findmnt: don't rely on st_dev for --target

The overlay filesystem does not provide usable st_dev (in traditional
UNIX way). It's necessary to search in /proc/self/mountinfo to detect
which path element is mountpoint.

$ findmnt --target /mnt/merged/dir-a/foo
TARGET      SOURCE  FSTYPE  OPTIONS
/mnt/merged overlay overlay rw,relatime,lowerdir=/mnt/low,upperdir=/mnt/high/data,workdir=/mnt/high/work

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

3 files changed, 20 insertions, 10 deletions

diff --git a/libmount/src/utils.c b/libmount/src/utils.c
index 25c762fdd..6f4d1a19f 100644
--- a/libmount/src/utils.c
+++ b/libmount/src/utils.c
@@ -960,6 +960,10 @@ int mnt_open_uniq_filename(const char *filename, char **name)
  * This function finds the mountpoint that a given path resides in. @path
  * should be canonicalized. The returned pointer should be freed by the caller.
  *
+ * WARNING: the function compares st_dev of the @path elements. This traditional
+ * way maybe be insufficient on filesystems like Linux "overlay". See also
+ * mnt_table_find_target().
+ *
  * Returns: allocated string with the target of the mounted device or NULL on error
  */
 char *mnt_get_mountpoint(const char *path)
diff --git a/misc-utils/findmnt.8 b/misc-utils/findmnt.8
index 45cb49b4a..31c614ae1 100644
--- a/misc-utils/findmnt.8
+++ b/misc-utils/findmnt.8
@@ -185,8 +185,12 @@ Search in
 .IR /etc/fstab .
 The output is in the list format (see \fB--list\fR).
 .TP
-.BR \-T , " \-\-target \fIdir\fP"
-Explicitly define the mount target (mountpoint directory).
+.BR \-T , " \-\-target \fIpath\fP"
+Explicitly define the mount target (mountpoint directory). If the \fIpath\fR
+is not a mountpoint file or directory than
+.B findmnt
+checks \fIpath\fR elements in reverse order for get the mountpoint (this feature is 
+supported only if search in kernel files and unsupported for \fB\-\-fstab\fP).
 .TP
 .BR \-t , " \-\-types \fIlist\fP"
 Limit the set of printed filesystems.  More than one type may be
diff --git a/misc-utils/findmnt.c b/misc-utils/findmnt.c
index 968512355..1adc23124 100644
--- a/misc-utils/findmnt.c
+++ b/misc-utils/findmnt.c
@@ -259,10 +259,12 @@ static void set_source_match(const char *data)
 		set_match(COL_SOURCE, data);
 }
 
-static void enable_extra_target_match(void)
+/* @tb has to be from kernel (so no fstab or so)! */
+static void enable_extra_target_match(struct libmnt_table *tb)
 {
-	char *cn = NULL, *mnt = NULL;
-	const char *tgt = NULL;
+	char *cn = NULL;
+	const char *tgt = NULL, *mnt = NULL;
+	struct libmnt_fs *fs;
 
 	/*
 	 * Check if match pattern is mountpoint, if not use the
@@ -276,11 +278,11 @@ static void enable_extra_target_match(void)
 			return;
 	}
 
-	mnt = mnt_get_mountpoint(tgt);
+	fs = mnt_table_find_mountpoint(tb, tgt, MNT_ITER_BACKWARD);
+	if (fs)
+		mnt = mnt_fs_get_target(fs);
 	if (mnt && strcmp(mnt, tgt) != 0)
-		set_match(COL_TARGET, mnt);	/* replace the current setting */
-	else
-		free(mnt);
+		set_match(COL_TARGET, xstrdup(mnt));	/* replace the current setting */
 
 	if (!cache)
 		free(cn);
@@ -1606,7 +1608,7 @@ int main(int argc, char *argv[])
 			 * try it again with extra functionality for target
 			 * match
 			 */
-			enable_extra_target_match();
+			enable_extra_target_match(tb);
 			rc = add_matching_lines(tb, table, direction);
 		}
 	}