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[PATCH] xfs: fix s_max_bytes to MAX_LFS_FILESIZE if needed

To: "xfs@xxxxxxxxxxx" <xfs@xxxxxxxxxxx>
Subject: [PATCH] xfs: fix s_max_bytes to MAX_LFS_FILESIZE if needed
From: Jeff Liu <jeff.liu@xxxxxxxxxx>
Date: Fri, 12 Apr 2013 18:26:40 +0800
Cc: "Michael L. Semon" <mlsemon35@xxxxxxxxx>
Delivered-to: xfs@xxxxxxxxxxx
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:16.0) Gecko/20121028 Thunderbird/16.0.2
From: Jie Liu <jeff.liu@xxxxxxxxxx>

On 32-bit machine, the s_maxbytes is larger than the MAX_LFS_FILESIZE limits if 
CONFIG_LBDAF is
not enabled.  Hence it's possible to create a huge file via buffered-IO write 
with a given offset
beyond this limitation. e.g.

# block_size=4096
# offset=$(((2**32 - 1) * $block_size))
# xfs_io -f -c "pwrite $offset $block_size" /storage/test_file

In this case, xfs_io will hang at the page writeback stage soon since the given 
offset would
cause an overflow at xfs_vm_writepage():

end_index = offset >> PAGE_CACHE_SHIFT;
last_index = (offset - 1) >> PAGE_CACHE_SHIFT;
if (page->index >= end_index) {
                unsigned offset_into_page = offset & (PAGE_CACHE_SIZE - 1);

                /*
                 * Just skip the page if it is fully outside i_size, e.g. due
                 * to a truncate operation that is in progress.
                 */
                if (page->index >= end_index + 1 || offset_into_page == 0) {
                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
                        unlock_page(page);
                        return 0;
                }
end_index is unsigned long so that the max value is '2^32-1 = 4294967295', and 
it
would be evaluated to the max value with the given offset(when writing the page 
offset
up to s_max_bytes) for above test case.  As a result, (page->index >= end_index 
+ 1) is
ok as (end_index + 1) is overflowed to ZERO.

Actually, create a file as above on 32-bit machine should be failed with EFBIG 
error returned
because there has strict check up at generic_write_checks() against the given 
offset with a
*correct* s_max_bytes.

This patch fix the s_max_bytes to MAX_LFS_FILESIZE if the pre-calculated value 
is greater
than it.

Reported-by: Michael L. Semon <mlsemon35@xxxxxxxxx>
Signed-off-by: Jie Liu <jeff.liu@xxxxxxxxxx>

---
 fs/xfs/xfs_super.c |    6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/fs/xfs/xfs_super.c b/fs/xfs/xfs_super.c
index ea341ce..0644d61 100644
--- a/fs/xfs/xfs_super.c
+++ b/fs/xfs/xfs_super.c
@@ -585,6 +585,7 @@ xfs_max_file_offset(
 {
        unsigned int            pagefactor = 1;
        unsigned int            bitshift = BITS_PER_LONG - 1;
+       __uint64_t              offset;
 
        /* Figure out maximum filesize, on Linux this can depend on
         * the filesystem blocksize (on 32 bit platforms).
@@ -610,7 +611,10 @@ xfs_max_file_offset(
 # endif
 #endif
 
-       return (((__uint64_t)pagefactor) << bitshift) - 1;
+       offset = (((__uint64_t)pagefactor) << bitshift) - 1;
+
+       /* Check against VM & VFS exposed limits */
+       return (offset > MAX_LFS_FILESIZE) ? MAX_LFS_FILESIZE : offset;
 }
 
 xfs_agnumber_t
-- 
1.7.9.5

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