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

[Andy Lutomirski]

On Fri, Nov 30, 2018 at 2:10 PM Arnd Bergmann <arnd@xxxxxxxx> wrote:
>
> On Fri, Nov 30, 2018 at 5:36 PM Andy Lutomirski <luto@xxxxxxxxxx> wrote:
> >
> > 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
>
> These seem unavoidable

Indeed, although, in hindsight, they should have had the same numbers.

>
> > 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
>
> Am I misreading the code? The way I understand it, setting the
> 0x40000000 bit means that both in_compat_syscall() and
> in_x32_syscall become() true, based on
>
> static inline bool in_x32_syscall(void)
> {
> #ifdef CONFIG_X86_X32_ABI
>         if (task_pt_regs(current)->orig_ax & __X32_SYSCALL_BIT)
>                 return true;
> #endif
>         return false;
> }

Yes.

>
> The '548 | 0x40000000' part seems to be the only sensible
> way to handle x32 here. What exactly would you propose to
> avoid defining the other entry points?

I would propose that it should be 335 | 0x40000000.  I can't see any
reasonable way to teach the kernel to reject 335 | 0x40000000 that
wouldn't work just as well to accept it and make it do the right
thing.  Currently we accept it and do the *wrong* thing, which is no
good.

>
> > 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.
>
> I'm not convinced that not assigning an x32 syscall number
> improves the situation, it just means that we now have one
> syscall that behaves completely differently from all others,
> in that the x32 version requires being called through a
> SYSCALL_DEFINE() entry point rather than a
> COMPAT_SYSCALL_DEFINE() one,

There's nothing particularly novel about this.  seccomp(), for
example, already works like this.

> and we have to
> add more complexity to the copy_siginfo_from_user()
> implementation to duplicate the hack that exists in
> copy_siginfo_from_user32().

What hack are you referring to here?

>
> Of course, the nicest option would be to completely remove
> x32 so we can stop worrying about it.

True :)