[PATCH 2/8] fs: btrfs: fix a bug which no data get read if the length is not 0

[Qu Wenruo]

[BUG]
When testing with unaligned read, if a specific length is passed in,
btrfs driver will read out nothing:

 => load host 0 $kernel_addr_r 5k_file 0x1000 0
 0 bytes read in 0 ms

But if no length is passed in, it works fine, even if we pass a non-zero
length:

 => load host 0 $kernel_addr_r 5k_file 0 0x1000
 1024 bytes read in 0 ms

[CAUSE]
In btrfs_read() if we have a larger size than our file, we will try to
truncate it using the file size.

However the real file size is not initialized if @len is not zero, thus
we always truncate our length to 0, and cause the problem.

[FIX]
Fix it by just always do the file size check.

In fact btrfs_size() always follow soft link, thus it will return the
real file size correctly.

Signed-off-by: Qu Wenruo <wqu at suse.com>
---
 fs/btrfs/btrfs.c | 17 +++++++++--------
 1 file changed, 9 insertions(+), 8 deletions(-)

diff --git a/fs/btrfs/btrfs.c b/fs/btrfs/btrfs.c
index 741c6e20f533..9145727058d4 100644
--- a/fs/btrfs/btrfs.c
+++ b/fs/btrfs/btrfs.c
@@ -246,16 +246,17 @@ 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 the length is 0 (meaning read the whole file) or the range is
+	 * beyond file size, truncate it to the end of the file.
+	 */
+	if (!len || len > real_size - offset)
 		len = real_size - offset;
 
 	ret = btrfs_file_read(root, ino, offset, len, buf);
-- 
2.36.1