Re: [PATCH v2] signal: add procfd_signal() syscall

[Andy Lutomirski]

On Fri, Nov 30, 2018 at 3:41 AM Arnd Bergmann <arnd@xxxxxxxx> wrote:
> siginfo_t as it is now still has a number of other downsides, and Andy in
> particular didn't like the idea of having three new variants on x86
> (depending on how you count). His alternative suggestion of having
> a single syscall entry point that takes a 'signfo_t __user *' but interprets
> it as compat_siginfo depending on in_compat_syscall()/in_x32_syscall()
> should work correctly, but feels wrong to me, or at least inconsistent
> with how we do this elsewhere.

If everyone else is okay with it, I can get on board with three
variants on x86.  What I can't get on board with is *five* variants on
x86, which would be:

procfd_signal via int80 / the 32-bit vDSO: the ia32 structure

syscall64 with nr == 335 (presumably): 64-bit

syscall64 with nr == 548 | 0x40000000: x32

syscall64 with nr == 548: 64-bit entry but in_compat_syscall() ==
true, behavior is arbitrary

syscall64 with nr == 335 | 0x40000000: x32 entry, but
in_compat_syscall() == false, behavior is arbitrary

This mess isn't really Christian's fault -- it's been there for a
while, but it's awful and I don't think we want to perpetuate it.

Obviously, I'd prefer a variant where the structure that's passed in
is always the same.

BTW, do we consider siginfo_t to be extensible?  If so, and if we pass
in a pointer, presumably we should pass a length as well.