* Brian Gerst <brge...@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
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