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Message-ID: <8df11af98033e4cb4d9b0f16d6e9d5b69110b036.1749724057.git.christophe.leroy@csgroup.eu>
Date: Thu, 12 Jun 2025 12:39:39 +0200
From: Christophe Leroy <christophe.leroy@...roup.eu>
To: Jaroslav Kysela <perex@...ex.cz>,
Takashi Iwai <tiwai@...e.com>
Cc: Christophe Leroy <christophe.leroy@...roup.eu>,
linux-kernel@...r.kernel.org,
linuxppc-dev@...ts.ozlabs.org,
linux-sound@...r.kernel.org,
Herve Codina <herve.codina@...tlin.com>,
Mark Brown <broonie@...nel.org>
Subject: [PATCH] ALSA: pcm: Convert snd_pcm_ioctl_sync_ptr_{compat/x32} to user_access_begin/user_access_end()
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 ioctl is a hot path that needs to be
optimised. To do that, perform user accesses by blocks using
user_access_begin/user_access_end() and unsafe_get_user()/
unsafe_put_user() and alike.
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@...roup.eu>
---
This is a lighter version of previous patch "[PATCH v2] ALSA: pcm: Convert multiple {get/put}_user to user_access_begin/user_access_end()" focussing on identified hot path.
Moved and nested the failure labels closer in order to increase readability
Link: https://patchwork.ozlabs.org/project/linuxppc-dev/patch/d2609397eafc2b55ec1f44a3f30ccec00e0c7f6e.1749455639.git.christophe.leroy@csgroup.eu/
sound/core/pcm_compat.c | 40 +++++++++++++++++++++++++++-------------
sound/core/pcm_native.c | 40 +++++++++++++++++++++++++++-------------
2 files changed, 54 insertions(+), 26 deletions(-)
diff --git a/sound/core/pcm_compat.c b/sound/core/pcm_compat.c
index a42ec7f5a1da..348a72e6499e 100644
--- a/sound/core/pcm_compat.c
+++ b/sound/core/pcm_compat.c
@@ -418,10 +418,18 @@ static int snd_pcm_ioctl_sync_ptr_x32(struct snd_pcm_substream *substream,
if (snd_BUG_ON(!runtime))
return -EINVAL;
- if (get_user(sflags, &src->flags) ||
- get_user(scontrol.appl_ptr, &src->c.control.appl_ptr) ||
- get_user(scontrol.avail_min, &src->c.control.avail_min))
+ if (!user_read_access_begin(src, sizeof(*src)))
return -EFAULT;
+ err = -EFAULT;
+ unsafe_get_user(sflags, &src->flags, Efault_rd);
+ unsafe_get_user(scontrol.appl_ptr, &src->c.control.appl_ptr, Efault_rd);
+ unsafe_get_user(scontrol.avail_min, &src->c.control.avail_min, Efault_rd);
+ err = 0;
+Efault_rd:
+ user_read_access_end();
+ if (err)
+ return err;
+
if (sflags & SNDRV_PCM_SYNC_PTR_HWSYNC) {
err = snd_pcm_hwsync(substream);
if (err < 0)
@@ -450,18 +458,24 @@ static int snd_pcm_ioctl_sync_ptr_x32(struct snd_pcm_substream *substream,
}
if (!(sflags & SNDRV_PCM_SYNC_PTR_APPL))
snd_pcm_dma_buffer_sync(substream, SNDRV_DMA_SYNC_DEVICE);
- if (put_user(sstatus.state, &src->s.status.state) ||
- put_user(sstatus.hw_ptr, &src->s.status.hw_ptr) ||
- put_user(sstatus.tstamp.tv_sec, &src->s.status.tstamp_sec) ||
- put_user(sstatus.tstamp.tv_nsec, &src->s.status.tstamp_nsec) ||
- put_user(sstatus.suspended_state, &src->s.status.suspended_state) ||
- put_user(sstatus.audio_tstamp.tv_sec, &src->s.status.audio_tstamp_sec) ||
- put_user(sstatus.audio_tstamp.tv_nsec, &src->s.status.audio_tstamp_nsec) ||
- put_user(scontrol.appl_ptr, &src->c.control.appl_ptr) ||
- put_user(scontrol.avail_min, &src->c.control.avail_min))
+
+ if (!user_write_access_begin(src, sizeof(*src)))
return -EFAULT;
+ err = -EFAULT;
+ unsafe_put_user(sstatus.state, &src->s.status.state, Efault_wr);
+ unsafe_put_user(sstatus.hw_ptr, &src->s.status.hw_ptr, Efault_wr);
