Re: [PATCH v2 2/4] ALSA: pcm: Convert SNDRV_PCM_IOCTL_SYNC_PTR to user_access_begin/user_access_end()

[Takashi Iwai <tiwai@suse.de>]

On Fri, 13 Jun 2025 17:37:09 +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.
> 
> SNDRV_PCM_IOCTL_SYNC_PTR is a hot path which is called really often
> and needs to run as fast as possible.
> 
> To improve performance, perform user accesses by blocks using
> user_access_begin/user_access_end() and unsafe_get_user()/
> unsafe_put_user().
> 
> 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().
> 
> Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>

Hm, with this patch, I got a compile warning:

sound/core/snd-pcm.o: warning: objtool: .altinstr_replacement+0x12: redundant UACCESS disable

It's with gcc-13.3.1.


Takashi

> ---
>  sound/core/pcm_native.c | 42 +++++++++++++++++++++++++----------------
>  1 file changed, 26 insertions(+), 16 deletions(-)
> 
> diff --git a/sound/core/pcm_native.c b/sound/core/pcm_native.c
> index 20326aa377b0..fd69eea935b2 100644
> --- a/sound/core/pcm_native.c
> +++ b/sound/core/pcm_native.c
> @@ -3053,30 +3053,40 @@ static inline int snd_pcm_hwsync(struct snd_pcm_substream *substream)
>  }
>  
>  #define snd_pcm_sync_ptr_get_user(__f, __c, __ptr) ({				\
> -	int __err = 0;								\
> +	__label__ failed;							\
> +	int __err = -EFAULT;							\
>  	typeof(*(__ptr)) __user *__src = (__ptr);					\
>  										\
> -	if (get_user(__f, &src->flags) ||					\
> -	    get_user(__c.appl_ptr, &__src->c.control.appl_ptr) ||		\
> -	    get_user(__c.avail_min, &__src->c.control.avail_min))		\
> -		__err = -EFAULT;						\
> +	if (!user_read_access_begin(__src, sizeof(*__src)))			\
> +		goto failed;							\
> +	unsafe_get_user(__f, &__src->flags, failed);				\
> +	unsafe_get_user(__c.appl_ptr, &__src->c.control.appl_ptr, failed);	\
> +	unsafe_get_user(__c.avail_min, &__src->c.control.avail_min, failed);	\
> +	__err = 0;								\
> +failed:										\
> +	user_read_access_end();							\
>  	__err;									\
>  })
>  
>  #define snd_pcm_sync_ptr_put_user(__s, __c, __ptr) ({				\
> -	int __err = 0;								\
> +	__label__ failed;							\
> +	int __err = -EFAULT;							\
>  	typeof(*(__ptr)) __user *__src = (__ptr);					\
>  										\
> -	if (put_user(__s.state, &__src->s.status.state) ||			\
> -	    put_user(__s.hw_ptr, &__src->s.status.hw_ptr) ||			\
> -	    put_user(__s.tstamp.tv_sec, &__src->s.status.tstamp_sec) ||		\
> -	    put_user(__s.tstamp.tv_nsec, &__src->s.status.tstamp_nsec) ||	\
> -	    put_user(__s.suspended_state, &__src->s.status.suspended_state) ||	\
> -	    put_user(__s.audio_tstamp.tv_sec, &__src->s.status.audio_tstamp_sec) ||	\
> -	    put_user(__s.audio_tstamp.tv_nsec, &__src->s.status.audio_tstamp_nsec) ||	\
> -	    put_user(__c.appl_ptr, &__src->c.control.appl_ptr) ||		\
> -	    put_user(__c.avail_min, &__src->c.control.avail_min))		\
> -		__err = -EFAULT;						\
> +	if (!user_write_access_begin(__src, sizeof(*__src)))			\
> +		goto failed;							\
> +	unsafe_put_user(__s.state, &__src->s.status.state, failed);		\
> +	unsafe_put_user(__s.hw_ptr, &__src->s.status.hw_ptr, failed);		\
> +	unsafe_put_user(__s.tstamp.tv_sec, &__src->s.status.tstamp_sec, failed);\
> +	unsafe_put_user(__s.tstamp.tv_nsec, &__src->s.status.tstamp_nsec, failed);		\
> +	unsafe_put_user(__s.suspended_state, &__src->s.status.suspended_state, failed);		\
> +	unsafe_put_user(__s.audio_tstamp.tv_sec, &__src->s.status.audio_tstamp_sec, failed);	\
> +	unsafe_put_user(__s.audio_tstamp.tv_nsec, &__src->s.status.audio_tstamp_nsec, failed);	\
> +	unsafe_put_user(__c.appl_ptr, &__src->c.control.appl_ptr, failed);	\
> +	unsafe_put_user(__c.avail_min, &__src->c.control.avail_min, failed);	\
> +	__err = 0;								\
> +failed:										\
> +	user_write_access_end();						\
>  	__err;									\
>  })
>  
> -- 
> 2.47.0
>