(bugfix) adding line function to clear out all the global 'free' information so that we can reset it after a failed traversal

Original issue found by thrasher8390

lfs.c

@@ -10,6 +10,8 @@
 #define LFS_BLOCK_NULL ((lfs_block_t)-1)
 #define LFS_BLOCK_INLINE ((lfs_block_t)-2)
 
+static void lfs_alloc_ack(lfs_t *lfs);
+
 /// Caching block device operations ///
 static inline void lfs_cache_drop(lfs_t *lfs, lfs_cache_t *rcache) {
     // do not zero, cheaper if cache is readonly or only going to be
@@ -24,12 +26,22 @@ static inline void lfs_cache_zero(lfs_t *lfs, lfs_cache_t *pcache) {
     pcache->block = LFS_BLOCK_NULL;
 }
 
+/// Invalidate the lookahead buffer. This is done during mounting and failed traversals ///
+static inline void lfs_setup_invalid_lookahead_buffer(lfs_t *lfs)
+{
+    lfs->free.off = lfs->seed % lfs->cfg->block_size;
+    lfs->free.size = 0;
+    lfs->free.i = 0;
+    lfs_alloc_ack(lfs);
+}
+
 static int lfs_bd_read(lfs_t *lfs,
         const lfs_cache_t *pcache, lfs_cache_t *rcache, lfs_size_t hint,
         lfs_block_t block, lfs_off_t off,
         void *buffer, lfs_size_t size) {
     uint8_t *data = buffer;
-    if ((off+size > lfs->cfg->block_size) || (block == LFS_BLOCK_NULL)) {
+    if (block >= lfs->cfg->block_count ||
+            off+size > lfs->cfg->block_size) {
         return LFS_ERR_CORRUPT;
     }
 
@@ -172,7 +184,7 @@ static int lfs_bd_prog(lfs_t *lfs,
         lfs_block_t block, lfs_off_t off,
         const void *buffer, lfs_size_t size) {
     const uint8_t *data = buffer;
-    LFS_ASSERT(block != LFS_BLOCK_NULL);
+    LFS_ASSERT(block == LFS_BLOCK_INLINE || block < lfs->cfg->block_count);
     LFS_ASSERT(off + size <= lfs->cfg->block_size);
 
     while (size > 0) {
@@ -476,6 +488,7 @@ static int lfs_alloc(lfs_t *lfs, lfs_block_t *block) {
         memset(lfs->free.buffer, 0, lfs->cfg->lookahead_size);
         int err = lfs_fs_traverseraw(lfs, lfs_alloc_lookahead, lfs, true);
         if (err) {
+            lfs_setup_invalid_lookahead_buffer(lfs);
             return err;
         }
     }
@@ -747,6 +760,12 @@ static lfs_stag_t lfs_dir_fetchmatch(lfs_t *lfs,
     // scanning the entire directory
     lfs_stag_t besttag = -1;
 
+    // if either block address is invalid we return LFS_ERR_CORRUPT here,
+    // otherwise later writes to the pair could fail
+    if (pair[0] >= lfs->cfg->block_count || pair[1] >= lfs->cfg->block_count) {
+        return LFS_ERR_CORRUPT;
+    }
+
     // find the block with the most recent revision
     uint32_t revs[2] = {0, 0};
     int r = 0;
@@ -2196,7 +2215,6 @@ static int lfs_ctz_find(lfs_t *lfs,
             return err;
         }
 
-        LFS_ASSERT(head >= 2 && head <= lfs->cfg->block_count);
         current -= 1 << skip;
     }
 
@@ -2216,7 +2234,6 @@ static int lfs_ctz_extend(lfs_t *lfs,
         if (err) {
             return err;
         }
-        LFS_ASSERT(nblock >= 2 && nblock <= lfs->cfg->block_count);
 
         {
             err = lfs_bd_erase(lfs, nblock);
@@ -2289,8 +2306,6 @@ static int lfs_ctz_extend(lfs_t *lfs,
                         return err;
                     }
                 }
-
-                LFS_ASSERT(nhead >= 2 && nhead <= lfs->cfg->block_count);
             }
 