+ unsafe_put_user(sstatus.tstamp.tv_sec, &src->s.status.tstamp_sec, Efault_wr);
+ unsafe_put_user(sstatus.tstamp.tv_nsec, &src->s.status.tstamp_nsec, Efault_wr);
+ unsafe_put_user(sstatus.suspended_state, &src->s.status.suspended_state, Efault_wr);
+ unsafe_put_user(sstatus.audio_tstamp.tv_sec, &src->s.status.audio_tstamp_sec, Efault_wr);
+ unsafe_put_user(sstatus.audio_tstamp.tv_nsec, &src->s.status.audio_tstamp_nsec, Efault_wr);
+ unsafe_put_user(scontrol.appl_ptr, &src->c.control.appl_ptr, Efault_wr);
+ unsafe_put_user(scontrol.avail_min, &src->c.control.avail_min), Efault_wr);
+ err = 0;
+Efault_wr:
+ user_write_access_end();
- return 0;
+ return err;
}
#endif /* CONFIG_X86_X32_ABI */
diff --git a/sound/core/pcm_native.c b/sound/core/pcm_native.c
index ecb71bf1859d..b104faddb6e3 100644
--- a/sound/core/pcm_native.c
+++ b/sound/core/pcm_native.c
@@ -3165,10 +3165,18 @@ static int snd_pcm_ioctl_sync_ptr_compat(struct snd_pcm_substream *substream,
if (snd_BUG_ON(!runtime))
return -EINVAL;
- if (get_user(sflags, &src->flags) ||
- get_user(scontrol.appl_ptr, &src->c.control.appl_ptr) ||
- get_user(scontrol.avail_min, &src->c.control.avail_min))
+ if (!user_read_access_begin(src, sizeof(*src)))
return -EFAULT;
+ err = -EFAULT;
+ unsafe_get_user(sflags, &src->flags, Efault_rd);
+ unsafe_get_user(scontrol.appl_ptr, &src->c.control.appl_ptr, Efault_rd);
+ unsafe_get_user(scontrol.avail_min, &src->c.control.avail_min, Efault_rd);
+ err = 0;
+Efault_rd:
+ user_read_access_end();
+ if (err)
+ return err;
+
if (sflags & SNDRV_PCM_SYNC_PTR_HWSYNC) {
err = snd_pcm_hwsync(substream);
if (err < 0)
@@ -3200,18 +3208,24 @@ static int snd_pcm_ioctl_sync_ptr_compat(struct snd_pcm_substream *substream,
}
if (!(sflags & SNDRV_PCM_SYNC_PTR_APPL))
snd_pcm_dma_buffer_sync(substream, SNDRV_DMA_SYNC_DEVICE);
- if (put_user(sstatus.state, &src->s.status.state) ||
- put_user(sstatus.hw_ptr, &src->s.status.hw_ptr) ||
- put_user(sstatus.tstamp.tv_sec, &src->s.status.tstamp_sec) ||
- put_user(sstatus.tstamp.tv_nsec, &src->s.status.tstamp_nsec) ||
- put_user(sstatus.suspended_state, &src->s.status.suspended_state) ||
- put_user(sstatus.audio_tstamp.tv_sec, &src->s.status.audio_tstamp_sec) ||
- put_user(sstatus.audio_tstamp.tv_nsec, &src->s.status.audio_tstamp_nsec) ||
- put_user(scontrol.appl_ptr, &src->c.control.appl_ptr) ||
- put_user(scontrol.avail_min, &src->c.control.avail_min))
+
+ if (!user_write_access_begin(src, sizeof(*src)))
return -EFAULT;
+ err = -EFAULT;
+ unsafe_put_user(sstatus.state, &src->s.status.state, Efault_wr);
+ unsafe_put_user(sstatus.hw_ptr, &src->s.status.hw_ptr, Efault_wr);
+ unsafe_put_user(sstatus.tstamp.tv_sec, &src->s.status.tstamp_sec, Efault_wr);
+ unsafe_put_user(sstatus.tstamp.tv_nsec, &src->s.status.tstamp_nsec, Efault_wr);
+ unsafe_put_user(sstatus.suspended_state, &src->s.status.suspended_state, Efault_wr);
+ unsafe_put_user(sstatus.audio_tstamp.tv_sec, &src->s.status.audio_tstamp_sec, Efault_wr);
+ unsafe_put_user(sstatus.audio_tstamp.tv_nsec, &src->s.status.audio_tstamp_nsec, Efault_wr);
+ unsafe_put_user(scontrol.appl_ptr, &src->c.control.appl_ptr, Efault_wr);
+ unsafe_put_user(scontrol.avail_min, &src->c.control.avail_min, Efault_wr);
+ err = 0;
+Efault_wr:
+ user_write_access_end();
- return 0;
+ return err;
}
#define __SNDRV_PCM_IOCTL_SYNC_PTR32 _IOWR('A', 0x23, struct snd_pcm_sync_ptr32)
--
2.47.0
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