Re: [PATCH v3 0/5] nolibc signal handling support

[Ammar Faizi <ammarfaizi2@gnuweeb.org>]

On Sun, Jan 08, 2023 at 07:49:30PM +0100, Willy Tarreau wrote:
> On Mon, Jan 09, 2023 at 01:31:17AM +0700, Ammar Faizi wrote:
> > I'll be pondering this code this week (to follow what actually the
> > rt_sigaction wants on i386 and arm):
> > 
> >   https://github.com/torvalds/linux/blob/v6.2-rc3/kernel/signal.c#L4404-L4434
> 
> Seems like it could simply be a matter of sigsetsize, which is the
> first one returning -EINVAL.
> 
> > Hopefully, I can get it sorted before the weekend.
> 
> OK!

I couldn't dedicate much time to this, but I looked into it, and here's
my report on the progress. I didn't manage to find a proper solution to
this. But yes, you're right. It's a matter of 'sizeof(sigset_t)'.

So here is my observation. Currently, nolibc's sys.h includes this:

    #include <asm/signal.h>

The definition of 'sigset_t' in that header is: 

    typedef unsigned long sigset_t;

On i386, 'sizeof(unsigned long)' is 4, but on x86-64 it's 8.

That is not the 'sigset_t' that the kernel wants. The kernel wants the
'sigset_t' that is in <asm-generic/signal.h>:

    #define _NSIG       64
    #define _NSIG_BPW   __BITS_PER_LONG      // this 64 on x86-64, but 32 on i386.
    #define _NSIG_WORDS (_NSIG / _NSIG_BPW)

    typedef struct {
        unsigned long sig[_NSIG_WORDS];
    } sigset_t;

The above struct is always 8 bytes in size. In other words:

    _NSIG_WORDS == 2 on i386
    _NSIG_WORDS == 1 on x86-64
    sizeof(unsigned long) == 4 on i386
    sizeof(unsigned long) == 8 on x86-64

Therefore, sizeof(unsigned long [_NSIG_WORDS]) is always 8 on both
architectures. That's the correct size.

I tried to #include <asm-generic/signal.h> but it conflicts with the
other 'sigset_t' definition. So I can't do that.

Why are there two different definitions of 'sigset_t'? I don't know.

I probably should read the story behind this syscall to get it
implemented right. Let me ponder this again on Monday. But at least I
tell what I have found so people can give some comments on it...

-- 
Ammar Faizi