[git commit] taskset: fix non-fancy cpuset printing on big-endian

[Denys Vlasenko]

commit: https://git.busybox.net/busybox/commit/?id=5fa9fefddce56fab75b2e6c88c4516e2b21d2f5a
branch: https://git.busybox.net/busybox/commit/?id=refs/heads/master

The non-fancy version of the from_cpuset uses CPU_SETSIZE as if it
represents the number of bytes in the cpuset, while it is actually
the number of bits. This leads to out-of-bounds accesses on the
cpu_set_t in the big-endian case. Basically all uses of CPU_SETSIZE
have to be divided by 8. This is done correctly in the fancy version
of from_cpuset.

In addition, the big-endian case is completely wrong to begin with.
All standard C libraries that I know of implement cpu_set_t as an
unsigned long array, so both for big and little endian, the least
significant bits are in the beginning of the array. Therefore, the
approach taken for the little endian case is equally valid. We only
need special handling for big endian when CPU_SETSIZE is large and
we use an unsigned long long to get more bits out.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout at mind.be>
Signed-off-by: Denys Vlasenko <vda.linux at googlemail.com>
---
 miscutils/taskset.c | 37 ++++++++++++++++++-------------------
 1 file changed, 18 insertions(+), 19 deletions(-)

diff --git a/miscutils/taskset.c b/miscutils/taskset.c
index 100b1d9..fb352ab 100644
--- a/miscutils/taskset.c
+++ b/miscutils/taskset.c
@@ -75,27 +75,26 @@ static char *from_cpuset(cpu_set_t *mask)
 #define TASKSET_PRINTF_MASK "%llx"
 static unsigned long long from_cpuset(cpu_set_t *mask)
 {
-	char *p = (void*)mask;
+	BUILD_BUG_ON(CPU_SETSIZE < 8*sizeof(int));
 
-	BUILD_BUG_ON(CPU_SETSIZE < sizeof(int));
-
-	/* Take the least significant bits. Careful!
-	 * Consider both CPU_SETSIZE=4 and CPU_SETSIZE=1024 cases
+	/* Take the least significant bits. Assume cpu_set_t is
+	 * implemented as an array of unsigned long or unsigned
+	 * int.
 	 */
-#if BB_BIG_ENDIAN
-	/* For big endian, it means LAST bits */
-	if (CPU_SETSIZE < sizeof(long))
-		p += CPU_SETSIZE - sizeof(int);
-	else if (CPU_SETSIZE < sizeof(long long))
-		p += CPU_SETSIZE - sizeof(long);
-	else
-		p += CPU_SETSIZE - sizeof(long long);
-#endif
-	if (CPU_SETSIZE < sizeof(long))
-		return *(unsigned*)p;
-	if (CPU_SETSIZE < sizeof(long long))
-		return *(unsigned long*)p;
-	return *(unsigned long long*)p;
+	if (CPU_SETSIZE < 8*sizeof(long))
+		return *(unsigned*)mask;
+	if (CPU_SETSIZE < 8*sizeof(long long))
+		return *(unsigned long*)mask;
+# if BB_BIG_ENDIAN
+	if (sizeof(long long) > sizeof(long)) {
+		/* We can put two long in the long long, but they have to
+		 * be swapped: the least significant word comes first in the
+		 * array */
+		unsigned long *p = (void*)mask;
+		return p[0] + ((unsigned long long)p[1] << (8*sizeof(long)));
+	}
+# endif
+	return *(unsigned long long*)mask;
 }
 #endif