             *block = nblock;
@@ -3661,7 +3676,15 @@ int lfs_mount(lfs_t *lfs, const struct lfs_config *cfg) {
 
     // scan directory blocks for superblock and any global updates
     lfs_mdir_t dir = {.tail = {0, 1}};
+    lfs_block_t cycle = 0;
     while (!lfs_pair_isnull(dir.tail)) {
+        if (cycle >= lfs->cfg->block_count/2) {
+            // loop detected
+            err = LFS_ERR_CORRUPT;
+            goto cleanup;
+        }
+        cycle += 1;
+
         // fetch next block in tail list
         lfs_stag_t tag = lfs_dir_fetchmatch(lfs, &dir, dir.tail,
                 LFS_MKTAG(0x7ff, 0x3ff, 0),
@@ -3761,10 +3784,7 @@ int lfs_mount(lfs_t *lfs, const struct lfs_config *cfg) {
     lfs->gdisk = lfs->gstate;
 
     // setup free lookahead
-    lfs->free.off = lfs->seed % lfs->cfg->block_size;
-    lfs->free.size = 0;
-    lfs->free.i = 0;
-    lfs_alloc_ack(lfs);
+    lfs_setup_invalid_lookahead_buffer(lfs);
 
     LFS_TRACE("lfs_mount -> %d", 0);
     return 0;
@@ -3803,7 +3823,14 @@ int lfs_fs_traverseraw(lfs_t *lfs,
     }
 #endif
 
+    lfs_block_t cycle = 0;
     while (!lfs_pair_isnull(dir.tail)) {
+        if (cycle >= lfs->cfg->block_count/2) {
+            // loop detected
+            return LFS_ERR_CORRUPT;
+        }
+        cycle += 1;
+
         for (int i = 0; i < 2; i++) {
             int err = cb(data, dir.tail[i]);
             if (err) {
@@ -3887,7 +3914,14 @@ static int lfs_fs_pred(lfs_t *lfs,
     // iterate over all directory directory entries
     pdir->tail[0] = 0;
     pdir->tail[1] = 1;
+    lfs_block_t cycle = 0;
     while (!lfs_pair_isnull(pdir->tail)) {
+        if (cycle >= lfs->cfg->block_count/2) {
+            // loop detected
+            return LFS_ERR_CORRUPT;
+        }
+        cycle += 1;
+
         if (lfs_pair_cmp(pdir->tail, pair) == 0) {
             return 0;
         }
@@ -3930,7 +3964,14 @@ static lfs_stag_t lfs_fs_parent(lfs_t *lfs, const lfs_block_t pair[2],
     // use fetchmatch with callback to find pairs
     parent->tail[0] = 0;
     parent->tail[1] = 1;
+    lfs_block_t cycle = 0;
     while (!lfs_pair_isnull(parent->tail)) {
+        if (cycle >= lfs->cfg->block_count/2) {
+            // loop detected
+            return LFS_ERR_CORRUPT;
+        }
+        cycle += 1;
+
         lfs_stag_t tag = lfs_dir_fetchmatch(lfs, parent, parent->tail,
                 LFS_MKTAG(0x7ff, 0, 0x3ff),
                 LFS_MKTAG(LFS_TYPE_DIRSTRUCT, 0, 8),
@@ -4562,6 +4603,7 @@ static int lfs1_mount(lfs_t *lfs, struct lfs1 *lfs1,
         lfs->lfs1->root[1] = LFS_BLOCK_NULL;
 
         // setup free lookahead
+        // TODO should this also call lfs_setup_invalid_lookahead_buffer(lfs); the free.off is different in the current version of lfs
         lfs->free.off = 0;
         lfs->free.size = 0;
         lfs->free.i = 0;

tests/test_alloc.toml

@@ -323,6 +323,90 @@ code = '''
     lfs_unmount(&lfs) => 0;
 '''
 
