[PATCH] fs: btrfs: fix reading when length specified

[Sam Edwards <cfsworks@gmail.com>]

The btrfs read function limits the read length to ensure that it
and the read offset do not together exceed the size of the file.
However, this size was only being queried if the read length was
passed a value of zero (meaning "whole file"), and the size is
defaulted to 0 otherwise. This means the clamp will just zero out
the length if one is specified, preventing reading of the file.

Fix this by checking the file size unconditionally, and unifying
the default length and clamping logic as a single range check instead.

This bug was discovered when trying to boot Linux with initrd= via
'bootefi' from a btrfs partition. The EFI stub entered an infinite
loop of zero-length reads while trying to read the initrd, and the
boot process stalled indefinitely.

Signed-off-by: Sam Edwards <CFSworks@gmail.com>
---
 fs/btrfs/btrfs.c | 15 ++++++---------
 1 file changed, 6 insertions(+), 9 deletions(-)

diff --git a/fs/btrfs/btrfs.c b/fs/btrfs/btrfs.c
index 4cdbbbe3d0..1149a3b200 100644
--- a/fs/btrfs/btrfs.c
+++ b/fs/btrfs/btrfs.c
@@ -228,7 +228,7 @@ int btrfs_read(const char *file, void *buf, loff_t offset, loff_t len,
 {
 	struct btrfs_fs_info *fs_info = current_fs_info;
 	struct btrfs_root *root;
-	loff_t real_size = 0;
+	loff_t real_size;
 	u64 ino;
 	u8 type;
 	int ret;
@@ -246,16 +246,13 @@ int btrfs_read(const char *file, void *buf, loff_t offset, loff_t len,
 		return -EINVAL;
 	}
 
-	if (!len) {
-		ret = btrfs_size(file, &real_size);
-		if (ret < 0) {
-			error("Failed to get inode size: %s", file);
-			return ret;
-		}
-		len = real_size;
+	ret = btrfs_size(file, &real_size);
+	if (ret < 0) {
+		error("Failed to get inode size: %s", file);
+		return ret;
 	}
 
-	if (len > real_size - offset)
+	if (!len || len > real_size - offset)
 		len = real_size - offset;
 
 	ret = btrfs_file_read(root, ino, offset, len, buf);
-- 
2.41.0