Re: [PATCH 1/3] ext3/ext4: Factor out disk addressability check

[Joel Becker <Joel.Becker@oracle.com>]

On Mon, Aug 16, 2010 at 09:44:35AM -0500, Eric Sandeen wrote:
> Joel Becker wrote:
> > How about:
> > 
> > 	u64 last_fs_page = last_fs_block >> (PAGE_CACHE_SHIFT - blocksize_bits);
> > 
> > 	... ||
> > 	(last_fs_page > (pgoff_t)(~0ULL))) {
> > 
> > Is that more readable?
> 
> To me, yes.  Maybe do similar for last_fs_sector.

	last_fs_sector would be shifting up, which could wrap a really
large last_fs_blocks.  So I'm going to keep the sector_t check as-is.
How's this:

>From 8de5cb9164cdc179ba84a07b282a895d0eb794b0 Mon Sep 17 00:00:00 2001
>From: Joel Becker <joel.becker@oracle.com>
Date: Mon, 16 Aug 2010 12:10:17 -0700
Subject: [PATCH] libfs: Fix shift bug in generic_check_addressable()

generic_check_addressable() erroneously shifts pages down by a block
factor when it should be shifting up.  To prevent overflow, we shift
blocks down to pages.

Signed-off-by: Joel Becker <joel.becker@oracle.com>
---
 fs/libfs.c |    8 ++++----
 1 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/fs/libfs.c b/fs/libfs.c
index 8debe7b..62baa03 100644
--- a/fs/libfs.c
+++ b/fs/libfs.c
@@ -925,6 +925,8 @@ EXPORT_SYMBOL(generic_file_fsync);
 int generic_check_addressable(unsigned blocksize_bits, u64 num_blocks)
 {
 	u64 last_fs_block = num_blocks - 1;
+	u64 last_fs_page =
+		last_fs_block >> (PAGE_CACHE_SHIFT - blocksize_bits);
 
 	if (unlikely(num_blocks == 0))
 		return 0;
@@ -932,10 +934,8 @@ int generic_check_addressable(unsigned blocksize_bits, u64 num_blocks)
 	if ((blocksize_bits < 9) || (blocksize_bits > PAGE_CACHE_SHIFT))
 		return -EINVAL;
 
-	if ((last_fs_block >
-	     (sector_t)(~0ULL) >> (blocksize_bits - 9)) ||
-	    (last_fs_block >
-	     (pgoff_t)(~0ULL) >> (PAGE_CACHE_SHIFT - blocksize_bits))) {
+	if ((last_fs_block > (sector_t)(~0ULL) >> (blocksize_bits - 9)) ||
+	    (last_fs_page > (pgoff_t)(~0ULL))) {
 		return -EFBIG;
 	}
 	return 0;
-- 
1.7.1

-- 

Life's Little Instruction Book #267

	"Lie on your back and look at the stars."

Joel Becker
Consulting Software Developer
Oracle
E-mail: joel.becker@oracle.com
Phone: (650) 506-8127