diff --git a/fs/fs.c b/fs/fs.c
index be1855d..9c2ef6b 100644
--- a/fs/fs.c
+++ b/fs/fs.c
@@ -75,6 +75,7 @@ static struct fstype_info fstypes[] = {
 		.close = fat_close,
 		.ls = file_fat_ls,
 		.read = fat_read_file,
+		.write = fs_write_unsupported,
 	},
 #endif
 #ifdef CONFIG_FS_EXT4
@@ -84,6 +85,7 @@ static struct fstype_info fstypes[] = {
 		.close = ext4fs_close,
 		.ls = ext4fs_ls,
 		.read = ext4_read_file,
+		.write = fs_write_unsupported,
 	},
 #endif
 #ifdef CONFIG_SANDBOX
@@ -212,16 +214,11 @@ int fs_write(const char *filename, ulong addr, int offset, int len)
 	void *buf;
 	int ret;
 
-	/*
-	 * We don't actually know how many bytes are being read, since len==0
-	 * means read the whole file.
-	 */
 	buf = map_sysmem(addr, len);
 	ret = info->write(filename, buf, offset, len);
 	unmap_sysmem(buf);
 
-	/* If we requested a specific number of bytes, check we got it */
-	if (ret >= 0 && len && ret != len) {
+	if (ret >= 0 && ret != len) {
 		printf("** Unable to write file %s **\n", filename);
 		ret = -1;
 	}
diff --git a/include/fs.h b/include/fs.h
index 7d9403e..97b0094 100644
--- a/include/fs.h
+++ b/include/fs.h
@@ -55,6 +55,16 @@ int fs_ls(const char *dirname);
 int fs_read(const char *filename, ulong addr, int offset, int len);
 
 /*
+ * Write file "filename" to the partition previously set by fs_set_blk_dev(),
+ * from address "addr", starting at byte offset "offset", and writing "len"
+ * bytes. "offset" may be 0 to write to the start of the file. Note that not
+ * all filesystem types support offset!=0.
+ *
+ * Returns number of bytes read on success. Returns <= 0 on error.
+ */
+int fs_write(const char *filename, ulong addr, int offset, int len);
+
+/*
  * Common implementation for various filesystem commands, optionally limited
  * to a specific filesystem type via the fstype parameter.
  */