libmount: add post-mount checks to detect ro/rw

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

libmount/src/context_loopdev.c

@@ -289,8 +289,7 @@ int mnt_context_setup_loopdev(struct libmnt_context *cxt)
 			 * mount planned read-write, but loopdev is read-only,
 			 * let's fix mount options...
 			 */
-			cxt->mountflags |= MS_RDONLY;
-
+			mnt_context_set_mflags(cxt, cxt->mountflags | MS_RDONLY);
 
 		/* we have to keep the device open until mount(1),
 		 * otherwise it will auto-cleared by kernel

libmount/src/context_mount.c

@@ -455,10 +455,6 @@ static int do_mount(struct libmnt_context *cxt, const char *try_type)
 			rc = mnt_fs_set_fstype(fs, try_type);
 	}
 
-	/* TODO: check if the result is really read-only/read-write
-	 *       and if necessary update cxt->mountflags
-	 */
-
 	return rc;
 }
 
@@ -591,6 +587,7 @@ int mnt_context_prepare_mount(struct libmnt_context *cxt)
 int mnt_context_do_mount(struct libmnt_context *cxt)
 {
 	const char *type;
+	int res;
 
 	assert(cxt);
 	assert(cxt->fs);
@@ -607,9 +604,40 @@ int mnt_context_do_mount(struct libmnt_context *cxt)
 
 	type = mnt_fs_get_fstype(cxt->fs);
 	if (type)
-		return do_mount(cxt, NULL);
+		res = do_mount(cxt, NULL);
+	else
+		res = do_mount_by_pattern(cxt, cxt->fstype_pattern);
 
-	return do_mount_by_pattern(cxt, cxt->fstype_pattern);
+	if (mnt_context_get_status(cxt)
+	    && !(cxt->flags & MNT_FL_FAKE)
+	    && !cxt->helper) {
+		/*
+		 * Mounted by mount(2), do some post-mount checks
+		 *
+		 * Kernel allows to use MS_RDONLY for bind mounts, but the
+		 * read-only request could be silently ignored. Check it to
+		 * avoid 'ro' in mtab and 'rw' in /proc/mounts.
+		 */
+		if ((cxt->mountflags & MS_BIND)
+		    && (cxt->mountflags & MS_RDONLY)
+		    && !mnt_is_readonly(mnt_context_get_target(cxt)))
+
+			mnt_context_set_mflags(cxt,
+					cxt->mountflags & ~MS_RDONLY);
+
+
+		/* Kernel can silently add MS_RDONLY flag when mounting file
+		 * system that does not have write support. Check this to avoid
+		 * 'ro' in /proc/mounts and 'rw' in mtab.
+		 */
+		if (!(cxt->mountflags & (MS_RDONLY | MS_PROPAGATION | MS_MOVE))
+		    && mnt_is_readonly(mnt_context_get_target(cxt)))
+
+			mnt_context_set_mflags(cxt,
+					cxt->mountflags | MS_RDONLY);
+	}
+
+	return res;
 }
 
 /**

libmount/src/mountP.h

@@ -129,6 +129,8 @@ extern int mnt_run_test(struct libmnt_test *tests, int argc, char *argv[]);
 extern int endswith(const char *s, const char *sx);
 extern int startswith(const char *s, const char *sx);
 
+extern int mnt_is_readonly(const char *path);
+
 extern int mnt_parse_offset(const char *str, size_t len, uintmax_t *res);
 
 extern int mnt_chdir_to_parent(const char *target, char **filename);

libmount/src/utils.c

@@ -145,6 +145,41 @@ err:
 	return rc;
 }
 
+/*
+ * Check if @path is on read-only filesystem independently on file permissions.
+ */
+int mnt_is_readonly(const char *path)
+{
+	if (access(path, W_OK) == 0)
+		return 0;
+	if (errno == EROFS)
+		return 1;
+	if (errno != EACCES)
+		return 0;
+
+#ifdef HAVE_FUTIMENS
+	/*
+	 * access(2) returns EACCES on read-only FS:
+	 *
+	 * - for set-uid application if one component of the path is not
+	 *   accessible for the current rUID. (Note that euidaccess(2) does not
+	 *   check for EROFS at all).
+	 *
+	 * - for read-write filesystem with read-only VFS node (aka -o remount,ro,bind)
+	 */
+	{
+		struct timespec times[2];
+
+		times[0].tv_nsec = UTIME_NOW;	/* atime */
+		times[1].tv_nsec = UTIME_OMIT;	/* mtime */
+
+		if (utimensat(AT_FDCWD, path, times, 0) == -1)
+			return errno == EROFS;
+	}
+#endif
+	return 0;
+}
+
 /**
  * mnt_mangle:
  * @str: string