[PATCH] mountpoint: check mntent instead of parent st_dev

[Dominique Martinet]

From: Dominique Martinet <dominique.martinet at atmark-techno.com>

Checking st_dev fails to identify bind mounts as mount points if the
bind mount target is in the same filesystem as its parent directory,
for example (simple 'mountpoint' command is util-linux's):
```
cd $(mktemp -d)
mkdir src target
mount --bind src target
busybox mountpoint target; echo $?
# target is not a mountpoint
# 1
mountpoint target; echo $?
# target is a mountpoint
# 0
umount target
rm -rf $PWD
```

Parsing /proc/mounts with getmntent (like util-linux implementation
does) fixes the issue.

function                                             old     new   delta
.rodata                                            79535   79529      -6
mountpoint_main                                      469     424     -45
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 0/2 up/down: 0/-51)             Total: -51 bytes
---
FWIW I've been working around this problem with a awk one-liner instead:
```
is_mountpoint() {
        ! awk -v dir="$1" '$5 == dir { exit 1 }' < /proc/self/mountinfo
}               
```
But this bit us somewhere else that wasn't using my wrapper, so figured
I'd try to fix this here... and it turns out to be a size saving as well
because we have a helper for mntent (find_mount_point).

I'll submit it to alpine in a few weeks depending on initial feedback.

 util-linux/mountpoint.c | 40 +++++++++++++++-------------------------
 1 file changed, 15 insertions(+), 25 deletions(-)

diff --git a/util-linux/mountpoint.c b/util-linux/mountpoint.c
index 87f4cc60d8c5..0a6df5160399 100644
--- a/util-linux/mountpoint.c
+++ b/util-linux/mountpoint.c
@@ -69,33 +69,23 @@ int mountpoint_main(int argc UNUSED_PARAM, char **argv)
 
 	errno = ENOTDIR;
 	if (S_ISDIR(st.st_mode)) {
-		dev_t st_dev = st.st_dev;
-		ino_t st_ino = st.st_ino;
-		char *p = xasprintf("%s/..", arg);
-
-		if (stat(p, &st) == 0) {
-			//int is_mnt = (st_dev != st.st_dev) || (st_dev == st.st_dev && st_ino == st.st_ino);
-			int is_not_mnt = (st_dev == st.st_dev) && (st_ino != st.st_ino);
-
-			if (opt & OPT_d)
-				printf("%u:%u\n", major(st_dev), minor(st_dev));
-			if (opt & OPT_n) {
-				const char *d = find_block_device(arg);
-				/* name is undefined, but device is mounted -> anonymous superblock! */
-				/* happens with btrfs */
-				if (!d) {
-					d = "UNKNOWN";
-					/* TODO: iterate /proc/mounts, or /proc/self/mountinfo
-					 * to find out the device name */
-				}
-				printf("%s %s\n", d, arg);
+		bool is_not_mnt = find_mount_point(arg, 0) == NULL;
+		if (opt & OPT_d)
+			printf("%u:%u\n", major(st.st_dev), minor(st.st_dev));
+		if (opt & OPT_n) {
+			const char *d = find_block_device(arg);
+			/* name is undefined, but device is mounted -> anonymous superblock! */
+			/* happens with btrfs */
+			if (!d) {
+				d = "UNKNOWN";
+				/* TODO: iterate /proc/mounts, or /proc/self/mountinfo
+				 * to find out the device name */
 			}
-			if (!(opt & (OPT_q | OPT_d | OPT_n)))
-				printf("%s is %sa mountpoint\n", arg, is_not_mnt ? "not " : "");
-			return is_not_mnt;
+			printf("%s %s\n", d, arg);
 		}
-		arg = p;
-		/* else: stat had set errno, just fall through */
+		if (!(opt & (OPT_q | OPT_d | OPT_n)))
+			printf("%s is %sa mountpoint\n", arg, is_not_mnt ? "not " : "");
+		return is_not_mnt;
 	}
 
  err:
-- 
2.39.2