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;
But there is also the SA_RESTORER check that needs this as well:

} else if (config_enabled(CONFIG_X86_32) &&
   !onsigstack &&
   (regs->ss & 0xffff) != __USER_DS &&
   !(ka->sa.sa_flags & SA_RESTORER) &&
   ka->sa.sa_restorer) {
		/* This is the legacy signal stack switching. */
		sp = (unsigned long) ka->sa.sa_restorer;

So I don't think you can that easily get rid of calling
on_sig_stack(), but definitely we can structure the code
so that gcc can better optimize it.

> 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.
I think Ingo replied to the stand-alone cleanup I sent a month
ago. Now I have also included it in an SS_AUTODISARM series, and
that led to confusion... Sorry about that.
But I think the above is not correct: if SS_AUTODISARM is used,
we should still look into sp, or sa_restorer will not work.
And in fact, someone could even call sigaltstack(SS_ONSTACK|SS_AUTODISARM)
inside the sighandler itself! In this case we still have to look
into sp as we did before.
So I don't think the optimization you suggest above, can work.

> 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.
Yes, but let me please do that later. :)
If I start merging also that into an existing series, we'll never
get any progress. So can we just get applied at least something? :)

> In general, the existing design is crap and it should always have
> worked the way that Stas is proposing with SS_AUTODISARM.
But how would you then return oss->ss_flags to user?
I think returning oss->ss_flags to user requires the current
design, and by specifying SS_AUTODISARM, the user accepts the
fact that oss->ss_flags is no longer reliable (never returns SS_ONSTACK).