Re: [PATCH] ALSA: firewire-lib: Fix 'amdtp_domain_start()' when no AMDTP_OUT_STREAM stream is found

[Takashi Sakamoto]

Hi,

On Thu, Jun 24, 2021 at 08:49:36PM +0200, Christophe JAILLET wrote:
> The intent here is to return an error code if we don't find what we are
> looking for in the 'list_for_each_entry()' loop.
> 
> 's' is not NULL if the list is empty or if we scan the complete list.
> Introduce a new 'found' variable to handle such cases.
> 
> Fixes: 60dd49298ec5 ("ALSA: firewire-lib: handle several AMDTP streams in callback handler of IRQ target")
> Signed-off-by: Christophe JAILLET <christophe.jaillet@xxxxxxxxxx>
> ---
> We could test with" if (list_entry_is_head(s, &d->streams, list))"
> instead, but I find it much less readable.
> ---
>  sound/firewire/amdtp-stream.c | 7 +++++--
>  1 file changed, 5 insertions(+), 2 deletions(-)
> 
> diff --git a/sound/firewire/amdtp-stream.c b/sound/firewire/amdtp-stream.c
> index aad9778d1c4d..9be2260e4ca2 100644
> --- a/sound/firewire/amdtp-stream.c
> +++ b/sound/firewire/amdtp-stream.c
> @@ -1943,6 +1943,7 @@ int amdtp_domain_start(struct amdtp_domain *d, unsigned int tx_init_skip_cycles,
>  	unsigned int events_per_period = d->events_per_period;
>  	unsigned int queue_size;
>  	struct amdtp_stream *s;
> +	bool found = false;
>  	int err;
>  
>  	if (replay_seq) {
> @@ -1955,10 +1956,12 @@ int amdtp_domain_start(struct amdtp_domain *d, unsigned int tx_init_skip_cycles,
>  
>  	// Select an IT context as IRQ target.
>  	list_for_each_entry(s, &d->streams, list) {
> -		if (s->direction == AMDTP_OUT_STREAM)
> +		if (s->direction == AMDTP_OUT_STREAM) {
> +			found = true;
>  			break;
> +		}
>  	}
> -	if (!s)
> +	if (!found)
>  		return -ENXIO;
>  	d->irq_target = s;
>  
> -- 
> 2.30.2

Indeed. Nice to catch it. The bug affects Linux kernel v5.5 or later.

Acked-by: Takashi Sakamoto <o-takashi@xxxxxxxxxxxxx>

In kernel v5.10 or later, we can use 'list_entry_is_head()' macro added
by a commit e130816164e2 ("include/linux/list.h: add a macro to test if
entry is pointing to the head"). However the development of ALSA firewire
stack is out-of-tree repository for my convenience to backport it to kernel
v4.17 or later (I need testers...), so I prefer your change.


Thanks

Takashi Sakamoto