[PATCH] xfs: fix s_maxbytes computation on 32-bit kernels

Darrick J. Wong
2019-12-22T16:37:16Z

From: Darrick J. Wong <darrick.wong@oracle.com>

I observed a hang in generic/308 while running fstests on a i686 kernel.
The hang occurred when trying to purge the pagecache on a large sparse
file that had a page created past MAX_LFS_FILESIZE, which caused an
integer overflow in the pagecache xarray and resulted in an infinite
loop.

I then noticed that Linus changed the definition of MAX_LFS_FILESIZE in
commit 0cc3b0ec23ce ("Clarify (and fix) MAX_LFS_FILESIZE macros") so
that it is now one page short of the maximum page index on 32-bit
kernels. Because the XFS function to compute max offset open-codes the
2005-era MAX_LFS_FILESIZE computation and neither the vfs nor mm perform
any sanity checking of s_maxbytes, the code in generic/308 can create a
page above the pagecache's limit and kaboom.

So, fix the function to return MAX_LFS_FILESIZE, but check that bmbt
record offsets have enough space to handle that many bytes. I have no
answer for why this seems to have been broken for years and nobody
noticed.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
---
fs/xfs/xfs_super.c | 37 ++++++++++++++++---------------------
1 file changed, 16 insertions(+), 21 deletions(-)

diff --git a/fs/xfs/xfs_super.c b/fs/xfs/xfs_super.c
index d9ae27ddf253..30a17e5ffa67 100644
--- a/fs/xfs/xfs_super.c
+++ b/fs/xfs/xfs_super.c
@@ -193,30 +193,25 @@ xfs_fs_show_options(
return 0;
}

-static uint64_t
+static loff_t
xfs_max_file_offset(
- unsigned int blockshift)
+ struct xfs_mount *mp)
{
- unsigned int pagefactor = 1;
- unsigned int bitshift = BITS_PER_LONG - 1;
-
- /* Figure out maximum filesize, on Linux this can depend on
- * the filesystem blocksize (on 32 bit platforms).
- * __block_write_begin does this in an [unsigned] long long...
- * page->index << (PAGE_SHIFT - bbits)
- * So, for page sized blocks (4K on 32 bit platforms),
- * this wraps at around 8Tb (hence MAX_LFS_FILESIZE which is
- * (((u64)PAGE_SIZE << (BITS_PER_LONG-1))-1)
- * but for smaller blocksizes it is less (bbits = log2 bsize).
+ /*
+ * XFS block mappings use 54 bits to store the logical block offset.
+ * This should suffice to handle the maximum file size that the VFS
+ * supports (currently 2^63 bytes on 64-bit and ULONG_MAX << PAGE_SHIFT
+ * bytes on 32-bit), but as XFS and VFS have gotten the s_maxbytes
+ * calculation wrong on 32-bit kernels in the past, we'll add a WARN_ON
+ * to check this assertion before returning MAX_LFS_FILESIZE.
+ *
+ * Avoid integer overflow by comparing the maximum bmbt offset to the
+ * maximum pagecache offset in units of fs blocks.
*/
+ WARN_ON(((1ULL << BMBT_STARTOFF_BITLEN) - 1) <
+ XFS_B_TO_FSBT(mp, MAX_LFS_FILESIZE));

-#if BITS_PER_LONG == 32
- ASSERT(sizeof(sector_t) == 8);
- pagefactor = PAGE_SIZE;
- bitshift = BITS_PER_LONG;
-#endif
-
- return (((uint64_t)pagefactor) << bitshift) - 1;
+ return MAX_LFS_FILESIZE;
}

/*
@@ -1435,7 +1430,7 @@ xfs_fc_fill_super(
sb->s_magic = XFS_SUPER_MAGIC;
sb->s_blocksize = mp->m_sb.sb_blocksize;
sb->s_blocksize_bits = ffs(sb->s_blocksize) - 1;
- sb->s_maxbytes = xfs_max_file_offset(sb->s_blocksize_bits);
+ sb->s_maxbytes = xfs_max_file_offset(mp);
sb->s_max_links = XFS_MAXLINK;
sb->s_time_gran = 1;
sb->s_time_min = S32_MIN;