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

[Takashi Iwai <tiwai@suse.de>]

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.


Takashi