1f # jump to label 1 if no summary overflow.
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#else
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#neg 3, 3 #negate the result (errno)
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#b 2f # jump to epilogue
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# this error check code at least does not spoil the clone call.
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#mfcr 0 # Check for an error
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#rlwinm 4, 0, 0, 3, 3 # by checking for bit 28.
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#cmplwi 0, 4, 0 # It is an error if non-zero.
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#beq 0, 1f # Jump if not an error.
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#neg 3, 3 # Negate the error number.
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#b 2f # jump to epilogue
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1:
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# compare sc result with 0
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cmpwi cr7, 3, 0
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# if not 0, jump to end
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bne cr7, 2f
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#else: we're the child
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#call funcptr
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# move arg (d) into r3
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mr 3, 31
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#move r30 (funcptr) into CTR reg
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mtctr 30
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# call CTR reg
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bctrl
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# mov SYS_exit into r0 (the exit param is already in r3)
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li 0, 1
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sc
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2:
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# restore stack
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lwz 30, 12(1)
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lwz 31, 8(1)
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addi 1, 1, 16
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blr
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