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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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Call SYS_exit on return from fn in __clone. This is the expected
behavior of this function. Without this the child task will crash on
return from fn, since it will return to nowhere.
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the workaround was for a bug that botched .gpword references to local
labels, applying a nonsensical random offset of -0x4000 to them.
this reverses commit 5e396fb996a80b035d0f6ecf7fed50f68aa3ebb7 and a
removes a similar hack that was added to syscall_cp.s in the later
commit 756c8af8589265e99e454fe3adcda1d0bc5e1963. it turns out one
additional instance of the same idiom, the GETFUNCSYM macro in
arch/mips/reloc.h, was still affected by the assembler bug and does
not admit an easy workaround without making assumptions about how the
macro is used. the previous workarounds made static linking work but
left the early-stage dynamic linker broken and thus had limited
usefulness.
instead, affected users (using binutils versions older than 2.20) will
need to fix the bug on the binutils side; the trivial patch is commit
453f5985b13e35161984bf1bf657bbab11515aa4 in the binutils-gdb
repository.
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the old __cp_cancel code path loaded the address of __cancel from the
GOT using the $gp register, which happened to be set to point to the
correct GOT by the calling C function, but there is no ABI requirement
that this happen. instead, go the roundabout way and compute the
address of __cancel via pc-relative and gp-relative addressing
starting with a fake return address generated by a bal instruction,
which is the same trick crt1 uses to bootstrap.
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linux kernel commit 46e12c07b3b9603c60fc1d421ff18618241cb081 caused
the mips syscall mechanism to fail with EFAULT when the userspace
stack pointer is invalid, breaking __unmapself used for detached
thread exit. the workaround is to set $sp to a known-valid, readable
address, and the simplest one to obtain is the address of the current
function, which is available (per o32 calling convention) in $25.
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otherwise disassemblers treat it as data.
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in a few places, non-hidden symbols were referenced from asm in ways
that assumed ld-time binding. while these is no semantic reason these
symbols need to be hidden, fixing the references without making them
hidden was going to be ugly, and hidden reduces some bloat anyway.
in the asm files, .global/.hidden directives have been moved to the
top to unclutter the actual code.
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this was resulting in crashes in posix_spawn on mips, and would have
affected applications calling clone too. since the prototype for
__clone has it as a variadic function, it may not assume that 16($sp)
is writable for use in making the syscall. instead, it needs to
allocate additional stack space, and then adjust the stack pointer
back in both of the code paths for the parent process/thread.
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unlike other archs, the mips version of clone was not doing anything
to align the stack pointer. this seems to have been the cause for some
SIGBUS crashes that were observed in posix_spawn.
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these fixes were already made to the normal syscall asm but not the
cancellation point version.
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the mips abi reserves stack space equal to the size of the in-register
args for the callee to save the args, if desired. this would cause the
beginning of the thread structure to be clobbered...
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the old code worked in qemu app-level emulation, but not on real
kernels where the clone syscall does not copy the register values to
the new thread. save arguments on the new thread stack instead.
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with this change, threads on mips seem to be working
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