kernel/groups.c: use sort library function

setgroups is not exactly a hot path, so we might as well use the library
function instead of open-coding the sorting.  Saves ~150 bytes.

Link: http://lkml.kernel.org/r/1497301378-22739-1-git-send-email-linux@rasmusvillemoes.dk
Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Cc: Matthew Wilcox <mawilcox@microsoft.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/groups.c

@@ -5,6 +5,7 @@
 #include <linux/export.h>
 #include <linux/slab.h>
 #include <linux/security.h>
+#include <linux/sort.h>
 #include <linux/syscalls.h>
 #include <linux/user_namespace.h>
 #include <linux/vmalloc.h>
@@ -76,32 +77,18 @@ static int groups_from_user(struct group_info *group_info,
 	return 0;
 }
 
-/* a simple Shell sort */
+static int gid_cmp(const void *_a, const void *_b)
+{
+	kgid_t a = *(kgid_t *)_a;
+	kgid_t b = *(kgid_t *)_b;
+
+	return gid_gt(a, b) - gid_lt(a, b);
+}
+
 static void groups_sort(struct group_info *group_info)
 {
-	int base, max, stride;
-	int gidsetsize = group_info->ngroups;
-
-	for (stride = 1; stride < gidsetsize; stride = 3 * stride + 1)
-		; /* nothing */
-	stride /= 3;
-
-	while (stride) {
-		max = gidsetsize - stride;
-		for (base = 0; base < max; base++) {
-			int left = base;
-			int right = left + stride;
-			kgid_t tmp = group_info->gid[right];
-
-			while (left >= 0 && gid_gt(group_info->gid[left], tmp)) {
-				group_info->gid[right] = group_info->gid[left];
-				right = left;
-				left -= stride;
-			}
-			group_info->gid[right] = tmp;
-		}
-		stride /= 3;
-	}
+	sort(group_info->gid, group_info->ngroups, sizeof(*group_info->gid),
+	     gid_cmp, NULL);
 }
 
 /* a simple bsearch */