Re: [PATCH] [Cleanup] x86: signal: unify the sigaltstack check with other arches

[Stas Sergeev]

10.03.2016 03:02, Andy Lutomirski ÐÐÑÐÑ:
> On Tue, Mar 8, 2016 at 8:20 AM, Ingo Molnar <mingo@xxxxxxxxxx> wrote:
> > * Stas Sergeev <stsp@xxxxxxx> wrote:
> >
> > > 25.02.2016 11:25, Ingo Molnar ÐÐÑÐÑ:
> > > > * Stas Sergeev <stsp@xxxxxxx> wrote:
> > > >
> > > > > Currently x86's get_sigframe() checks for "current->sas_ss_size"
> > > > > to determine whether there is a need to switch to sigaltstack.
> > > > > The common practice used by all other arches is to check for
> > > > > sas_ss_flags(sp) == 0
> > > > >
> > > > > This patch makes the code consistent with other arches.
> > > > > The slight complexity of the patch is added by the optimization on
> > > > > !sigstack check that was requested by Andy Lutomirski: sas_ss_flags(sp)==0
> > > > > already implies that we are not on a sigstack, so the code is shuffled
> > > > > to avoid the duplicate checking.
> > > > So this changelog is missing an analysis about what effect this change will have
> > > > on applications. Can any type of user-space code see a change in behavior? If yes,
> > > > what will happen and is that effect desirable?
> > > This is a clean-up, and as such, there is no visible effect.
> > > If there is - it is a bug.
> > > The purpose of this patch is only to unify the x86 code with
> > > what all the other arches do. It was initially the part of the
> > > rejected series, but now it is just a clean-up.
> > Ok, so AFAICS the relevant change is:
> >
> > -               if (current->sas_ss_size)
> > -                       sp = current->sas_ss_sp + current->sas_ss_size;
> > +               if (sas_ss_flags(sp) == 0)
> > +                       sp = current->sas_ss_sp + current->sas_ss_size;
> >
> > and since sas_ss_flags() is defined as:
> >
> > static inline int sas_ss_flags(unsigned long sp)
> > {
> >          if (!current->sas_ss_size)
> >                  return SS_DISABLE;
> >
> >          return on_sig_stack(sp) ? SS_ONSTACK : 0;
> > }
> >
> > sas_ss_flags() returns 0 iff current->sas_ss_size && !on_sig_stack().
> >
> > But we already have on_sig_stack(sp) calculated. Why not write that as:
> >
> > +               if (current->sas_ss_size && !onsigstack)
> > +                       sp = current->sas_ss_sp + current->sas_ss_size;
> >
> > and since we check '!onsigstack' in both branches, we might as well factor it out
> > into a single condition ... and arrive to the exact code that we began with.
> ISTM it's silly for us to be unconditionally computing onsigstack.
> We're doing it because we need it later for this:
>
>          /*
>           * If we are on the alternate signal stack and would overflow it, don't.
>           * Return an always-bogus address instead so we will die with SIGSEGV.
>           */
>          if (onsigstack && !likely(on_sig_stack(sp)))
>                  return (void __user *)-1L;
>
> This seems basically useless to me.  Sure, it's nice to send SIGSEGV
> if we overflow due to signal delivery.  But we're almost as likely to
> overflow in the signal handler as we are to overflow during delivery,
> and we don't even try to catch that.
>
> Anyway, I think we should make two changes to the sig_on_stack thing:
>
> 1. If SS_AUTODISARM, then we're not on the stack, regardless of what sp says.
>
> 2. If !user_64bit_mode(regs) && (regs->ss & 0x4), then we're not on
> the signal stack.  This will happen if we're running on an LDT stack
> and we coincidentally have the ESP part of SS:ESP matching the signal
> stack.
>
> In general, the existing design is crap and it should always have
> worked the way that Stas is proposing with SS_AUTODISARM.
Andy, so can I use your Acked-by to this patch?
I think 2 is an entirely separate problem and can be implemented
later. Do you have any objections to the patch in its current form?
Do you think that 1 should be implemented, or can we agree that
it is not possible/unsafe? Please, state your optinion explicitly, because
I am completely puzzled with why this patch cannot be applied.