+[[case]] # what if we have a bad block during an allocation scan?
+in = "lfs.c"
+define.LFS_ERASE_CYCLES = 0xffffffff
+define.LFS_BADBLOCK_BEHAVIOR = 'LFS_TESTBD_BADBLOCK_READERROR'
+code = '''
+    lfs_format(&lfs, &cfg) => 0;
+    lfs_mount(&lfs, &cfg) => 0;
+    // first fill to exhaustion to find available space
+    lfs_file_open(&lfs, &file, "pacman", LFS_O_WRONLY | LFS_O_CREAT) => 0;
+    strcpy((char*)buffer, "waka");
+    size = strlen("waka");
+    lfs_size_t filesize = 0;
+    while (true) {
+        lfs_ssize_t res = lfs_file_write(&lfs, &file, buffer, size);
+        assert(res == (lfs_ssize_t)size || res == LFS_ERR_NOSPC);
+        if (res == LFS_ERR_NOSPC) {
+            break;
+        }
+        filesize += size;
+    }
+    lfs_file_close(&lfs, &file) => 0;
+    // now fill all but a couple of blocks of the filesystem with data
+    filesize -= 3*LFS_BLOCK_SIZE;
+    lfs_file_open(&lfs, &file, "pacman", LFS_O_WRONLY | LFS_O_CREAT) => 0;
+    strcpy((char*)buffer, "waka");
+    size = strlen("waka");
+    for (lfs_size_t i = 0; i < filesize/size; i++) {
+        lfs_file_write(&lfs, &file, buffer, size) => size;
+    }
+    lfs_file_close(&lfs, &file) => 0;
+    // also save head of file so we can error during lookahead scan
+    lfs_block_t fileblock = file.ctz.head;
+    lfs_unmount(&lfs) => 0;
+
+    // remount to force an alloc scan
+    lfs_mount(&lfs, &cfg) => 0;
+
+    // but mark the head of our file as a "bad block", this is force our
+    // scan to bail early
+    lfs_testbd_setwear(&cfg, fileblock, 0xffffffff) => 0;
+    lfs_file_open(&lfs, &file, "ghost", LFS_O_WRONLY | LFS_O_CREAT) => 0;
+    strcpy((char*)buffer, "chomp");
+    size = strlen("chomp");
+    while (true) {
+        lfs_ssize_t res = lfs_file_write(&lfs, &file, buffer, size);
+        assert(res == (lfs_ssize_t)size || res == LFS_ERR_CORRUPT);
+        if (res == LFS_ERR_CORRUPT) {
+            break;
+        }
+    }
+    lfs_file_close(&lfs, &file) => 0;
+
+    // now reverse the "bad block" and try to write the file again until we
+    // run out of space
+    lfs_testbd_setwear(&cfg, fileblock, 0) => 0;
+    lfs_file_open(&lfs, &file, "ghost", LFS_O_WRONLY | LFS_O_CREAT) => 0;
+    strcpy((char*)buffer, "chomp");
+    size = strlen("chomp");
+    while (true) {
+        lfs_ssize_t res = lfs_file_write(&lfs, &file, buffer, size);
+        assert(res == (lfs_ssize_t)size || res == LFS_ERR_NOSPC);
+        if (res == LFS_ERR_NOSPC) {
+            break;
+        }
+    }
+    lfs_file_close(&lfs, &file) => 0;
+
+    lfs_unmount(&lfs) => 0;
+
+    // check that the disk isn't hurt
+    lfs_mount(&lfs, &cfg) => 0;
+    lfs_file_open(&lfs, &file, "pacman", LFS_O_RDONLY) => 0;
+    strcpy((char*)buffer, "waka");
+    size = strlen("waka");
+    for (lfs_size_t i = 0; i < filesize/size; i++) {
+        uint8_t rbuffer[4];
+        lfs_file_read(&lfs, &file, rbuffer, size) => size;
+        assert(memcmp(rbuffer, buffer, size) == 0);
+    }
+    lfs_file_close(&lfs, &file) => 0;
+    lfs_unmount(&lfs) => 0;
+'''
+
+
 # Below, I don't like these tests. They're fragile and depend _heavily_
 # on the geometry of the block device. But they are valuable. Eventually they
 # should be removed and replaced with generalized tests.

