From ded106036984a9dd766e7e1451b02e2d15c81438 Mon Sep 17 00:00:00 2001 From: Henrik Enquist Date: Sat, 3 Oct 2026 15:26:11 +0200 Subject: [PATCH] ALSA: aloop: Don't constrain playback params in notify mode With notify enabled (the pcm_notify module parameter or the "PCM Notify" control), the playback side of a cable may change format, rate and channels while a capture is running. loopback_check_format() then stops the capture and notifies the "PCM Slave" controls, so that the capture application can reopen with the new parameters. For this reason loopback_open() still gives a notify mode playback the unconstrained loopback_pcm_hardware. Since commit 898dfe4687f4 ("ALSA: aloop: Fix racy hw constraints adjustment") the hw rules refine against the shared cable->hw instead of the substream's own runtime->hw. This pins the playback side to the parameters of an open capture whatever the notify setting is. A player probing the device sees only the capture's rate, format and channels, and the format change path in loopback_check_format() can't be reached by a new playback. Skip the format, rate and channels rules for the playback substream when notify is enabled. Skip the period bytes rule as well: with a sound timer source, params_change() already takes the period size from a notify mode playback, and keeping the old capture's period bytes leaves a playback at another rate with a period that doesn't match the timer. The capture side stays constrained to the cable parameters. The capture stop this makes reachable again was hardened by commit 826af7fa62e3 ("ALSA: aloop: Fix racy access at PCM trigger") and commit e5c33cdc6f40 ("ALSA: aloop: Fix peer runtime UAF during format-change stop"), which need to be applied first when backporting. Reproducer, with notify enabled on the cable: arecord -D hw:Loopback,1,0 -r 44100 -f S32_LE -c 2 /dev/null & aplay -D hw:Loopback,0,0 96k.wav Before: aplay is forced to 44100 Hz and the capture keeps running. After: aplay runs at 96000 Hz, the capture is stopped and "PCM Slave Rate" reports 96000. Fixes: 898dfe4687f4 ("ALSA: aloop: Fix racy hw constraints adjustment") Reported-by: Pavel Hofman Closes: https://lore.kernel.org/all/b4af9071-f8d7-5b47-4d7a-c5743bd67394@ivitera.com/ Cc: stable@vger.kernel.org Assisted-by: Claude:claude-opus-5-5 Signed-off-by: Henrik Enquist Signed-off-by: Takashi Iwai Link: https://patch.msgid.link/20261003-aloop-notify-fix-v1-1-ba5dfb831bf5@gmail.com --- sound/drivers/aloop.c | 20 ++++++++++++++++++++ 1 file changed, 20 insertions(+) diff --git a/sound/drivers/aloop.c b/sound/drivers/aloop.c index 81cb1f59f703..5f0e2dc96769 100644 --- a/sound/drivers/aloop.c +++ b/sound/drivers/aloop.c @@ -1073,6 +1073,18 @@ static unsigned int get_cable_index(struct snd_pcm_substream *substream) return !substream->stream; } +/* + * In notify mode the playback side may change format, rate, channels and, + * with a sound timer, the period size. A running capture that no longer + * matches is stopped by loopback_check_format(). + * call in loopback->cable_lock + */ +static bool loopback_playback_unconstrained(struct loopback_pcm *dpcm) +{ + return dpcm->substream->stream == SNDRV_PCM_STREAM_PLAYBACK && + get_notify(dpcm); +} + static int rule_format(struct snd_pcm_hw_params *params, struct snd_pcm_hw_rule *rule) { @@ -1082,6 +1094,8 @@ static int rule_format(struct snd_pcm_hw_params *params, snd_mask_none(&m); scoped_guard(mutex, &dpcm->loopback->cable_lock) { + if (loopback_playback_unconstrained(dpcm)) + return 0; m.bits[0] = (u_int32_t)cable->hw.formats; m.bits[1] = (u_int32_t)(cable->hw.formats >> 32); } @@ -1096,6 +1110,8 @@ static int rule_rate(struct snd_pcm_hw_params *params, struct snd_interval t; scoped_guard(mutex, &dpcm->loopback->cable_lock) { + if (loopback_playback_unconstrained(dpcm)) + return 0; t.min = cable->hw.rate_min; t.max = cable->hw.rate_max; } @@ -1112,6 +1128,8 @@ static int rule_channels(struct snd_pcm_hw_params *params, struct snd_interval t; scoped_guard(mutex, &dpcm->loopback->cable_lock) { + if (loopback_playback_unconstrained(dpcm)) + return 0; t.min = cable->hw.channels_min; t.max = cable->hw.channels_max; } @@ -1128,6 +1146,8 @@ static int rule_period_bytes(struct snd_pcm_hw_params *params, struct snd_interval t; scoped_guard(mutex, &dpcm->loopback->cable_lock) { + if (loopback_playback_unconstrained(dpcm)) + return 0; t.min = cable->hw.period_bytes_min; t.max = cable->hw.period_bytes_max; }