linux-kernel - Re: [PATCH] x86/signal: Remove pax argument from restore

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Message-ID: <CAMzpN2jbnUnAjqLK+TiA5t8-MgjoV5vdnkO4G6hGcULmGW7n+A@mail.gmail.com>
Date:	Mon, 6 Apr 2015 08:00:40 -0400
From:	Brian Gerst <brgerst@...il.com>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	"H. Peter Anvin" <hpa@...or.com>,
	Andy Lutomirski <luto@...capital.net>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Borislav Petkov <bp@...en8.de>,
	"the arch/x86 maintainers" <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86/signal: Remove pax argument from restore_sigcontext

On Mon, Apr 6, 2015 at 3:03 AM, Ingo Molnar <mingo@...nel.org> wrote:
>
> * Brian Gerst <brgerst@...il.com> wrote:
>
>> > I'm wondering what the original reason for adding the extra
>> > handling of regs->ax was. Maybe something changed regs->ax - but I
>> > cannot find such code path anymore.
>> >
>> > It would be nice to try to do a bit of Git archeology to figure
>> > out the origins of this complication - maybe it's something subtle
>> > - or it's something that has changed meanwhile.
>>
>> It goes all the way back to 2.1.106pre1, when restore_sigcontext()
>> was changed to return an error code instead of EAX directly.
>>
>> https://git.kernel.org/cgit/linux/kernel/git/history/history.git/diff/arch/i386/kernel/signal.c?id=9a8f8b7ca3f319bd668298d447bdf32730e51174
>
> Indeed: restore_sigcontext() used to return eax as a return value,
> without copying it into regs->ax.
>
> Then in 2007

Version 2.1.106 was released on Jun 13, 1998.

> sigaltstack syscall support was added, where the return
> value of restore_sigcontext() was changed to carry the memory-copying
> failure code. But instead of putting 'ax' into regs->ax, it was
> carried in via a pointer and then returned, where the generic syscall
> return code copied it to regs->ax.
>
> So there was never any deeper reason for this suboptimal pattern, it
> was simply never noticed after being introduced.
>
> (Btw., the regs->ax we return will be copied back to regs->ax after
> the syscall straight away once again - but I guess this cannot be
> helped.)

The 64-bit stub could skip saving it back to regs.  However 32-bit
does not have a special stub so sys_rt_sigreturn() still needs to
return regs->ax.

--
Brian Gerst
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