Re: [RFC PATCH v1 0/8] nolibc signal handling support

[Ammar Faizi]

On 12/28/22 1:49 AM, Willy Tarreau wrote:
> I'll try to do it but do not want to make you wait too long in case it
> gets delayed. In the worst case we should only postpone the getauxval()
> patch and not the other ones.

I will split it into 2 patchset then.

> BTW, do you think your arch-specific changes for sigaction() will be
> easily portable to other architectures ? I feel a bit wary of starting
> to have different features per architecture given the purpose of the
> lib, so the more uniform the coverage the better.

The 'rt_sigaction()' itself doesn't seem to be an arch specific, but
the way it resumes the execution needs to call 'rt_sigreturn()' which
is arch specific. I took a look at the kernel source code, most
architectures read 'struct rt_sigframe' from the stack pointer.

https://github.com/torvalds/linux/blob/631aa744423173bf921191ba695bbc7c1aabd9e0/arch/x86/kernel/signal_32.c#L145
https://github.com/torvalds/linux/blob/631aa744423173bf921191ba695bbc7c1aabd9e0/arch/x86/kernel/signal_64.c#L243-L271
https://github.com/torvalds/linux/blob/a6b450573b912316ad36262bfc70e7c3870c56d1/arch/arm64/kernel/signal.c#L668-L699
https://github.com/torvalds/linux/blob/a6b450573b912316ad36262bfc70e7c3870c56d1/arch/arm64/kernel/signal32.c#L259
https://github.com/torvalds/linux/blob/eb67d239f3aa1711afb0a42eab50459d9f3d672e/arch/riscv/kernel/signal.c#L101

On the x86-64 arch, the implementation is just like this:

   __arch_restore_rt:
       #
       # ((%rsp - sizeof(long)) must point to 'struct rt_sigframe')
       #
       # 'struct rt_sigframe' is automatically constructed by
       # the kernel when a signal is caught.
       #
       movl       $0xf, %eax // __NR_rt_sigreturn == 0xf
       syscall

I believe aarch64 and RISCV don't behave differently, but different
registers.

Not sure what PowerPC does here, it seems a bit different:
https://github.com/torvalds/linux/blob/1612c382ffbdf1f673caec76502b1c00e6d35363/arch/powerpc/kernel/signal_64.c#L744

I haven't taken a look at other archs.

What do you think? Is it affordable for nolibc to implement all of
these?

--
Ammar Faizi