Improve ABI backward & forward compatibillity

 - Padd the compressor config union
    - 128 bytes aught to be enough for everyone, i.e. future compressors.
    - Insist that the padding space is initialized to 0. If a field gets
      added to an existing compressor, it can test for 0 as a sentinel
      value.
 - Add a size field to the hook structure, aka "the Microsoft way".
    - The explanation is in the comment.
    - Don't make the Microsoft mistake of checking for >=, insist on *exact*
      size match. Future users will need a fallback if their hooks are
      rejected. But at least they will be rejected instead of silently not
      being used.
 - Add an unsupported flag check to the dir tree reader.
 - Add a basic abi unit test that, for now, checks the size of the compressor
   config struct fields.

Signed-off-by: David Oberhollenzer <david.oberhollenzer@sigma-star.at>

4 files changed, 40 insertions, 2 deletions

diff --git a/lib/sqfs/comp/compressor.c b/lib/sqfs/comp/compressor.c
index bfc20b7..902ce92 100644
--- a/lib/sqfs/comp/compressor.c
+++ b/lib/sqfs/comp/compressor.c
@@ -87,12 +87,42 @@ bool sqfs_compressor_exists(E_SQFS_COMPRESSOR id)
 
 sqfs_compressor_t *sqfs_compressor_create(const sqfs_compressor_config_t *cfg)
 {
+	sqfs_u8 padd0[sizeof(cfg->opt)];
+	int ret;
+
 	if (cfg == NULL || cfg->id < SQFS_COMP_MIN || cfg->id > SQFS_COMP_MAX)
 		return NULL;
 
 	if (compressors[cfg->id] == NULL)
 		return NULL;
 
+	memset(padd0, 0, sizeof(padd0));
+
+	switch (cfg->id) {
+	case SQFS_COMP_XZ:
+		ret = memcmp(cfg->opt.xz.padd0, padd0,
+			     sizeof(cfg->opt.xz.padd0));
+		break;
+	case SQFS_COMP_LZO:
+		ret = memcmp(cfg->opt.lzo.padd0, padd0,
+			     sizeof(cfg->opt.lzo.padd0));
+		break;
+	case SQFS_COMP_ZSTD:
+		ret = memcmp(cfg->opt.zstd.padd0, padd0,
+			     sizeof(cfg->opt.zstd.padd0));
+		break;
+	case SQFS_COMP_GZIP:
+		ret = memcmp(cfg->opt.gzip.padd0, padd0,
+			     sizeof(cfg->opt.gzip.padd0));
+		break;
+	default:
+		ret = memcmp(cfg->opt.padd0, padd0, sizeof(cfg->opt.padd0));
+		break;
+	}
+
+	if (ret != 0)
+		return NULL;
+
 	return compressors[cfg->id](cfg);
 }
 
diff --git a/lib/sqfs/data_writer/common.c b/lib/sqfs/data_writer/common.c
index 059ef41..a19eb6d 100644
--- a/lib/sqfs/data_writer/common.c
+++ b/lib/sqfs/data_writer/common.c
@@ -157,9 +157,13 @@ int sqfs_data_writer_write_fragment_table(sqfs_data_writer_t *proc,
 	return 0;
 }
 
-void sqfs_data_writer_set_hooks(sqfs_data_writer_t *proc, void *user_ptr,
-				const sqfs_block_hooks_t *hooks)
+int sqfs_data_writer_set_hooks(sqfs_data_writer_t *proc, void *user_ptr,
+			       const sqfs_block_hooks_t *hooks)
 {
+	if (hooks->size != sizeof(*hooks))
+		return SQFS_ERROR_UNSUPPORTED;
+
 	proc->hooks = hooks;
 	proc->user_ptr = user_ptr;
+	return 0;
 }
diff --git a/lib/sqfs/read_tree.c b/lib/sqfs/read_tree.c
index 46c7ec0..3a43769 100644
--- a/lib/sqfs/read_tree.c
+++ b/lib/sqfs/read_tree.c
@@ -199,6 +199,9 @@ int sqfs_dir_reader_get_full_hierarchy(sqfs_dir_reader_t *rd,
 	const char *ptr;
 	int ret;
 
+	if (flags & ~SQFS_TREE_ALL_FLAGS)
+		return SQFS_ERROR_UNSUPPORTED;
+
 	ret = sqfs_dir_reader_get_root_inode(rd, &inode);
 	if (ret)
 		return ret;
diff --git a/lib/sqfshelper/statistics.c b/lib/sqfshelper/statistics.c
index 3f9f595..fb70615 100644
--- a/lib/sqfshelper/statistics.c
+++ b/lib/sqfshelper/statistics.c
@@ -55,6 +55,7 @@ static void notify_fragment_discard(void *user, const sqfs_block_t *block)
 }
 
 static const sqfs_block_hooks_t hooks = {
+	.size = sizeof(hooks),
 	.post_block_write = post_block_write,
 	.pre_fragment_store = pre_fragment_store,
 	.notify_blocks_erased = notify_blocks_erased,