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

[Ingo Molnar <mingo@kernel.org>]

* Brian Gerst <brgerst@gmail.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 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.)

I've added this information to the changelog.

Thanks,

	Ingo