Re: [PATCH v6 1/3] x86_64: Set the syscall register right before doing the syscall.

[Noah Goldstein <goldstein.w.n@gmail.com>]

On Fri, May 26, 2023 at 5:59 AM Joe Simmons-Talbott <josimmon@redhat.com> wrote:
>
> On Fri, May 26, 2023 at 09:04:06AM +0200, Florian Weimer wrote:
> > * Noah Goldstein via Libc-alpha:
> >
> > > I'm minorly opposed to this patch. Even if GLIBC guarantees all
> > > syscalls will set the number the instruction before, that's no guarantee
> > > for the entire program. Furthermore in the event of:
> > >    `movl $VAL, %eax; syscall`
> > > It's still not safe to *always* assume that `VAL` correspond to the
> > > syscall number as a jump (direct or indirect) could still go between
> > > the instructions (i.e there is no guarantee in the assembly that the
> > > `mov` dominates the `syscall).
> > > So at the end of the day, we are bloating the library without, AFAICT,
> > > providing any real guarantee. Maybe I'm missing something?
> >
> > Joe, is there a size change to libc.so.6 as the result of this change?
>
> No, the size is the same with and with out this patchset on x86_64.
>
There aren't many syscalls so it's only a minor cost (hence the only
minor opposition), but I don't see the value this provides given that it
still won't be safe to assume the syscall number is always set the
instruction beforehand for any robust purpose. So it still feels like
why take any cost at all?

> Thanks,
> Joe
>