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

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Message-ID: <795db5f1-c266-4fb3-a51b-c2b3745d334b@csgroup.eu>
Date: Fri, 6 Sep 2024 16:26:32 +0200
From: Christophe Leroy <christophe.leroy@...roup.eu>
To: "Jason A. Donenfeld" <Jason@...c4.com>
Cc: linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
 Michael Ellerman <mpe@...erman.id.au>, Nicholas Piggin <npiggin@...il.com>,
 Naveen N Rao <naveen@...nel.org>,
 Vincenzo Frascino <vincenzo.frascino@....com>,
 Andrei Vagin <avagin@...il.com>
Subject: Re: [PATCH 2/2] Fixup for 3279be36b671 ("powerpc/vdso: Wire up
 getrandom() vDSO implementation on VDSO32")

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@...roup.eu>
>> ---
>>   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