extmod/vfs: Check block 0 and 1 when auto-detecting littlefs.

The superblock for littlefs is in block 0 and 1, but block 0 may be erased
or partially written, so block 1 must be checked if block 0 does not have a
valid littlefs superblock in it.

Prior to this commit, the mount of a block device which auto-detected the
filysystem type would fail for littlefs if block 0 did not contain a valid
superblock.  That is now fixed.

Signed-off-by: Damien George <damien@micropython.org>

extmod/vfs.c

@@ -160,15 +160,17 @@ STATIC mp_obj_t mp_vfs_autodetect(mp_obj_t bdev_obj) {
     #if MICROPY_VFS_LFS1 || MICROPY_VFS_LFS2
     nlr_buf_t nlr;
     if (nlr_push(&nlr) == 0) {
-        mp_obj_t vfs = MP_OBJ_NULL;
+        // The superblock for littlefs is in both block 0 and 1, but block 0 may be erased
+        // or partially written, so search both blocks 0 and 1 for the littlefs signature.
         mp_vfs_blockdev_t blockdev;
         mp_vfs_blockdev_init(&blockdev, bdev_obj);
         uint8_t buf[44];
-        mp_vfs_blockdev_read_ext(&blockdev, 0, 8, sizeof(buf), buf);
+        for (size_t block_num = 0; block_num <= 1; ++block_num) {
+            mp_vfs_blockdev_read_ext(&blockdev, block_num, 8, sizeof(buf), buf);
             #if MICROPY_VFS_LFS1
             if (memcmp(&buf[32], "littlefs", 8) == 0) {
                 // LFS1
-            vfs = mp_type_vfs_lfs1.make_new(&mp_type_vfs_lfs1, 1, 0, &bdev_obj);
+                mp_obj_t vfs = mp_type_vfs_lfs1.make_new(&mp_type_vfs_lfs1, 1, 0, &bdev_obj);
                 nlr_pop();
                 return vfs;
             }
@@ -176,11 +178,12 @@ STATIC mp_obj_t mp_vfs_autodetect(mp_obj_t bdev_obj) {
             #if MICROPY_VFS_LFS2
             if (memcmp(&buf[0], "littlefs", 8) == 0) {
                 // LFS2
-            vfs = mp_type_vfs_lfs2.make_new(&mp_type_vfs_lfs2, 1, 0, &bdev_obj);
+                mp_obj_t vfs = mp_type_vfs_lfs2.make_new(&mp_type_vfs_lfs2, 1, 0, &bdev_obj);
                 nlr_pop();
                 return vfs;
             }
             #endif
+        }
         nlr_pop();
     } else {
         // Ignore exception (eg block device doesn't support extended readblocks)

tests/extmod/vfs_lfs_superblock.py

@@ -0,0 +1,47 @@
+# Test for VfsLfs using a RAM device, when the first superblock does not exist
+
+try:
+    import uos
+
+    uos.VfsLfs2
+except (ImportError, AttributeError):
+    print("SKIP")
+    raise SystemExit
+
+
+class RAMBlockDevice:
+    def __init__(self, block_size, data):
+        self.block_size = block_size
+        self.data = data
+
+    def readblocks(self, block, buf, off):
+        addr = block * self.block_size + off
+        for i in range(len(buf)):
+            buf[i] = self.data[addr + i]
+
+    def ioctl(self, op, arg):
+        if op == 4:  # block count
+            return len(self.data) // self.block_size
+        if op == 5:  # block size
+            return self.block_size
+        if op == 6:  # erase block
+            return 0
+
+
+# This is a valid littlefs2 filesystem with a block size of 64 bytes.
+# The first block (where the first superblock is stored) is fully erased.
+lfs2_data = b"\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\x02\x00\x00\x00\xf0\x0f\xff\xf7littlefs/\xe0\x00\x10\x00\x00\x02\x00@\x00\x00\x00\x04\x00\x00\x00\xff\x00\x00\x00\xff\xff\xff\x7f\xfe\x03\x00\x00p\x1f\xfc\x08\x1b\xb4\x14\xa7\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\x01\x00\x00\x00\xff\xef\xff\xf7test.txt \x00\x00\x08p\x1f\xfc\x08\x83\xf1u\xba\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00"
+
+# Create the block device from the static data (it will be read-only).
+bdev = RAMBlockDevice(64, lfs2_data)
+
+# Create the VFS explicitly, no auto-detection is needed for this.
+vfs = uos.VfsLfs2(bdev)
+print(list(vfs.ilistdir()))
+
+# Mount the block device directly; this relies on auto-detection.
+uos.mount(bdev, "/userfs")
+print(uos.listdir("/userfs"))
+
+# Clean up.
+uos.umount("/userfs")

tests/extmod/vfs_lfs_superblock.py.exp

@@ -0,0 +1,2 @@
+[]
+[]