Re: [PATCH] x86/signal: Remove pax argument from restore_sigcontext

[Brian Gerst]

On Mon, Apr 6, 2015 at 3:03 AM, Ingo Molnar <mingo@xxxxxxxxxx> wrote:
>
> * Brian Gerst <brgerst@xxxxxxxxx> 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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