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

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()'.

Fixes: ed78b7b ("vdso/gettimeofday: Add a helper to read the sequence lock of a time namespace aware clock")
Reported-by: Ricardo Ribalda <ribalda@chromium.org>
Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Tested-by: Ricardo Ribalda <ribalda@chromium.org>
Reviewed-by: Christophe Leroy (CS GROUP) <chleroy@kernel.org>
Link: https://patch.msgid.link/20260422-vdso-aux-timens-loop-v1-1-e2dd8c7164cc@linutronix.de
Closes: https://lore.kernel.org/lkml/CANiDSCsOy0P1if-gJZqOM5pTJ0RDcwVfru1B7KFbTOEMqjPKJw@mail.gmail.com/

Author: t-8ch

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? */