[PATCH] mdev: don't follow deprecated sysfs /sys/block symlinks

[Gregory Fong]

/sys/block will only be scanned with CONFIG_SYSFS_DEPRECATED=y and
deprecated sysfs enabled (using CONFIG_SYSFS_DEPRECATED_V2=y or the
related kernel boot param).  In that case, all of /sys/block/* will be
a real directory, so we don't need to follow symlinks.  Additionally,
following symlinks causes a bug for mtd in this case, e.g. several
people have been seeing that /dev/mtd0 was getting created by this
traversal as a block device.  Subsequent investigation revealed that
was caused by this pass following the /sys/block/mtdblock0/device
symlink and resulting in both
- /sys/block/mtdblock0/dev
- /sys/block/mtdblock0/device/dev
being used here, and the second results in that incorrect /dev/mtd0.
Change to not follow symlinks while traversing /sys/block to fix this.

Fixes BZ #6806.

Signed-off-by: Gregory Fong <gregory.0xf0 at gmail.com>
Cc: Denys Vlasenko <vda.linux at googlemail.com>
Cc: Simon Edlund <simon at edlund.nl>
---
 util-linux/mdev.c | 19 ++++++++++++-------
 1 file changed, 12 insertions(+), 7 deletions(-)

diff --git a/util-linux/mdev.c b/util-linux/mdev.c
index ca4b915..af986a2 100644
--- a/util-linux/mdev.c
+++ b/util-linux/mdev.c
@@ -1063,22 +1063,27 @@ int mdev_main(int argc UNUSED_PARAM, char **argv)
 
 		putenv((char*)"ACTION=add");
 
-		/* ACTION_FOLLOWLINKS is needed since in newer kernels
-		 * /sys/block/loop* (for example) are symlinks to dirs,
-		 * not real directories.
-		 * (kernel's CONFIG_SYSFS_DEPRECATED makes them real dirs,
-		 * but we can't enforce that on users)
-		 */
 		if (access("/sys/class/block", F_OK) != 0) {
 			/* Scan obsolete /sys/block only if /sys/class/block
 			 * doesn't exist. Otherwise we'll have dupes.
 			 * Also, do not complain if it doesn't exist.
 			 * Some people configure kernel to have no blockdevs.
+			 *
+			 * Don't use ACTION_FOLLOWLINKS here. This will only be
+			 * run if CONFIG_SYSFS_DEPRECATED=y, in which case
+			 * everything at the top-level will be a real dir, and
+			 * following symlinks can result in the 'device'
+			 * symlink being followed and resulting in e.g. mtd0
+			 * being set up as a block dev instead of a char dev.
 			 */
 			recursive_action("/sys/block",
-				ACTION_RECURSE | ACTION_FOLLOWLINKS | ACTION_QUIET,
+				ACTION_RECURSE | ACTION_QUIET,
 				fileAction, dirAction, temp, 0);
 		}
+		/* ACTION_FOLLOWLINKS is needed since in newer kernels
+		 * /sys/class/block/loop* (for example) are symlinks to dirs,
+		 * not real directories.
+		 */
 		recursive_action("/sys/class",
 			ACTION_RECURSE | ACTION_FOLLOWLINKS,
 			fileAction, dirAction, temp, 0);
-- 
1.9.1