[PATCH 3/8] fs: btrfs: fix a crash if specified range is beyond file size

[Qu Wenruo]

[BUG]
When try to read a range beyond file size, btrfs driver will cause
crash/segfault:

 => load host 0 $kernel_addr_r 5k_file 0 0x2000
 SEGFAULT

[CAUSE]
In btrfs_read(), if @len is 0, we will truncated it to file end, but if
file end is beyond our file size, this truncation will underflow @len,
making it -3K in this case.

And later that @len is used to memzero the output buffer, resulting
above crash.

[FIX]
Just error out if @offset is already beyond our file size.

Now it will fail properly with correct error message:

 => load host 0 $kernel_addr_r 5m_origin 0 0x2000
 BTRFS: Read range beyond file size, offset 8192 file size 5120

 Failed to load '5m_origin'

Signed-off-by: Qu Wenruo <wqu at suse.com>
---
 fs/btrfs/btrfs.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/fs/btrfs/btrfs.c b/fs/btrfs/btrfs.c
index 9145727058d4..bf9e1f2f17cf 100644
--- a/fs/btrfs/btrfs.c
+++ b/fs/btrfs/btrfs.c
@@ -252,6 +252,12 @@ int btrfs_read(const char *file, void *buf, loff_t offset, loff_t len,
 		return ret;
 	}
 
+	if (offset >= real_size) {
+		error("Read range beyond file size, offset %llu file size %llu",
+			offset, real_size);
+		return -EINVAL;
+	}
+
 	/*
 	 * 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.
-- 
2.36.1