tests/test_evil.toml

@@ -0,0 +1,288 @@
+# Tests for recovering from conditions which shouldn't normally
+# happen during normal operation of littlefs
+
+# invalid pointer tests (outside of block_count)
+
+[[case]] # invalid tail-pointer test
+define.TAIL_TYPE = ['LFS_TYPE_HARDTAIL', 'LFS_TYPE_SOFTTAIL']
+define.INVALSET = [0x3, 0x1, 0x2]
+in = "lfs.c"
+code = '''
+    // create littlefs
+    lfs_format(&lfs, &cfg) => 0;
+
+    // change tail-pointer to invalid pointers
+    lfs_init(&lfs, &cfg) => 0;
+    lfs_mdir_t mdir;
+    lfs_dir_fetch(&lfs, &mdir, (lfs_block_t[2]){0, 1}) => 0;
+    lfs_dir_commit(&lfs, &mdir, LFS_MKATTRS(
+            {LFS_MKTAG(LFS_TYPE_HARDTAIL, 0x3ff, 8),
+                (lfs_block_t[2]){
+                    (INVALSET & 0x1) ? 0xcccccccc : 0,
+                    (INVALSET & 0x2) ? 0xcccccccc : 0}})) => 0;
+    lfs_deinit(&lfs) => 0;
+
+    // test that mount fails gracefully
+    lfs_mount(&lfs, &cfg) => LFS_ERR_CORRUPT;
+'''
+
+[[case]] # invalid dir pointer test
+define.INVALSET = [0x3, 0x1, 0x2]
+in = "lfs.c"
+code = '''
+    // create littlefs
+    lfs_format(&lfs, &cfg) => 0;
+    // make a dir
+    lfs_mount(&lfs, &cfg) => 0;
+    lfs_mkdir(&lfs, "dir_here") => 0;
+    lfs_unmount(&lfs) => 0;
+
+    // change the dir pointer to be invalid
+    lfs_init(&lfs, &cfg) => 0;
+    lfs_mdir_t mdir;
+    lfs_dir_fetch(&lfs, &mdir, (lfs_block_t[2]){0, 1}) => 0;
+    // make sure id 1 == our directory
+    lfs_dir_get(&lfs, &mdir,
+            LFS_MKTAG(0x700, 0x3ff, 0),
+            LFS_MKTAG(LFS_TYPE_NAME, 1, strlen("dir_here")), buffer)
+                => LFS_MKTAG(LFS_TYPE_DIR, 1, strlen("dir_here"));
+    assert(memcmp((char*)buffer, "dir_here", strlen("dir_here")) == 0);
+    // change dir pointer
+    lfs_dir_commit(&lfs, &mdir, LFS_MKATTRS(
+            {LFS_MKTAG(LFS_TYPE_DIRSTRUCT, 1, 8),
+                (lfs_block_t[2]){
+                    (INVALSET & 0x1) ? 0xcccccccc : 0,
+                    (INVALSET & 0x2) ? 0xcccccccc : 0}})) => 0;
+    lfs_deinit(&lfs) => 0;
+
+    // test that accessing our bad dir fails, note there's a number
+    // of ways to access the dir, some can fail, but some don't
+    lfs_mount(&lfs, &cfg) => 0;
+    lfs_stat(&lfs, "dir_here", &info) => 0;
+    assert(strcmp(info.name, "dir_here") == 0);
+    assert(info.type == LFS_TYPE_DIR);
+
+    lfs_dir_open(&lfs, &dir, "dir_here") => LFS_ERR_CORRUPT;
+    lfs_stat(&lfs, "dir_here/file_here", &info) => LFS_ERR_CORRUPT;
+    lfs_dir_open(&lfs, &dir, "dir_here/dir_here") => LFS_ERR_CORRUPT;
+    lfs_file_open(&lfs, &file, "dir_here/file_here",
+            LFS_O_RDONLY) => LFS_ERR_CORRUPT;
