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

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Date:	Tue, 8 Mar 2016 17:20:46 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	Stas Sergeev <stsp@...t.ru>
Cc:	linux-kernel@...r.kernel.org,
	Andy Lutomirski <luto@...capital.net>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
	Borislav Petkov <bp@...e.de>, Brian Gerst <brgerst@...il.com>,
	Oleg Nesterov <oleg@...hat.com>,
	Richard Weinberger <richard@....at>,
	Stas Sergeev <stsp@...rs.sourceforge.net>
Subject: Re: [PATCH] [Cleanup] x86: signal: unify the sigaltstack check with
 other arches


* Stas Sergeev <stsp@...t.ru> wrote:

> 25.02.2016 11:25, Ingo Molnar пишет:
> > 
> > * Stas Sergeev <stsp@...t.ru> 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.

So what happened is that every other arch has a non-optimal version of this 
function.

And if you look at the x86-32 defconfig build size difference:

   text    data     bss     dec     hex filename
   4155       0       0    4155    103b signal.o.before
   4299       0       0    4299    10cb signal.o.after

i.e. your patch increases the generated code size. So I don't see the upside.

If this is really duplicated across architectures then we should perhaps try to 
factor out this check into kernel/signal.c or so, and share it between 
architectures more seriously?

Thanks,

	Ingo