[PATCH V2] xfsprogs: be careful about what we stat in platform_check_mount

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After we lost ustat(2) in commit 4e7a824, we ended up with a slightly
bonkers method to determine if our target block device was mounted:
it goes through every entry returned by getmntent and stats the dir
to see if its underlying device matches ours.

Unfortunately that dir might be a hung nfs server and sadness ensues.

So just do a really simple sanity check before we try to stat the
mountpoint: does its device start with a / ?  If not, skip it.

Fixes: 4e7a824 ("libxfs/linux.c: Replace use of ustat by stat")
Signed-off-by: Eric Sandeen <sandeen@xxxxxxxxxx>
---

V2: just look for fully-qualified device paths vs expecting an actual
block device.

diff --git a/libxfs/linux.c b/libxfs/linux.c
index 0bace3e..5c8602e 100644
--- a/libxfs/linux.c
+++ b/libxfs/linux.c
@@ -77,7 +77,17 @@ platform_check_mount(char *name, char *block, struct stat *s, int flags)
 		    progname, name);
 		return 1;
 	}
+	/*
+	 * This whole business is to work out if our block device is mounted
+	 * after we lost ustat(2), see:
+	 * 	4e7a824 libxfs/linux.c: Replace use of ustat by stat
+	 * We don't really want to stat every single mounted directory,
+	 * as that may include tmpfs, cgroups, procfs or - worst - hung nfs
+	 * servers.  So first, a simple check: does the "dev" start with "/" ?
+	 */
 	while ((mnt = getmntent(f)) != NULL) {
+		if (mnt->mnt_fsname[0] != '/')
+			continue;
 		if (stat(mnt->mnt_dir, &mst) < 0)
 			continue;
 		if (mst.st_dev != s->st_rdev)

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