[PATCH v2] ALSA: pcm: Convert multiple {get/put}_user to user_access_begin/user_access_end()

[Christophe Leroy]

Le 09/06/2025 à 13:00, Takashi Iwai a écrit :
> On Mon, 09 Jun 2025 12:02:00 +0200,
> Christophe Leroy wrote:
>>
>>
>>
>> Le 09/06/2025 à 10:10, Takashi Iwai a écrit :
>>> On Mon, 09 Jun 2025 10:00:38 +0200,
>>> Christophe Leroy wrote:
>>>>
>>>> With user access protection (Called SMAP on x86 or KUAP on powerpc)
>>>> each and every call to get_user() or put_user() performs heavy
>>>> operations to unlock and lock kernel access to userspace.
>>>>
>>>> To avoid that, perform user accesses by blocks using
>>>> user_access_begin/user_access_end() and unsafe_get_user()/
>>>> unsafe_put_user() and alike.
>>>>
>>>> As an exemple, before the patch the 9 calls to put_user() at the
>>>> end of snd_pcm_ioctl_sync_ptr_compat() imply the following set of
>>>> instructions about 9 times (access_ok - enable user - write - disable
>>>> user):
>>>>       0.00 :   c057f858:       3d 20 7f ff     lis     r9,32767
>>>>       0.29 :   c057f85c:       39 5e 00 14     addi    r10,r30,20
>>>>       0.77 :   c057f860:       61 29 ff fc     ori     r9,r9,65532
>>>>       0.32 :   c057f864:       7c 0a 48 40     cmplw   r10,r9
>>>>       0.36 :   c057f868:       41 a1 fb 58     bgt     c057f3c0 <snd_pcm_ioctl+0xbb0>
>>>>       0.30 :   c057f86c:       3d 20 dc 00     lis     r9,-9216
>>>>       1.95 :   c057f870:       7d 3a c3 a6     mtspr   794,r9
>>>>       0.33 :   c057f874:       92 8a 00 00     stw     r20,0(r10)
>>>>       0.27 :   c057f878:       3d 20 de 00     lis     r9,-8704
>>>>       0.28 :   c057f87c:       7d 3a c3 a6     mtspr   794,r9
>>>> ...
>>>>
>>>> A perf profile shows that in total the 9 put_user() represent 36% of
>>>> the time spent in snd_pcm_ioctl() and about 80 instructions.
>>>>
>>>> With this patch everything is done in 13 instructions and represent
>>>> only 15% of the time spent in snd_pcm_ioctl():
>>>>
>>>>       0.57 :   c057f5dc:       3d 20 dc 00     lis     r9,-9216
>>>>       0.98 :   c057f5e0:       7d 3a c3 a6     mtspr   794,r9
>>>>       0.16 :   c057f5e4:       92 7f 00 04     stw     r19,4(r31)
>>>>       0.63 :   c057f5e8:       93 df 00 0c     stw     r30,12(r31)
>>>>       0.16 :   c057f5ec:       93 9f 00 10     stw     r28,16(r31)
>>>>       4.95 :   c057f5f0:       92 9f 00 14     stw     r20,20(r31)
>>>>       0.19 :   c057f5f4:       92 5f 00 18     stw     r18,24(r31)
>>>>       0.49 :   c057f5f8:       92 bf 00 1c     stw     r21,28(r31)
>>>>       0.27 :   c057f5fc:       93 7f 00 20     stw     r27,32(r31)
>>>>       5.88 :   c057f600:       93 36 00 00     stw     r25,0(r22)
>>>>       0.11 :   c057f604:       93 17 00 00     stw     r24,0(r23)
>>>>       0.00 :   c057f608:       3d 20 de 00     lis     r9,-8704
>>>>       0.79 :   c057f60c:       7d 3a c3 a6     mtspr   794,r9
>>>>
>>>> Note that here the access_ok() in user_write_access_begin() is skipped
>>>> because the exact same verification has already been performed at the
>>>> beginning of the fonction with the call to user_read_access_begin().
>>>>
>>>> A couple more can be converted as well but require
>>>> unsafe_copy_from_user() which is not defined on x86 and arm64, so
>>>> those are left aside for the time being and will be handled in a
>>>> separate patch.
>>>>
>>>> Signed-off-by: Christophe Leroy <christophe.leroy at csgroup.eu>
>>>> ---
>>>> v2: Split out the two hunks using copy_from_user() as unsafe_copy_from_user() is not implemented on x86 and arm64 yet.
>>>
>>> Thanks for the patch.
>>>
>>> The idea looks interesting, but the implementations with
>>> unsafe_get_user() leads to very ugly goto lines, and that's too bad;
>>> it makes the code flow much more difficult to follow.
>>>
>>> I guess that, in most cases this patch tries to cover, we just use
>>> another temporary variable for compat struct, copy fields locally,
>>> then run copy_to_user() in a shot instead.
>>
>> Thanks for looking.
>>
>> I'll give it a try but I think going through a local intermediate will
>> be less performant than direct copy with unsafe_get/put_user().
> 

I have now tried going through a temporary struct and the result is 
awful, even worth than the current implementation, because 
snd_pcm_ioctl_sync_ptr_compat() is not inlined anymore into 
snd_pcm_ioctl(), and the call to copy_from_user() and copy_to_user() 
also show up in the top 10 functions in the perf profile.

> Yes, but the code readability is often more important than minor
> optimizations unless it's in a hot path.

So let's focus on the identified hot path: the SNDRV_PCM_IOCTL_SYNC_PTR 
iotcl.

I will send a patch focussing only on that part. I tweaked it a bit to 
increase readability by nesting the failure labels closer to the 
actions. Let me know whether that patch is more acceptable for you.

In parallel I will send a RFC series that reworks a bit deaper the 
SNDRV_PCM_IOCTL_SYNC_PTR functions with a helper macro, maybe you will 
prefer that allthough the churn is bigger.

Christophe