btrfs-progs: add basename wrappers for unified semantics

What basename(3) does with the argument depends on _GNU_SOURCE and
inclusion of libgen.h. This is problematic on Musl (1.2.5) as reported.

We want the GNU semantics that does not modify the argument. Common way
to make it portable is to add own helper. This is now implemented in
path_basename() that does not use the libc provided basename but preserves
the semantics. The path_dirname() is just for parity, otherwise same as
dirname().

Sources:
- https://bugs.gentoo.org/926288
- https://git.musl-libc.org/cgit/musl/commit/?id=725e17ed6dff4d0cd22487bb64470881e86a92e7

Issue: #778
Signed-off-by: David Sterba <dsterba@suse.com>

cmds/subvolume.c

@@ -24,7 +24,6 @@
 #include <string.h>
 #include <unistd.h>
 #include <errno.h>
-#include <libgen.h>
 #include <getopt.h>
 #include <dirent.h>
 #include <stdbool.h>
@@ -149,7 +148,7 @@ static int create_one_subvolume(const char *dst, struct btrfs_qgroup_inherit *in
 	int	fddst = -1;
 	char	*dupname = NULL;
 	char	*dupdir = NULL;
-	char	*newname;
+	const char *newname;
 	char	*dstdir;
 
 	ret = path_is_dir(dst);
@@ -170,15 +169,15 @@ static int create_one_subvolume(const char *dst, struct btrfs_qgroup_inherit *in
 		ret = -ENOMEM;
 		goto out;
 	}
-	newname = basename(dupname);
+	newname = path_basename(dupname);
 
 	dupdir = strdup(dst);
 	if (!dupdir) {
 		error_msg(ERROR_MSG_MEMORY, "duplicating %s", dst);
 		ret = -ENOMEM;
 		goto out;
 	}
-	dstdir = dirname(dupdir);
+	dstdir = path_dirname(dupdir);
 
 	if (!test_issubvolname(newname)) {
 		error("invalid subvolume name: %s", newname);
@@ -364,7 +363,8 @@ static int cmd_subvolume_delete(const struct cmd_struct *cmd, int argc, char **a
 	int res, ret = 0;
 	int cnt;
 	int fd = -1;
-	char	*dname, *vname, *cpath;
+	char	*dname, *cpath;
+	const char *vname;
 	char	*dupdname = NULL;
 	char	*dupvname = NULL;
 	char	*path = NULL;
@@ -482,9 +482,9 @@ static int cmd_subvolume_delete(const struct cmd_struct *cmd, int argc, char **a
 		goto out;
 	}
 	dupdname = strdup(cpath);
-	dname = dirname(dupdname);
+	dname = path_dirname(dupdname);
 	dupvname = strdup(cpath);
-	vname = basename(dupvname);
+	vname = path_basename(dupvname);
 	free(cpath);
 
 	/* When subvolid is passed, <path> will point to the mount point */
@@ -670,7 +670,7 @@ static int cmd_subvolume_snapshot(const struct cmd_struct *cmd, int argc, char *
 	bool readonly = false;
 	char	*dupname = NULL;
 	char	*dupdir = NULL;
-	char	*newname;
+	const char *newname;
 	char	*dstdir;
 	enum btrfs_util_error err;
 	struct btrfs_ioctl_vol_args_v2	args;
@@ -727,13 +727,13 @@ static int cmd_subvolume_snapshot(const struct cmd_struct *cmd, int argc, char *
 
 	if (res > 0) {
 		dupname = strdup(subvol);
-		newname = basename(dupname);
+		newname = path_basename(dupname);
 		dstdir = dst;
 	} else {
 		dupname = strdup(dst);
-		newname = basename(dupname);
+		newname = path_basename(dupname);
 		dupdir = strdup(dst);
-		dstdir = dirname(dupdir);
+		dstdir = path_dirname(dupdir);
 	}
 
 	if (!test_issubvolname(newname)) {
@@ -1557,7 +1557,7 @@ static int cmd_subvolume_show(const struct cmd_struct *cmd, int argc, char **arg
 	struct btrfs_util_subvolume_iterator *iter;
 	struct btrfs_util_subvolume_info subvol;
 	char *subvol_path = NULL;
-	char *subvol_name = NULL;
+	const char *subvol_name = NULL;
 	enum btrfs_util_error err;
 	struct btrfs_qgroup_stats stats;
 	unsigned int unit_mode;
@@ -1669,7 +1669,7 @@ static int cmd_subvolume_show(const struct cmd_struct *cmd, int argc, char **arg
 		subvol_path = strdup("/");
 		subvol_name = "<FS_TREE>";
 	} else {
-		subvol_name = basename(subvol_path);
+		subvol_name = path_basename(subvol_path);
 	}
 
 	if (bconf.output_format == CMD_FORMAT_JSON) {

common/device-utils.c

@@ -343,14 +343,14 @@ static u64 device_get_partition_size_sysfs(const char *dev)
 	char path[PATH_MAX] = {};
 	char sysfs[PATH_MAX] = {};
 	char sizebuf[128] = {};
-	char *name = NULL;
+	const char *name = NULL;
 	int sysfd;
 	unsigned long long size = 0;
 
 	name = realpath(dev, path);
 	if (!name)
 		return 0;
-	name = basename(path);
+	name = path_basename(path);
 
 	ret = path_cat3_out(sysfs, "/sys/class/block", name, "size");
 	if (ret < 0)

common/path-utils.c

@@ -28,6 +28,11 @@
 #include <string.h>
 #include <errno.h>
 #include <ctype.h>
+/*
+ * For dirname() and basename(), but never use basename directly, there's
+ * path_basename() with unified GNU behaviour regardless of the includes and
+ * conditional defines. See basename(3) for more.
+ */
 #include <libgen.h>
 #include <limits.h>
 #include "common/path-utils.h"
@@ -482,3 +487,26 @@ int test_issubvolname(const char *name)
 		strcmp(name, ".") && strcmp(name, "..");
 }
 
+/*
+ * Unified GNU semantics basename helper, never changing the argument. Always
+ * use this instead of basename().
+ */
+const char *path_basename(const char *path)
+{
+	const char *tmp = strrchr(path, '/');
+
+	/* Special case when the whole path is just "/". */
+	if (path[0] == '/' && path[1] == 0)
+		return path;
+
+	return tmp ? tmp + 1 : path;
+}
+
+/*
+ * Return dirname component of path, may change the argument.
+ * Own helper for parity with path_basename().
+ */
+char *path_dirname(char *path)
+{
+	return dirname(path);
+}

common/path-utils.h

@@ -39,6 +39,8 @@ int path_is_dir(const char *path);
 int is_same_loop_file(const char *a, const char *b);
 int path_is_reg_or_block_device(const char *filename);
 int path_is_in_dir(const char *parent, const char *path);
+const char *path_basename(const char *path);
+char *path_dirname(char *path);
 
 int test_issubvolname(const char *name);