Btrfs-progs: check return value of realpath(3)

I hit a segfault when deleting a subvolume with very long name(>4096),
it's because cmd_subvol_delete() calls strdup() and passes NULL as
argument, which is returned by realpath(3).

I used the following script to reproduce

	#!/bin/bash
	mnt=$1
	i=1
	path=$mnt/subvol_$i

	# Create very deep subvolumes
	while btrfs sub create $path;do
	      ((i++))
	      path="$path/subvol_$i"
	done
	last_vol=$(dirname $path)
	dir=$(dirname $last_vol)
	vol=$(basename $last_vol)

	# Try to delete tha last one, this would get segfault
	pushd $dir
	btrfs sub delete $vol
	popd

Fix it by checking return value of realpath(3), also fix the one in
find_mount_root().

Signed-off-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: David Sterba <dsterba@suse.cz>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>

cmds-send.c

@@ -63,6 +63,7 @@ int find_mount_root(const char *path, char **mount_root)
 	int fd;
 	struct mntent *ent;
 	int len;
+	int ret;
 	int longest_matchlen = 0;
 	char *longest_match = NULL;
 
@@ -95,10 +96,13 @@ int find_mount_root(const char *path, char **mount_root)
 		return -ENOENT;
 	}
 
+	ret = 0;
 	*mount_root = realpath(longest_match, NULL);
-	free(longest_match);
+	if (!mount_root)
+		ret = -errno;
 
-	return 0;
+	free(longest_match);
+	return ret;
 }
 
 static int get_root_id(struct btrfs_send *s, const char *path, u64 *root_id)

cmds-subvolume.c

@@ -236,6 +236,12 @@ again:
 	}
 
 	cpath = realpath(path, NULL);
+	if (!cpath) {
+		ret = errno;
+		fprintf(stderr, "ERROR: finding real path for '%s': %s\n",
+			path, strerror(errno));
+		goto out;
+	}
 	dupdname = strdup(cpath);
 	dname = dirname(dupdname);
 	dupvname = strdup(cpath);