[PATCH] __bitmap_parselist: fix bug in empty string handling

[Chris Metcalf <cmetcalf@ezchip.com>]

bitmap_parselist("", &mask, nmaskbits) will erroneously set bit
zero in the mask.  The same bug is visible in cpumask_parselist()
since it is layered on top of the bitmask code, e.g. if you boot with
"isolcpus=", you will actually end up with cpu zero isolated.

The bug was introduced in commit 4b060420a596 ("bitmap, irq: add
smp_affinity_list interface to /proc/irq") when bitmap_parselist()
was generalized to support userspace as well as kernelspace.

Signed-off-by: Chris Metcalf <cmetcalf@ezchip.com>
Cc: stable@vger.kernel.org
---
 lib/bitmap.c | 17 +++++++++--------
 1 file changed, 9 insertions(+), 8 deletions(-)

diff --git a/lib/bitmap.c b/lib/bitmap.c
index d456f4c15a9f..c04448bf1271 100644
--- a/lib/bitmap.c
+++ b/lib/bitmap.c
@@ -536,12 +536,12 @@ static int __bitmap_parselist(const char *buf, unsigned int buflen,
 	unsigned a, b;
 	int c, old_c, totaldigits;
 	const char __user __force *ubuf = (const char __user __force *)buf;
-	int exp_digit, in_range;
+	int at_start, in_range;
 
 	totaldigits = c = 0;
 	bitmap_zero(maskp, nmaskbits);
 	do {
-		exp_digit = 1;
+		at_start = 1;
 		in_range = 0;
 		a = b = 0;
 
@@ -570,11 +570,10 @@ static int __bitmap_parselist(const char *buf, unsigned int buflen,
 				break;
 
 			if (c == '-') {
-				if (exp_digit || in_range)
+				if (at_start || in_range)
 					return -EINVAL;
 				b = 0;
 				in_range = 1;
-				exp_digit = 1;
 				continue;
 			}
 
@@ -584,16 +583,18 @@ static int __bitmap_parselist(const char *buf, unsigned int buflen,
 			b = b * 10 + (c - '0');
 			if (!in_range)
 				a = b;
-			exp_digit = 0;
+			at_start = 0;
 			totaldigits++;
 		}
 		if (!(a <= b))
 			return -EINVAL;
 		if (b >= nmaskbits)
 			return -ERANGE;
-		while (a <= b) {
-			set_bit(a, maskp);
-			a++;
+		if (!at_start) {
+			while (a <= b) {
+				set_bit(a, maskp);
+				a++;
+			}
 		}
 	} while (buflen && c == ',');
 	return 0;
-- 
2.1.2