Re: [PATCH] ALSA: firewire-tascam: Do not drop unread control events

["Cássio Gabriel Monteiro Pires" <cassiogabrielcontato@gmail.com>]

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On 5/3/26 08:29, Takashi Sakamoto wrote:
> Hi,
> 
> On Fri, May 01, 2026 at 03:22:58PM -0300, Cássio Gabriel wrote:
>> tscm_hwdep_read_queue() copies as many queued control events as fit in
>> the userspace buffer. When the buffer is smaller than the current
>> contiguous queue segment, length is rounded down to the number of bytes
>> that can be copied.
>>
>> However, after copying that shortened length, the code advances pull_pos
>> to tail_pos, marking the whole contiguous segment as consumed. Any events
>> between the copied portion and tail_pos are lost.
>>
>> Advance pull_pos by the number of entries actually copied instead. When
>> the whole segment fits, this is equivalent to the old tail_pos update;
>> when the buffer is smaller, the remaining events stay queued for the next
>> read.
>>
>> Fixes: a8c0d13267a4 ("ALSA: firewire-tascam: notify events of change of state for userspace applications")
>> Cc: stable@vger.kernel.org
>> Signed-off-by: Cássio Gabriel <cassiogabrielcontato@gmail.com>
>> ---
>>  sound/firewire/tascam/tascam-hwdep.c | 4 +++-
>>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> Thank you for catching the issue.
> 
> I agree that this is indeed a bug. While your approach makes sense, I
> wonder if it could be simplified by updating tail_pos instead.
> 
> Would you mind trying the following change?
> 
> ======== 8< --------
> diff --git a/sound/firewire/tascam/tascam-hwdep.c b/sound/firewire/tascam/tascam-hwdep.c
> index 867b4ea1096e..6270263e7bf4 100644
> --- a/sound/firewire/tascam/tascam-hwdep.c
> +++ b/sound/firewire/tascam/tascam-hwdep.c
> @@ -73,6 +73,7 @@ static long tscm_hwdep_read_queue(struct snd_tscm *tscm, char __user *buf,
>                         length = rounddown(remained, sizeof(*entries));
>                 if (length == 0)
>                         break;
> +               tail_pos = head_pos + length / sizeof(*entries);
> 
>                 spin_unlock_irq(&tscm->lock);
>                 if (copy_to_user(pos, &entries[head_pos], length))
> ======== 8< --------
> 
> 
> Thanks
> 
> Takashi Sakamoto

Hi,

Thank you for the suggestion.

I'll send a v2 patch.

-- 
Thanks,
Cássio


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