[alsa-devel] [PATCH] alsa-lib/tlv: fix handling of raw value ranges

[Benoît Thébaudeau]

Takashi Iwai wrote:
> Benoît Thébaudeau wrote:
> >
> > Clemens Ladisch wrote:
> > > I'll see if I can cobble together some TLV checking code ...
> >
> > Great.
> >
> > > > Because it's useless to wait until the end of the loop to
> > > > return
> > > > -EINVAL,
> > >
> > > It is not necessary to optimize for error cases.
> >
> > It's not an optimization, but a simplification. Otherwise, pos
> > would have to be
> > tested again after the loop to differentiate the loop-skipped case
> > from the
> > at-least-one-loop-iteration cases to return either -EINVAL or the
> > prev submax.
>
> Unless it's really a hot path, such a micro-optimization doesn't give
> much value.
>
> Rather better to keep the standard style that makes people easier to
> read the code.  A do-while loop with a large block is usually less
> readable than a normal while loop.

Then, the patch becomes:

--- alsa-lib/src/control/tlv.c
+++ alsa-lib/src/control/tlv.c
@@ -291,41 +291,37 @@ int snd_tlv_convert_from_dB(unsigned int
 {
 	switch (tlv[0]) {
 	case SND_CTL_TLVT_DB_RANGE: {
-		long dbmin, dbmax, prev_rangemax;
+		long dbmin, dbmax, prev_submax;
 		unsigned int pos, len;
 		len = int_index(tlv[1]);
-		if (len > MAX_TLV_RANGE_SIZE)
-			return -EINVAL;
-		if (snd_tlv_get_dB_range(tlv, rangemin, rangemax,
-					 &dbmin, &dbmax))
+		if (len < 6 || len > MAX_TLV_RANGE_SIZE)
 			return -EINVAL;
-		if (db_gain <= dbmin) {
-			*value = rangemin;
-			return 0;
-		} else if (db_gain >= dbmax) {
-			*value = rangemax;
-			return 0;
-		}
 		pos = 2;
-		prev_rangemax = 0;
+		prev_submax = 0;
 		while (pos + 4 <= len) {
-			rangemin = (int)tlv[pos];
-			rangemax = (int)tlv[pos + 1];
+			long submin, submax;
+			submin = (int)tlv[pos];
+			submax = (int)tlv[pos + 1];
+			if (rangemax < submax)
+				submax = rangemax;
 			if (!snd_tlv_get_dB_range(tlv + pos + 2,
-						  rangemin, rangemax,
+						  submin, submax,
 						  &dbmin, &dbmax) &&
 			    db_gain >= dbmin && db_gain <= dbmax)
 				return snd_tlv_convert_from_dB(tlv + pos + 2,
-							       rangemin, rangemax,
+							       submin, submax,
 							       db_gain, value, xdir);
 			else if (db_gain < dbmin) {
-				*value = xdir ? rangemin : prev_rangemax;
+				*value = xdir || pos == 2 ? submin : prev_submax;
 				return 0;
 			}
-			prev_rangemax = rangemax;
+			prev_submax = submax;
+			if (rangemax == submax)
+				break;
 			pos += int_index(tlv[pos + 3]) + 4;
 		}
-		return -EINVAL;
+		*value = prev_submax;
+		return 0;
 	}
 	case SND_CTL_TLVT_DB_SCALE: {
 		int min, step, max;