Re: [PATCH] vdso/gettimeofday: Reload sequence counter after switch to time page in do_aux()

[Ricardo Ribalda]

Hi Thomas.

Thanks for the patch. Any idea when do you plan to land this?

Regards!


On Wed, 22 Apr 2026 at 11:49, Christophe Leroy (CS GROUP)
<chleroy@xxxxxxxxxx> wrote:
>
>
>
> Le 22/04/2026 à 11:42, Thomas Weißschuh a écrit :
> > After switching to the real data pages, the sequence counter needs to be
> > reloaded from there. The code using vdso_read_begin_timens() assumed
> > this worked by 'continue' jumping to the *beginning* of the do-while
> > retry loop. However the 'continue' jumps to the *end* of said loop,
> > evaluating the exit condition. If the data page has a sequence counter
> > of '1' it will match the one from the time namespace page and prematurely
> > exit the retry loop. This would result in garbage returned to the caller.
> >
> > Reload the sequence counter after switching the pages by using an inner
> > while loop again, which will loop at most once.
> >
> > The loop generates slightly better code than an explicit reload through
> > 'seq = vdso_read_begin()'.
> >
> > Reported-by: Ricardo Ribalda <ribalda@xxxxxxxxxxxx>
> > Closes: https://eur01.safelinks.protection.outlook.com/?url=https%3A%2F%2Flore.kernel.org%2Flkml%2FCANiDSCsOy0P1if-gJZqOM5pTJ0RDcwVfru1B7KFbTOEMqjPKJw%40mail.gmail.com%2F&data=05%7C02%7Cchristophe.leroy%40csgroup.eu%7C8090d715bf644286a5e908dea0538801%7C8b87af7d86474dc78df45f69a2011bb5%7C0%7C0%7C639124477796711565%7CUnknown%7CTWFpbGZsb3d8eyJFbXB0eU1hcGkiOnRydWUsIlYiOiIwLjAuMDAwMCIsIlAiOiJXaW4zMiIsIkFOIjoiTWFpbCIsIldUIjoyfQ%3D%3D%7C0%7C%7C%7C&sdata=yQti2tvfaExxkn%2BoD7wzPhAkFTYQ7Sr%2FV3e3fRdJYVY%3D&reserved=0
> > Fixes: ed78b7b2c5ae ("vdso/gettimeofday: Add a helper to read the sequence lock of a time namespace aware clock")
> > Signed-off-by: Thomas Weißschuh <thomas.weissschuh@xxxxxxxxxxxxx>
>
> Reviewed-by: Christophe Leroy (CS GROUP) <chleroy@xxxxxxxxxx>
>
> > ---
> >   lib/vdso/gettimeofday.c | 5 ++---
> >   1 file changed, 2 insertions(+), 3 deletions(-)
> >
> > diff --git a/lib/vdso/gettimeofday.c b/lib/vdso/gettimeofday.c
> > index a5798bd26d20..da224011fafd 100644
> > --- a/lib/vdso/gettimeofday.c
> > +++ b/lib/vdso/gettimeofday.c
> > @@ -248,11 +248,10 @@ bool do_aux(const struct vdso_time_data *vd, clockid_t clock, struct __kernel_ti
> >       vc = &vd->aux_clock_data[idx];
> >
> >       do {
> > -             if (vdso_read_begin_timens(vc, &seq)) {
> > +             while (vdso_read_begin_timens(vc, &seq)) {
> > +                     /* Re-read from the real time data page, reload seq by looping */
> >                       vd = __arch_get_vdso_u_timens_data(vd);
> >                       vc = &vd->aux_clock_data[idx];
> > -                     /* Re-read from the real time data page */
> > -                     continue;
> >               }
> >
> >               /* Auxclock disabled? */
> >
> > ---
> > base-commit: aa70a22882fe51fadd2392b480864d15e1b9ab3d
> > change-id: 20260422-vdso-aux-timens-loop-acbaa5a2434e
> >
> > Best regards,
> > --
> > Thomas Weißschuh <thomas.weissschuh@xxxxxxxxxxxxx>
> >
>


--
Ricardo Ribalda