Re: [PATCH] ALSA: control: Verify put() result when in debug mode

[Takashi Iwai <tiwai@suse.de>]

On Fri, 30 Jan 2026 15:12:58 +0100,
Mark Brown wrote:
> > +	struct snd_ctl_elem_value original;
> 
> > +	ret = kctl->get(kctl, &original);
> > +	if (ret)
> > +		return ret;
> 
> > +	retcmp = memcmp(&original.value.bytes.data[0], &control->value.bytes.data[0],
> > +			sizeof(original.value.bytes.data[0]));
> > +	if (retcmp)
> > +		retcmp = 1;
> 
> original was just allocated from the stack so who knows what values it
> had originally, and the get() is only going to write the part of the
> value that has data since the normal get() path has a memset() in the
> core.  Similarly with the new value coming in from userspace there's no
> requirement for userspace to set anything that isn't part of the value
> being written to any particular value.  This means we're liable to get
> spurious mismatches.

Yes, and if I understand correctly, the above memcmpy() just compare
the single byte from original and the result?  Then it'll lead to
false-positive outputs.

We'll need to query the control info and check the relevant values
for each info->type and count.

Also, better to name it snd_ctl_put_verify() instead of
snd_ctl_write_verify(); it's an equivalent with snd_ctl_put() having
an additional verification.


thanks,

Takashi