libext2fs: avoid unnecessary stat(2) calls on mountpoints

If the device name in the mtab or /proc/mounts file does not match
with the device passed into ext2fs_check_if_mounted() or
ext2fs_check_mount_point(), skip the stat(2) call on the mountpoint,
since we never use the results of the stat(2) in that case.  Not only
does this provide a slight performance win, but it the stat calls on
the mountpoints could potentially trigger some SELinux denials that
could stress some sysadmins out.

Google-Bug-Id:  193137337
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

diff --git a/lib/ext2fs/ismounted.c b/lib/ext2fs/ismounted.c
index 46d330d..c9e6a9d 100644 (file)
--- a/lib/ext2fs/ismounted.c
+++ b/lib/ext2fs/ismounted.c
@@ -128,14 +128,14 @@ static errcode_t check_mntent_file(const char *mtab_file, const char *file,
        while ((mnt = getmntent (f)) != NULL) {
                if (mnt->mnt_fsname[0] != '/')
                        continue;
-               if (stat(mnt->mnt_dir, &st_buf) != 0)
-                       continue;
                if (strcmp(file, mnt->mnt_fsname) == 0) {
+                       if (stat(mnt->mnt_dir, &st_buf) != 0)
+                               continue;
                        if (file_rdev && (file_rdev != st_buf.st_dev)) {
 #ifdef DEBUG
                                printf("Bogus entry in %s!  "
-                                      "(%s does not exist)\n",
-                                      mtab_file, mnt->mnt_dir);
+                                      "(%s is not mounted on %s)\n",
+                                      mtab_file, file, mnt->mnt_dir);
 #endif /* DEBUG */
                                continue;
                        }