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

[Christophe Leroy <christophe.leroy@csgroup.eu>]

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@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().

Christophe