Add -o readonly option to FUSE driver

doc/dwarfs.md

@@ -64,6 +64,16 @@ options:
     will also consume more memory to hold the hardlink count table.
     This will be 4 bytes for every regular file inode.
 
+  * `-o readonly`
+    Show all file system entries as read-only. By default, DwarFS
+    will preserve the original writeability, which is obviously a
+    lie as it's a read-only file system. However, this is needed
+    for overlays to work correctly, as otherwise directories are
+    seen as read-only by the overlay and it'll be impossible to
+    create new files even in a writeable overlay. If you don't use
+    overlays and want the file system to reflect its read-only
+    state, you can set this option.
+
   * `-o (no_)cache_image`
     By default, `dwarfs` tries to ensure that the compressed file
     system image will not be cached by the kernel (i.e. the default

include/dwarfs/options.h

@@ -38,6 +38,7 @@ struct block_cache_options {
 
 struct metadata_options {
   bool enable_nlink{false};
+  bool readonly{false};
 };
 
 struct filesystem_options {

src/dwarfs.cpp

@@ -53,6 +53,7 @@ struct options {
   const char* mlock_str;            // TODO: const?? -> use string?
   const char* decompress_ratio_str; // TODO: const?? -> use string?
   int enable_nlink;
+  int readonly;
   int cache_image;
   int cache_files;
   size_t cachesize;
@@ -79,6 +80,7 @@ const struct fuse_opt dwarfs_opts[] = {
     DWARFS_OPT("mlock=%s", mlock_str, 0),
     DWARFS_OPT("decratio=%s", decompress_ratio_str, 0),
     DWARFS_OPT("enable_nlink", enable_nlink, 1),
+    DWARFS_OPT("readonly", readonly, 1),
     DWARFS_OPT("cache_image", cache_image, 1),
     DWARFS_OPT("no_cache_image", cache_image, 0),
     DWARFS_OPT("cache_files", cache_files, 1),
@@ -105,6 +107,7 @@ void op_init(void* /*userdata*/, struct fuse_conn_info* /*conn*/) {
     fsopts.block_cache.decompress_ratio = s_opts.decompress_ratio;
     fsopts.block_cache.mm_release = !s_opts.cache_image;
     fsopts.metadata.enable_nlink = bool(s_opts.enable_nlink);
+    fsopts.metadata.readonly = bool(s_opts.readonly);
     s_fs = std::make_shared<filesystem_v2>(
         s_lgr, std::make_shared<mmap>(s_opts.fsimage), fsopts,
         &s_opts.stat_defaults, FUSE_ROOT_ID);
@@ -400,6 +403,7 @@ void usage(const char* progname) {
             << "    -o mlock=NAME          mlock mode: (none), try, must\n"
             << "    -o decratio=NUM        ratio for full decompression (0.8)\n"
             << "    -o enable_nlink        show correct hardlink numbers\n"
+            << "    -o readonly            show read-only file system\n"
             << "    -o (no_)cache_image    (don't) keep image in kernel cache\n"
             << "    -o (no_)cache_files    (don't) keep files in kernel cache\n"
             << "    -o debuglevel=NAME     error, warn, (info), debug, trace\n"

src/dwarfs/metadata_v2.cpp

@@ -95,6 +95,8 @@ void analyze_frozen(std::ostream& os,
   os << '\n';
 }
 
+const uint16_t READ_ONLY_MASK = ~(S_IWUSR | S_IWGRP | S_IWOTH);
+
 } // namespace
 
 template <typename LoggerPolicy>
@@ -637,6 +639,10 @@ int metadata_<LoggerPolicy>::getattr(entry_view entry,
 
   stbuf->st_mode = mode;
 
+  if (options_.readonly) {
+    stbuf->st_mode &= READ_ONLY_MASK;
+  }
+
   stbuf->st_size = S_ISDIR(mode) ? make_directory_view(entry).entry_count()
                                  : file_size(entry, mode);
   stbuf->st_ino = inode + inode_offset_;