[PATCH] lib/vdso: use CLOCK_REALTIME_COARSE for time()

[PATCH] lib/vdso: use CLOCK_REALTIME_COARSE for time()

[Mark Salyzyn]

From: Chiawei Wang <chiaweiwang@google.com>

CLOCK_REALTIME in vdso data won't be updated if
__arch_use_vsyscall() returns false. It will let time()
return an incorrect time value. Since time() is designed
to return the seconds since the Epoch, using
CLOCK_REALTIME_COARSE can still fulfill the request and
never fails.

Signed-off-by: Chiawei Wang <chiaweiwang@google.com>
Signed-off-by: Mark Salyzyn <salyzyn@android.com>
Cc: linux-kernel@vger.kernel.org
Cc: kernel-team@android.com
Cc: stable@vger.kernel.org # 5.4+
---
 lib/vdso/gettimeofday.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/lib/vdso/gettimeofday.c b/lib/vdso/gettimeofday.c
index a2909af4b924..7ea22096cbe2 100644
--- a/lib/vdso/gettimeofday.c
+++ b/lib/vdso/gettimeofday.c
@@ -324,7 +324,7 @@ __cvdso_time_data(const struct vdso_data *vd, __kernel_old_time_t *time)
 	    vd->clock_mode == VDSO_CLOCKMODE_TIMENS)
 		vd = __arch_get_timens_vdso_data();
 
-	t = READ_ONCE(vd[CS_HRES_COARSE].basetime[CLOCK_REALTIME].sec);
+	t = READ_ONCE(vd[CS_HRES_COARSE].basetime[CLOCK_REALTIME_COARSE].sec);
 
 	if (time)
 		*time = t;
-- 
2.27.0.278.ge193c7cf3a9-goog

Re: [PATCH] lib/vdso: use CLOCK_REALTIME_COARSE for time()

[Thomas Gleixner]

Mark Salyzyn <salyzyn@android.com> writes:
> From: Chiawei Wang <chiaweiwang@google.com>
>
> CLOCK_REALTIME in vdso data won't be updated if
> __arch_use_vsyscall() returns false.

Errm!

# git grep __arch_use_vsyscall
#

Aside of that update_vsyscall() updates CLOCK_REALTIME data
unconditionally. No idea what this patch is solving.

> Cc: stable@vger.kernel.org # 5.4+

This # 5.4+ is pointless. You really want to add a fixes tag which pin
points the commit which introduced the wreckage.

But thats moot as this is not fixing anything not even in 5.4.

I assume this was developed against some Frankenkernel which has a messy
backport or a snapshot of some development version of that vdso stuff.

Not that I want to know, but please make sure that something you send my
way makes sense on sane kernels.

Oh well.

Thanks,

        tglx

Re: [PATCH] lib/vdso: use CLOCK_REALTIME_COARSE for time()

[Mark Salyzyn]

On Jun 11, 2020, at 12:34 PM, Thomas Gleixner <tglx@linutronix.de> wrote:
> 
> Mark Salyzyn <salyzyn@android.com> writes:
>> From: Chiawei Wang <chiaweiwang@google.com>
>> 
>> CLOCK_REALTIME in vdso data won't be updated if
>> __arch_use_vsyscall() returns false.
> 
> Errm!
> 
> # git grep __arch_use_vsyscall
> #
> 
> Aside of that update_vsyscall() updates CLOCK_REALTIME data
> unconditionally. No idea what this patch is solving.
> 
>> Cc: stable@vger.kernel.org # 5.4+
> 
> This # 5.4+ is pointless. You really want to add a fixes tag which pin
> points the commit which introduced the wreckage.
> 
> But thats moot as this is not fixing anything not even in 5.4.
> 
> I assume this was developed against some Frankenkernel which has a messy
> backport or a snapshot of some development version of that vdso stuff.

Yes, problem code was removed with 52338415cf4d4064ae6b8dd972dadbda841da4fa not applied.

4.19 back port without time namespaces, chip vendor using __arch_use_vsyscall to solve a chip errata.

> 
> Not that I want to know, but please make sure that something you send my
> way makes sense on sane kernels.
> 
> Oh well.
> 
> Thanks,
> 
>        tglx

Sorry for your troubles.

Sincerely — Mark Salyzyn