+    lfs_file_open(&lfs, &file, "dir_here/file_here",
+            LFS_O_WRONLY | LFS_O_CREAT) => LFS_ERR_CORRUPT;
+    lfs_unmount(&lfs) => 0;
+'''
+
+[[case]] # invalid file pointer test
+in = "lfs.c"
+define.SIZE = [10, 1000, 100000] # faked file size
+code = '''
+    // create littlefs
+    lfs_format(&lfs, &cfg) => 0;
+    // make a file
+    lfs_mount(&lfs, &cfg) => 0;
+    lfs_file_open(&lfs, &file, "file_here",
+            LFS_O_WRONLY | LFS_O_CREAT) => 0;
+    lfs_file_close(&lfs, &file) => 0;
+    lfs_unmount(&lfs) => 0;
+
+    // change the file pointer to be invalid
+    lfs_init(&lfs, &cfg) => 0;
+    lfs_mdir_t mdir;
+    lfs_dir_fetch(&lfs, &mdir, (lfs_block_t[2]){0, 1}) => 0;
+    // make sure id 1 == our file
+    lfs_dir_get(&lfs, &mdir,
+            LFS_MKTAG(0x700, 0x3ff, 0),
+            LFS_MKTAG(LFS_TYPE_NAME, 1, strlen("file_here")), buffer)
+                => LFS_MKTAG(LFS_TYPE_REG, 1, strlen("file_here"));
+    assert(memcmp((char*)buffer, "file_here", strlen("file_here")) == 0);
+    // change file pointer
+    lfs_dir_commit(&lfs, &mdir, LFS_MKATTRS(
+            {LFS_MKTAG(LFS_TYPE_CTZSTRUCT, 1, sizeof(struct lfs_ctz)),
+                &(struct lfs_ctz){0xcccccccc, lfs_tole32(SIZE)}})) => 0;
+    lfs_deinit(&lfs) => 0;
+
+    // test that accessing our bad file fails, note there's a number
+    // of ways to access the dir, some can fail, but some don't
+    lfs_mount(&lfs, &cfg) => 0;
+    lfs_stat(&lfs, "file_here", &info) => 0;
+    assert(strcmp(info.name, "file_here") == 0);
+    assert(info.type == LFS_TYPE_REG);
+    assert(info.size == SIZE);
+
+    lfs_file_open(&lfs, &file, "file_here", LFS_O_RDONLY) => 0;
+    lfs_file_read(&lfs, &file, buffer, SIZE) => LFS_ERR_CORRUPT;
+    lfs_file_close(&lfs, &file) => 0;
+
+    // any allocs that traverse CTZ must unfortunately must fail
+    if (SIZE > 2*LFS_BLOCK_SIZE) {
+        lfs_mkdir(&lfs, "dir_here") => LFS_ERR_CORRUPT;
+    }
+    lfs_unmount(&lfs) => 0;
+'''
+
+[[case]] # invalid pointer in CTZ skip-list test
+define.SIZE = ['2*LFS_BLOCK_SIZE', '3*LFS_BLOCK_SIZE', '4*LFS_BLOCK_SIZE']
+in = "lfs.c"
+code = '''
+    // create littlefs
+    lfs_format(&lfs, &cfg) => 0;
+    // make a file
+    lfs_mount(&lfs, &cfg) => 0;
+    lfs_file_open(&lfs, &file, "file_here",
+            LFS_O_WRONLY | LFS_O_CREAT) => 0;
+    for (int i = 0; i < SIZE; i++) {
+        char c = 'c';
+        lfs_file_write(&lfs, &file, &c, 1) => 1;
+    }
+    lfs_file_close(&lfs, &file) => 0;
+    lfs_unmount(&lfs) => 0;
+    // change pointer in CTZ skip-list to be invalid
+    lfs_init(&lfs, &cfg) => 0;
