From 74b1df2e84e836a1710561f52075d51f20cd5c78 Mon Sep 17 00:00:00 2001
From: Matthew <matthew@matthew-cash.com>
Date: Fri, 23 Feb 2024 22:56:49 -0800
Subject: [PATCH] Passthrough options starting with "x-" to mtab (#894)

This implements #651, tested with bindfs.

"x-*" options are comments meant to be interpreted by userspace.

#651 is about some 3rd party mount options like 'x-gvfs-notrash'.

This also removes the test if /etc/mtab is a symlink.

This test was added in commit 5f28cd15ab43c741f6d116be4d3a9aa5d82ab385
and the corresponding ChangeLog entry in this commit points to mount
issues for read-only mtab.

However, in all recent Linux distributions /etc/mtab is a symlink to
/proc/self/mounts and never writable. In fact, util-linux 2.39
(libmount) entirely removed support for a writable mtab.

At least since util-linux 2.19 (10-Feb-2011) /run/mount/utab is used
as replacement for userspace mount entries..
---
 lib/mount.c       |  6 ++++++
 lib/mount_util.c  |  4 ----
 util/fusermount.c | 50 +++++++++++++++++++++++++++++++++++++++++++++--
 3 files changed, 54 insertions(+), 6 deletions(-)

diff --git a/lib/mount.c b/lib/mount.c
index d71e6fc..f98a8bb 100644
--- a/lib/mount.c
+++ b/lib/mount.c
@@ -209,6 +209,12 @@ static int fuse_mount_opt_proc(void *data, const char *arg, int key,
 
 	case KEY_MTAB_OPT:
 		return fuse_opt_add_opt(&mo->mtab_opts, arg);
+
+	/* Third party options like 'x-gvfs-notrash' */
+	case FUSE_OPT_KEY_OPT:
+		return (strncmp("x-", arg, 2) == 0) ?
+			fuse_opt_add_opt(&mo->mtab_opts, arg) :
+			1;
 	}
 
 	/* Pass through unknown options */
diff --git a/lib/mount_util.c b/lib/mount_util.c
index 8027a2e..dd32276 100644
--- a/lib/mount_util.c
+++ b/lib/mount_util.c
@@ -54,7 +54,6 @@ static int mtab_needs_update(const char *mnt)
 	 * Skip mtab update if /etc/mtab:
 	 *
 	 *  - doesn't exist,
-	 *  - is a symlink,
 	 *  - is on a read-only filesystem.
 	 */
 	res = lstat(_PATH_MOUNTED, &stbuf);
@@ -65,9 +64,6 @@ static int mtab_needs_update(const char *mnt)
 		uid_t ruid;
 		int err;
 
-		if (S_ISLNK(stbuf.st_mode))
-			return 0;
-
 		ruid = getuid();
 		if (ruid != 0)
 			setreuid(0, -1);
diff --git a/util/fusermount.c b/util/fusermount.c
index a2dd8e1..fe75631 100644
--- a/util/fusermount.c
+++ b/util/fusermount.c
@@ -97,6 +97,30 @@ static struct mntent *GETMNTENT(FILE *stream)
 #define GETMNTENT getmntent
 #endif // GETMNTENT_NEEDS_UNESCAPING
 
+/*
+ * Take a ',' separated option string and extract "x-" options
+ */
+static void extract_x_options(const char *original, char *non_x_opts, char *x_opts)
+{
+	size_t len;
+	const char *opt, *opt_end;
+	char *opt_buf;
+
+	len = strlen(original);
+
+	for (opt = original; opt < original + len; opt = opt_end + 1) {
+		opt_end = strchr(opt, ',');
+		if (opt_end == NULL)
+			opt_end = original + len;
+
+		opt_buf = strncmp(opt, "x-", 2) ? non_x_opts : x_opts;
+
+		if (strlen(opt_buf) > 1)
+			strncat(opt_buf, ",", 2);
+
+		strncat(opt_buf, opt, opt_end - opt);
+	}
+}
 
 static const char *get_user_name(void)
 {
@@ -1135,6 +1159,11 @@ static int mount_fuse(const char *mnt, const char *opts, const char **type)
 	char *mnt_opts = NULL;
 	const char *real_mnt = mnt;
 	int mountpoint_fd = -1;
+	size_t opts_size;
+	char *do_mount_opts;
+	char *x_opts;
+	size_t add_mount_opts_size, add_mount_opts_len;
+	char *add_mount_opts;
 
 	fd = open_fuse_device(&dev);
 	if (fd == -1)
@@ -1151,11 +1180,17 @@ static int mount_fuse(const char *mnt, const char *opts, const char **type)
 		}
 	}
 
+	// Extract any options starting with "x-"
+	opts_size = strlen(opts) + 1;
+	do_mount_opts = malloc(opts_size);
+	x_opts = malloc(opts_size);
+	extract_x_options(opts, do_mount_opts, x_opts);
+
 	res = check_perm(&real_mnt, &stbuf, &mountpoint_fd);
 	restore_privs();
 	if (res != -1)
 		res = do_mount(real_mnt, type, stbuf.st_mode & S_IFMT,
-			       fd, opts, dev, &source, &mnt_opts);
+			       fd, do_mount_opts, dev, &source, &mnt_opts);
 
 	if (mountpoint_fd != -1)
 		close(mountpoint_fd);
@@ -1170,7 +1205,18 @@ static int mount_fuse(const char *mnt, const char *opts, const char **type)
 	}
 
 	if (geteuid() == 0) {
-		res = add_mount(source, mnt, *type, mnt_opts);
+		// Add back the options starting with "x-" to opts from do_mount
+		add_mount_opts_size = strlen(mnt_opts) + strlen(x_opts) + 2;
+		add_mount_opts = malloc(add_mount_opts_size);
+
+		strncpy(add_mount_opts, mnt_opts, add_mount_opts_size - 1);
+		add_mount_opts_len = strlen(add_mount_opts);
+		if (add_mount_opts_len > 0)
+			strncat(add_mount_opts, ",", 2);
+		strncat(add_mount_opts, x_opts,
+			add_mount_opts_size - add_mount_opts_len - 2);
+
+		res = add_mount(source, mnt, *type, add_mount_opts);
 		if (res == -1) {
 			/* Can't clean up mount in a non-racy way */
 			goto fail_close_fd;
-- 
2.30.2