+/* Can the specified WIM resource be used as the source of an external backing
+ * for the wof.sys WIM provider? */
+static bool
+is_resource_valid_for_external_backing(const struct wim_resource_descriptor *rdesc,
+ struct win32_apply_ctx *ctx)
+{
+ /* Must be the original WIM file format. This check excludes pipable
+ * resources and solid resources. It also excludes other resources
+ * contained in such files even if they would be otherwise compatible.
+ */
+ if (rdesc->wim->hdr.magic != WIM_MAGIC ||
+ rdesc->wim->hdr.wim_version != WIM_VERSION_DEFAULT)
+ {
+ ctx->wimboot.have_wrong_version_wims = true;
+ return false;
+ }
+
+ /*
+ * Whitelist of compression types and chunk sizes supported by
+ * Microsoft's WOF driver.
+ *
+ * Notes:
+ * - Uncompressed WIMs result in BSOD. However, this only applies to
+ * the WIM file itself, not to uncompressed resources in a WIM file
+ * that is otherwise compressed.
+ * - XPRESS 64K sometimes appears to work, but sometimes it causes
+ * reads to fail with STATUS_UNSUCCESSFUL.
+ */
+ switch (rdesc->compression_type) {
+ case WIMLIB_COMPRESSION_TYPE_NONE:
+ if (rdesc->wim->compression_type == WIMLIB_COMPRESSION_TYPE_NONE) {
+ ctx->wimboot.have_uncompressed_wims = true;
+ return false;
+ }
+ break;
+ case WIMLIB_COMPRESSION_TYPE_XPRESS:
+ switch (rdesc->chunk_size) {
+ case 4096:
+ case 8192:
+ case 16384:
+ case 32768:
+ break;
+ default:
+ ctx->wimboot.have_unsupported_compressed_resources = true;
+ return false;
+ }
+ break;
+ case WIMLIB_COMPRESSION_TYPE_LZX:
+ switch (rdesc->chunk_size) {
+ case 32768:
+ break;
+ default:
+ ctx->wimboot.have_unsupported_compressed_resources = true;
+ return false;
+ }
+ break;
+ default:
+ ctx->wimboot.have_unsupported_compressed_resources = true;
+ return false;
+ }
+
+ /* Microsoft's WoF driver errors out if it tries to satisfy a read with
+ * ending offset >= 4 GiB from an externally backed file. */
+ if (rdesc->uncompressed_size > 4200000000) {
+ ctx->wimboot.have_huge_resources = true;
+ return false;
+ }
+
+ return true;
+}
+
+#define EXTERNAL_BACKING_NOT_ENABLED -1
+#define EXTERNAL_BACKING_NOT_POSSIBLE -2
+#define EXTERNAL_BACKING_EXCLUDED -3
+
+/*
+ * Determines whether the specified file will be externally backed. Returns a
+ * negative status code if no, 0 if yes, or a positive wimlib error code on
+ * error. If the file is excluded from external backing based on its path, then
+ * *excluded_dentry_ret is set to the dentry for the path that matched the
+ * exclusion rule.
+ *
+ * Note that this logic applies to both types of "external backing":
+ *
+ * - WIM backing ("WIMBoot" - Windows 8.1 and later)
+ * - File backing ("System Compression" - Windows 10 and later)
+ *
+ * However, in the case of WIM backing we also need to validate that the WIM
+ * resource that would be the source of the backing is supported by the wof.sys
+ * WIM provider.
+ */
+static int
+will_externally_back_inode(struct wim_inode *inode, struct win32_apply_ctx *ctx,
+ const struct wim_dentry **excluded_dentry_ret,
+ bool wimboot_mode)
+{
+ struct wim_dentry *dentry;
+ struct blob_descriptor *blob;
+ int ret;
+
+ if (load_prepopulate_pats(ctx) == WIMLIB_ERR_NOMEM)
+ return WIMLIB_ERR_NOMEM;
+
+ if (inode->i_can_externally_back)
+ return 0;
+
+ /* This may do redundant checks because the cached value
+ * i_can_externally_back is 2-state (as opposed to 3-state:
+ * unknown/no/yes). But most files can be externally backed, so this
+ * way is fine. */
+
+ if (inode->i_attributes & (FILE_ATTRIBUTE_DIRECTORY |
+ FILE_ATTRIBUTE_REPARSE_POINT |
+ FILE_ATTRIBUTE_ENCRYPTED))
+ return EXTERNAL_BACKING_NOT_POSSIBLE;
+
+ blob = inode_get_blob_for_unnamed_data_stream_resolved(inode);
+
+ if (!blob)
+ return EXTERNAL_BACKING_NOT_POSSIBLE;
+
+ if (wimboot_mode &&
+ (blob->blob_location != BLOB_IN_WIM ||
+ !is_resource_valid_for_external_backing(blob->rdesc, ctx)))
+ return EXTERNAL_BACKING_NOT_POSSIBLE;
+
+ /*
+ * We need to check the patterns in [PrepopulateList] against every name
+ * of the inode, in case any of them match.
+ */
+
+ inode_for_each_extraction_alias(dentry, inode) {
+
+ ret = calculate_dentry_full_path(dentry);
+ if (ret)
+ return ret;
+
+ if (!can_externally_back_path(dentry->d_full_path, ctx)) {
+ if (excluded_dentry_ret)
+ *excluded_dentry_ret = dentry;
+ return EXTERNAL_BACKING_EXCLUDED;
+ }
+ }
+
+ inode->i_can_externally_back = 1;
+ return 0;
+}
+
+/*
+ * Determines if the unnamed data stream of a file will be created as a WIM
+ * external backing (a "WIMBoot pointer file"), as opposed to a standard
+ * extraction.
+ */