+    lfs_mdir_t mdir;
+    lfs_dir_fetch(&lfs, &mdir, (lfs_block_t[2]){0, 1}) => 0;
+    // make sure id 1 == our file and get our CTZ structure
+    lfs_dir_get(&lfs, &mdir,
+            LFS_MKTAG(0x700, 0x3ff, 0),
+            LFS_MKTAG(LFS_TYPE_NAME, 1, strlen("file_here")), buffer)
+                => LFS_MKTAG(LFS_TYPE_REG, 1, strlen("file_here"));
+    assert(memcmp((char*)buffer, "file_here", strlen("file_here")) == 0);
+    struct lfs_ctz ctz;
+    lfs_dir_get(&lfs, &mdir,
+            LFS_MKTAG(0x700, 0x3ff, 0),
+            LFS_MKTAG(LFS_TYPE_STRUCT, 1, sizeof(struct lfs_ctz)), &ctz)
+                => LFS_MKTAG(LFS_TYPE_CTZSTRUCT, 1, sizeof(struct lfs_ctz));
+    lfs_ctz_fromle32(&ctz);
+    // rewrite block to contain bad pointer
+    uint8_t bbuffer[LFS_BLOCK_SIZE];
+    cfg.read(&cfg, ctz.head, 0, bbuffer, LFS_BLOCK_SIZE) => 0;
+    uint32_t bad = lfs_tole32(0xcccccccc);
+    memcpy(&bbuffer[0], &bad, sizeof(bad));
+    memcpy(&bbuffer[4], &bad, sizeof(bad));
+    cfg.erase(&cfg, ctz.head) => 0;
+    cfg.prog(&cfg, ctz.head, 0, bbuffer, LFS_BLOCK_SIZE) => 0;
+    lfs_deinit(&lfs) => 0;
+
+    // test that accessing our bad file fails, note there's a number
+    // of ways to access the dir, some can fail, but some don't
+    lfs_mount(&lfs, &cfg) => 0;
+    lfs_stat(&lfs, "file_here", &info) => 0;
+    assert(strcmp(info.name, "file_here") == 0);
+    assert(info.type == LFS_TYPE_REG);
+    assert(info.size == SIZE);
+
+    lfs_file_open(&lfs, &file, "file_here", LFS_O_RDONLY) => 0;
+    lfs_file_read(&lfs, &file, buffer, SIZE) => LFS_ERR_CORRUPT;
+    lfs_file_close(&lfs, &file) => 0;
+
+    // any allocs that traverse CTZ must unfortunately must fail
+    if (SIZE > 2*LFS_BLOCK_SIZE) {
+        lfs_mkdir(&lfs, "dir_here") => LFS_ERR_CORRUPT;
+    }
+    lfs_unmount(&lfs) => 0;
+'''
+
+
+[[case]] # invalid gstate pointer
+define.INVALSET = [0x3, 0x1, 0x2]
+in = "lfs.c"
+code = '''
+    // create littlefs
+    lfs_format(&lfs, &cfg) => 0;
+
+    // create an invalid gstate
+    lfs_init(&lfs, &cfg) => 0;
+    lfs_mdir_t mdir;
+    lfs_dir_fetch(&lfs, &mdir, (lfs_block_t[2]){0, 1}) => 0;
+    lfs_fs_prepmove(&lfs, 1, (lfs_block_t [2]){
+            (INVALSET & 0x1) ? 0xcccccccc : 0,
+            (INVALSET & 0x2) ? 0xcccccccc : 0});
+    lfs_dir_commit(&lfs, &mdir, NULL, 0) => 0;
+    lfs_deinit(&lfs) => 0;
+
+    // test that mount fails gracefully
+    // mount may not fail, but our first alloc should fail when
+    // we try to fix the gstate
+    lfs_mount(&lfs, &cfg) => 0;
+    lfs_mkdir(&lfs, "should_fail") => LFS_ERR_CORRUPT;
+    lfs_unmount(&lfs) => 0;
