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The ABI requires a stack frame to, at minimum, consist of the backchain slot and
the LR save slot at sp+0 and sp+4 respectively. The old code spilled r30/r31
into those slots, meaning that a backchain-based unwinder would see a nonsense
value as the backchain pointer and go on a wild goose chase.
It's admittedly a very small window where this is possible -- a thread that's
stopped in the middle of the clone() parent body -- but fixing it just requires
shifting the r30/r31 spill slots down by 8 bytes and storing the old sp in the
backchain slot, so seems reasonable to do.
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these are not a public interface and are not intended to be callable
from anywhere but the public clone function or other places in libc.
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per the powerpc psabi, offset 4 of the stack at call time belongs to
the callee and is used for spilling lr (return address). in addition,
offset 0 on the stack must contain a pointer to the previous stack
frame, or a null pointer for the initial stack frame of a thread.
__clone failed to setup any stack frame on the new thread's stack,
thereby allowing the start function it called to clobber offset 4 of
the new thread's struct __pthread, which contains the dtv pointer.
add code to setup a proper stack frame and align the stack pointer to
a multiple of 16 (also an abi requirement) if it was not already
aligned.
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sigsetjmp: store temporaries in jmp_buf rather than on stack.
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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.
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