[alsa-devel] [patch] ALSA: bits vs bytes bug in snd_card_create()

[Takashi Iwai]

At Thu, 23 Jan 2014 09:55:31 +0100,
walter harms wrote:
> 
> 
> Am 23.01.2014 09:21, schrieb Dan Carpenter:
> > The test here is intended intended to prevent shift wrapping bugs when
> > we do "1U << idx2".  We should consider the number of bits in a u32
> > instead of the number of bytes.
> > 
> > Fixes: 7bb2491b35a2 ('ALSA: Add kconfig to specify the max card numbers')
> > Signed-off-by: Dan Carpenter <dan.carpenter at oracle.com>
> > ---
> > WARNING:  Please check this one carefully because I might be wrong.
> > 
> > diff --git a/sound/core/init.c b/sound/core/init.c
> > index 1351f22f651c..e9e3db321e64 100644
> > --- a/sound/core/init.c
> > +++ b/sound/core/init.c
> > @@ -183,7 +183,7 @@ int snd_card_create(int idx, const char *xid,
> >  	if (idx < 0) {
> >  		for (idx2 = 0; idx2 < SNDRV_CARDS; idx2++) {
> >  			/* idx == -1 == 0xffff means: take any free slot */
> > -			if (idx2 < sizeof(int) && !(idx & (1U << idx2)))
> > +			if (idx2 < 32 && !(idx & (1U << idx2)))
> >  				continue;
> >  			if (!test_bit(idx2, snd_cards_lock)) {
> >  				if (!slots[idx2] || !*slots[idx2]) {
> > --
> 
> Funny bug,
> to be 100% sure you could multiply by CHAR_BIT ;)
> 
> other code simple does (1UL << (nr & 31))
> on the other side a check like:
> 
> #if SNDRV_CARDS > 32
> # error to much sound
> #endif
> 
> whould give the user a hint what is going on instead of silently ignoring
> that error.

SNDRV_CARDS can be more than 32.  That's why the check was
introduced.


Takashi