fs: Add support for mount flags

The flags field has been added to fs_mount_t structure, accompanied
with two new flags:
  FS_MOUNT_FLAG_READ_ONLY -- mount fs as read only
  FS_MOUNT_FLAG_NO_FORMAT -- do not format volume when system not found

Code supporting the flags has been added to FS layer and drivers for
LittleFS and FAT FS.

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>

include/fs/fs.h

@@ -73,6 +73,11 @@ enum {
 	FS_TYPE_EXTERNAL_BASE,
 };
 
+/** Flag prevents formatting device if requested file system not found */
+#define FS_MOUNT_FLAG_NO_FORMAT BIT(0)
+/** Flag makes mounted file system read-only */
+#define FS_MOUNT_FLAG_READ_ONLY BIT(1)
+
 /**
  * @brief File system mount info structure
  *
@@ -83,6 +88,7 @@ enum {
  * @param storage_dev Pointer to backend storage device
  * @param mountp_len Length of Mount point string
  * @param fs Pointer to File system interface of the mount point
+ * @param flags Mount flags
  */
 struct fs_mount_t {
 	sys_dnode_t node;
@@ -93,6 +99,7 @@ struct fs_mount_t {
 	/* fields filled by file system core */
 	size_t mountp_len;
 	const struct fs_file_system_t *fs;
+	uint8_t flags;
 };
 
 /**
@@ -201,6 +208,9 @@ struct fs_statvfs {
  *
  * @retval 0 on success;
  * @retval -EINVAL when a bad file name is given;
+ * @retval -EROFS when opening read-only file for write, or attempting to
+ *	   create a file on a system that has been mounted with the
+ *	   FS_MOUNT_FLAG_READ_ONLY flag;
  * @retval -ENOENT when the file path is not possible (bad mount point);
  * @retval <0 an other negative errno code, depending on a file system back-end.
  */
@@ -226,6 +236,8 @@ int fs_close(struct fs_file_t *zfp);
  * @param path Path to the file or directory to delete
  *
  * @retval 0 on success;
+ * @retval -EROFS if file is read-only, or when file system has been mounted
+ *	   with the FS_MOUNT_FLAG_READ_ONLY flag;
  * @retval -ENOTSUP when not implemented by underlying file system driver;
  * @retval <0 an other negative errno code on error.
  */
@@ -435,7 +447,9 @@ int fs_closedir(struct fs_dir_t *zdp);
  * @retval 0 on success;
  * @retval -ENOENT when file system type has not been registered;
  * @retval -ENOTSUP when not supported by underlying file system driver;
- * @retval <0 a other negative errno code on error.
+ * @retval -EROFS if system requires formatting but @c FS_MOUNT_FLAG_READ_ONLY
+ *	   has been set;
+ * @retval <0 an other negative errno code on error.
  */
 int fs_mount(struct fs_mount_t *mp);
 

subsys/fs/fat_fs.c

@@ -410,7 +410,8 @@ static int fatfs_mount(struct fs_mount_t *mountp)
 
 #if defined(CONFIG_FS_FATFS_MOUNT_MKFS)
 	/* If no file system found then create one */
-	if (res == FR_NO_FILESYSTEM) {
+	if (res == FR_NO_FILESYSTEM &&
+	    !(mountp->flags & FS_MOUNT_FLAG_NO_FORMAT)) {
 		uint8_t work[_MAX_SS];
 
 		res = f_mkfs(&mountp->mnt_point[1],

subsys/fs/fs.c

@@ -150,6 +150,10 @@ int fs_open(struct fs_file_t *zfp, const char *file_name, fs_mode_t flags)
 	}
 
 	zfp->mp = mp;
+	if (((mp->flags & FS_MOUNT_FLAG_READ_ONLY) != 0) &&
+	    (flags & FS_O_CREATE || flags & FS_O_WRITE)) {
+		return -EROFS;
+	}
 
 	CHECKIF(zfp->mp->fs->open == NULL) {
 		return -ENOTSUP;
@@ -473,6 +477,10 @@ int fs_mkdir(const char *abs_path)
 		return rc;
 	}
 
+	if (mp->flags & FS_MOUNT_FLAG_READ_ONLY) {
+		return -EROFS;
+	}
+
 	CHECKIF(mp->fs->mkdir == NULL) {
 		return -ENOTSUP;
 	}
@@ -502,6 +510,10 @@ int fs_unlink(const char *abs_path)
 		return rc;
 	}
 
+	if (mp->flags & FS_MOUNT_FLAG_READ_ONLY) {
+		return -EROFS;
+	}
+
 	CHECKIF(mp->fs->unlink == NULL) {
 		return -ENOTSUP;
 	}
@@ -532,6 +544,10 @@ int fs_rename(const char *from, const char *to)
 		return rc;
 	}
 
+	if (mp->flags & FS_MOUNT_FLAG_READ_ONLY) {
+		return -EROFS;
+	}
+
 	/* Make sure both files are mounted on the same path */
 	if (strncmp(from, to, match_len) != 0) {
 		LOG_ERR("mount point not same!!");

subsys/fs/littlefs_fs.c

@@ -677,7 +677,7 @@ static int littlefs_mount(struct fs_mount_t *mountp)
 
 	/* Mount it, formatting if needed. */
 	ret = lfs_mount(&fs->lfs, &fs->cfg);
-	if (ret < 0) {
+	if (ret < 0 && !(mountp->flags & FS_MOUNT_FLAG_NO_FORMAT)) {
 		LOG_WRN("can't mount (LFS %d); formatting", ret);
 		ret = lfs_format(&fs->lfs, &fs->cfg);
 		if (ret < 0) {