Add config flag to disable block count check on mount

When the block count check was added to `lfs_mount` in [#584][]
it was mentionned to have a mount flag to disable this check in the future.

Therefore, this PR:

- Adds a new `flags` field to lfs_config
- Defines a new `lfs_fs_flags` enum that contains these flags
- Defines one flag `LFS_CFG_DISABLE_BLOCK_COUNT_CHECK` that disables
  the check inserted in [#584][]

[#584]: #584

Author: sosthene-nitrokey

lfs.c

@@ -4592,15 +4592,21 @@ static int lfs_mount_(lfs_t *lfs, const struct lfs_config *cfg) {
             }
 
             // this is where we get the block_count from disk if block_count=0
-            if (lfs->cfg->block_count
+
+            if ((lfs->cfg->flags & LFS_CFG_DISABLE_BLOCK_COUNT_CHECK) == 0
+                    && lfs->cfg->block_count
                     && superblock.block_count != lfs->cfg->block_count) {
                 LFS_ERROR("Invalid block count (%"PRIu32" != %"PRIu32")",
                         superblock.block_count, lfs->cfg->block_count);
                 err = LFS_ERR_INVAL;
                 goto cleanup;
             }
 
-            lfs->block_count = superblock.block_count;
+            if (lfs->cfg->block_count) {
+                lfs->block_count = lfs->cfg->block_count;
+            } else {
+                lfs->block_count = superblock.block_count;
+            }
 
             if (superblock.block_size != lfs->cfg->block_size) {
                 LFS_ERROR("Invalid block size (%"PRIu32" != %"PRIu32")",

lfs.h

@@ -283,6 +283,11 @@ struct lfs_config {
     // Set to -1 to disable inlined files.
     lfs_size_t inline_max;
 
+    // Configuration flags for the filesystem
+    //
+    // See variants of lfs_fs_flags
+    uint32_t flags;
+
 #ifdef LFS_MULTIVERSION
     // On-disk version to use when writing in the form of 16-bit major version
     // + 16-bit minor version. This limiting metadata to what is supported by
@@ -292,6 +297,10 @@ struct lfs_config {
 #endif
 };
 
+enum lfs_fs_flags {
+    LFS_CFG_DISABLE_BLOCK_COUNT_CHECK = 1, // Allow mounting a filesystem with a different block count in the config and the superblock
+};
+
 // File info structure
 struct lfs_info {
     // Type of the file, either LFS_TYPE_REG or LFS_TYPE_DIR

tests/test_superblocks.toml

@@ -465,7 +465,13 @@ code = '''
     lfs_unmount(&lfs) => 0;
 
     if (KNOWN_BLOCK_COUNT) {
+        cfg->block_count = BLOCK_COUNT_2 + 1;
+        lfs_mount(&lfs, cfg) => LFS_ERR_INVAL;
+        cfg->flags |= LFS_CFG_DISABLE_BLOCK_COUNT_CHECK;
+        lfs_mount(&lfs, cfg) => 0;
+        lfs_unmount(&lfs) => 0;
         cfg->block_count = BLOCK_COUNT_2;
+        cfg->flags &= ~LFS_CFG_DISABLE_BLOCK_COUNT_CHECK;
     } else {
         cfg->block_count = 0;
     }