+'''
+
+# cycle detection/recovery tests
+
+[[case]] # metadata-pair threaded-list loop test
+in = "lfs.c"
+code = '''
+    // create littlefs
+    lfs_format(&lfs, &cfg) => 0;
+
+    // change tail-pointer to point to ourself
+    lfs_init(&lfs, &cfg) => 0;
+    lfs_mdir_t mdir;
+    lfs_dir_fetch(&lfs, &mdir, (lfs_block_t[2]){0, 1}) => 0;
+    lfs_dir_commit(&lfs, &mdir, LFS_MKATTRS(
+            {LFS_MKTAG(LFS_TYPE_HARDTAIL, 0x3ff, 8),
+                (lfs_block_t[2]){0, 1}})) => 0;
+    lfs_deinit(&lfs) => 0;
+
+    // test that mount fails gracefully
+    lfs_mount(&lfs, &cfg) => LFS_ERR_CORRUPT;
+'''
+
+[[case]] # metadata-pair threaded-list 2-length loop test
+in = "lfs.c"
+code = '''
+    // create littlefs with child dir
+    lfs_format(&lfs, &cfg) => 0;
+    lfs_mount(&lfs, &cfg) => 0;
+    lfs_mkdir(&lfs, "child") => 0;
+    lfs_unmount(&lfs) => 0;
+
+    // find child
+    lfs_init(&lfs, &cfg) => 0;
+    lfs_mdir_t mdir;
+    lfs_block_t pair[2];
+    lfs_dir_fetch(&lfs, &mdir, (lfs_block_t[2]){0, 1}) => 0;
+    lfs_dir_get(&lfs, &mdir,
+            LFS_MKTAG(0x7ff, 0x3ff, 0),
+            LFS_MKTAG(LFS_TYPE_DIRSTRUCT, 1, sizeof(pair)), pair)
+                => LFS_MKTAG(LFS_TYPE_DIRSTRUCT, 1, sizeof(pair));
+    lfs_pair_fromle32(pair);
+    // change tail-pointer to point to root
+    lfs_dir_fetch(&lfs, &mdir, pair) => 0;
+    lfs_dir_commit(&lfs, &mdir, LFS_MKATTRS(
+            {LFS_MKTAG(LFS_TYPE_HARDTAIL, 0x3ff, 8),
+                (lfs_block_t[2]){0, 1}})) => 0;
+    lfs_deinit(&lfs) => 0;
+
+    // test that mount fails gracefully
+    lfs_mount(&lfs, &cfg) => LFS_ERR_CORRUPT;
+'''
+
+[[case]] # metadata-pair threaded-list 1-length child loop test
+in = "lfs.c"
+code = '''
+    // create littlefs with child dir
+    lfs_format(&lfs, &cfg) => 0;
+    lfs_mount(&lfs, &cfg) => 0;
+    lfs_mkdir(&lfs, "child") => 0;
+    lfs_unmount(&lfs) => 0;
+
+    // find child
+    lfs_init(&lfs, &cfg) => 0;
+    lfs_mdir_t mdir;
+    lfs_block_t pair[2];
+    lfs_dir_fetch(&lfs, &mdir, (lfs_block_t[2]){0, 1}) => 0;
+    lfs_dir_get(&lfs, &mdir,
+            LFS_MKTAG(0x7ff, 0x3ff, 0),
+            LFS_MKTAG(LFS_TYPE_DIRSTRUCT, 1, sizeof(pair)), pair)
+                => LFS_MKTAG(LFS_TYPE_DIRSTRUCT, 1, sizeof(pair));
+    lfs_pair_fromle32(pair);
+    // change tail-pointer to point to ourself
+    lfs_dir_fetch(&lfs, &mdir, pair) => 0;
+    lfs_dir_commit(&lfs, &mdir, LFS_MKATTRS(
+            {LFS_MKTAG(LFS_TYPE_HARDTAIL, 0x3ff, 8), pair})) => 0;
+    lfs_deinit(&lfs) => 0;
+
+    // test that mount fails gracefully
+    lfs_mount(&lfs, &cfg) => LFS_ERR_CORRUPT;
+'''