From: Karel Zak <kzak@redhat.com>
Date: Thu, 26 Sep 2024 12:44:36 +0000 (+0200)
Subject: libmount: support bind symlink over symlink
X-Git-Tag: v2.42-start~187^2~2
X-Git-Url: http://git.ipfire.org/?a=commitdiff_plain;h=22cda2f71cdf2c75ac01cdbedf3a57b146186812;p=thirdparty%2Futil-linux.git

libmount: support bind symlink over symlink

The new mount API allows for the use of AT_SYMLINK_NOFOLLOW when
opening a mount tree (aka the "mount source" for libmount).
As a result, you can now replace one symlink with another by using a
bind mount.

By default, the mount(8) command follows symlinks and canonicalizes
all paths. However, with the X-mount.nocanonicalize=source option, it
is possible to open the symlink itself. Similarly, with the
X-mount.nocanonicalize=target option, the path of the mount point can
be kept as the original symlink. (Using X-mount.nocanonicalize without
any argument works for both the "source" and "target".)

Example:

 # file /mnt/test/symlinkA /mnt/test/symlinkB
 /mnt/test/symlinkA: symbolic link to /mnt/test/fileA
 /mnt/test/symlinkB: symbolic link to /mnt/test/fileB

 # strace -e open_tree,move_mount \
   ./mount --bind -o X-mount.nocanonicalize /mnt/test/symlinkA /mnt/test/symlinkB
 ...
 open_tree(AT_FDCWD, "/mnt/test/symlinkA", OPEN_TREE_CLONE|OPEN_TREE_CLOEXEC|AT_SYMLINK_NOFOLLOW) = 3
 move_mount(3, "", AT_FDCWD, "/mnt/test/symlinkB", MOVE_MOUNT_F_EMPTY_PATH) = 0

 # ls -la  /mnt/test/symlinkB
 lrwxrwxrwx 1 root root 15 Sep 26 13:41 /mnt/test/symlinkB -> /mnt/test/fileA

The result is that 'symlinkB' is still a symlink, but it now points to
a different file.

This commit also modifies umount(8) because it does not work with
symlinks by default. The solution is to call umount2(UMOUNT_NOFOLLOW)
for symlinks after a failed regular umount(). For example:

 # strace -e umount,umount2 \
   ./umount /mnt/test/symlinkB
 ...
 umount2("/mnt/test/symlinkB", 0)        = -1 EINVAL (Invalid argument)
 umount2("/mnt/test/symlinkB", UMOUNT_NOFOLLOW) = 0

Signed-off-by: Karel Zak <kzak@redhat.com>
---

diff --git a/libmount/src/context.c b/libmount/src/context.c
index 28cce65e8..96eb32d02 100644
--- a/libmount/src/context.c
+++ b/libmount/src/context.c
@@ -1854,6 +1854,9 @@ int mnt_context_open_tree(struct libmnt_context *cxt, const char *path, unsigned
 	if (cxt->force_clone)
 		oflg |= OPEN_TREE_CLONE;
 
+	if (mnt_context_is_xnocanonicalize(cxt, "source"))
+		oflg |= AT_SYMLINK_NOFOLLOW;
+
 	DBG(CXT, ul_debugobj(cxt, "open_tree(path=%s%s%s)", path,
 				oflg & OPEN_TREE_CLONE ? " clone" : "",
 				oflg & AT_RECURSIVE ? " recursive" : ""));
diff --git a/libmount/src/context_umount.c b/libmount/src/context_umount.c
index 499cc43ac..64a1d884b 100644
--- a/libmount/src/context_umount.c
+++ b/libmount/src/context_umount.c
@@ -887,7 +887,17 @@ static int do_umount(struct libmnt_context *cxt)
 	if (mnt_context_is_fake(cxt))
 		rc = 0;
 	else {
+		struct stat st;
+
 		rc = flags ? umount2(target, flags) : umount(target);
+
+		if (rc < 0
+		    && errno == EINVAL
+		    && !(flags & UMOUNT_NOFOLLOW)
+		    && !mnt_context_is_restricted(cxt)
+		    && mnt_safe_lstat(target, &st) == 0 && S_ISLNK(st.st_mode))
+			rc = umount2(target, flags | UMOUNT_NOFOLLOW);
+
 		if (rc < 0)
 			cxt->syscall_status = -errno;
 		free(tgtbuf);