Daniel Bertalan db71c36657 Kernel: Properly align stack for signal handlers ...

The System V ABI requires that the stack is 16-byte aligned on function
call. Confusingly, however, they mean that the stack must be aligned
this way **before** the `CALL` instruction is executed. That instruction
pushes the return value onto the stack, so the callee will actually see
the stack pointer as a value `sizeof(FlatPtr)` smaller.

The signal trampoline was written with this in mind, but `setup_stack`
aligned the entire stack, *including the return address* to a 16-byte
boundary. Because of this, the trampoline subtracted too much from the
stack pointer, thus misaligning it.

This was not a problem on i686 because we didn't execute any
instructions from signal handlers that would require memory operands to
be aligned to more than 4 bytes. This is not the case, however, on
x86_64, where SSE instructions are enabled by default and they require
16-byte aligned operands. Running such instructions raised a GP fault,
immediately killing the offending program with a SIGSEGV signal.

This issue caused TestKernelAlarm to fail in LibC when ran locally, and
at one point, the zsh port was affected too.

Fixes #9291

2021-10-24 21:54:51 +02:00

44 KiB

Raw Blame History

View Raw