Re: [PATCH 2/2] Fixup for 3279be36b671 ("powerpc/vdso: Wire up getrandom() vDSO implementation on VDSO32")

From: Christophe Leroy
Date: Fri Sep 06 2024 - 10:26:45 EST




Le 06/09/2024 à 16:07, Jason A. Donenfeld a écrit :
On Fri, Sep 06, 2024 at 10:33:44AM +0200, Christophe Leroy wrote:
Use the new get_realdatapage macro instead of get_datapage

Signed-off-by: Christophe Leroy <christophe.leroy@xxxxxxxxxx>
---
arch/powerpc/kernel/vdso/getrandom.S | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/powerpc/kernel/vdso/getrandom.S b/arch/powerpc/kernel/vdso/getrandom.S
index a957cd2b2b03..f3bbf931931c 100644
--- a/arch/powerpc/kernel/vdso/getrandom.S
+++ b/arch/powerpc/kernel/vdso/getrandom.S
@@ -31,7 +31,7 @@
PPC_STL r2, PPC_MIN_STKFRM + STK_GOT(r1)
.cfi_rel_offset r2, PPC_MIN_STKFRM + STK_GOT
#endif
- get_datapage r8
+ get_realdatapage r8, r11
addi r8, r8, VDSO_RNG_DATA_OFFSET
bl CFUNC(DOTSYM(\funct))
PPC_LL r0, PPC_MIN_STKFRM + PPC_LR_STKOFF(r1)

I tested that this is working as intended on powerpc, powerpc64, and
powerpc64le. Thanks for writing the patch so quickly.

You are welcome.

And thanks for playing up with it while I was sleeping and getting ideas too.

Did you learn powerpc assembly during the night or did you know it already ?

At the end I ended up with something which I think is simple enough for a backport to stable.

On the long run I wonder if we should try to find a more generic solution for getrandom instead of requiring each architecture to handle it. On gettimeofday the selection of the right page is embeded in the generic part, see for instance :

static __maybe_unused __kernel_old_time_t
__cvdso_time_data(const struct vdso_data *vd, __kernel_old_time_t *time)
{
__kernel_old_time_t t;

if (IS_ENABLED(CONFIG_TIME_NS) &&
vd->clock_mode == VDSO_CLOCKMODE_TIMENS)
vd = __arch_get_timens_vdso_data(vd);

t = READ_ONCE(vd[CS_HRES_COARSE].basetime[CLOCK_REALTIME].sec);

if (time)
*time = t;

return t;
}

and powerpc just provides:

static __always_inline
const struct vdso_data *__arch_get_timens_vdso_data(const struct vdso_data *vd)
{
return (void *)vd + (1U << CONFIG_PAGE_SHIFT);
}


I know it may not be that simple for getrandom but its probably worth trying.

Or another solution could be to put random data in a third page that is always at the same place regardless of timens ?

Christophe