cbfs: add cbfs_read()

Allow for reading from cbfs media without having a handle
to a non-CBFS_DEFAULT_MEDIA cbfs_media. In conjunction with
cbfs_locate_file() one can locate and cbfs_read() a file
without bringing the entire file through a potentially
temporary buffer (non-memory-mappable cbfs media platforms).

BUG=chrome-os-partner:29922
BRANCH=None
TEST=Built.

Change-Id: Ib5d965334bce1267650fc23c9e9f496675cf8450
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/205991
Reviewed-by: Furquan Shaikh <furquan@chromium.org>

src/include/cbfs_core.h

@@ -233,6 +233,10 @@ struct cbfs_media {
 ssize_t cbfs_locate_file(struct cbfs_media *media, struct cbfs_file *file,
 				const char *name);
 
+/* Read count bytes at offset into dest. Return number of bytes read. */
+size_t cbfs_read(struct cbfs_media *media, void *dest, size_t offset,
+			size_t count);
+
 /* returns pointer to a file entry inside CBFS or NULL */
 struct cbfs_file *cbfs_get_file(struct cbfs_media *media, const char *name);
 

src/lib/cbfs_core.c

@@ -192,6 +192,22 @@ ssize_t cbfs_locate_file(struct cbfs_media *media, struct cbfs_file *file,
 	return -1;
 }
 
+size_t cbfs_read(struct cbfs_media *media, void *dest, size_t offset,
+			size_t count)
+{
+	struct cbfs_media default_media;
+	size_t nread;
+
+	if (init_media(&media, &default_media))
+		return 0;
+
+	media->open(media);
+	nread = media->read(media, dest, offset, count);
+	media->close(media);
+
+	return nread;
+}
+
 struct cbfs_file *cbfs_get_file(struct cbfs_media *media, const char *name)
 {
 	struct cbfs_media default_media;