Commit 932befe3 authored by Darrick J. Wong's avatar Darrick J. Wong
Browse files

xfs: fix s_maxbytes computation on 32-bit kernels 



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 0cc3b0ec ("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.

Fix all this by setting s_maxbytes to MAX_LFS_FILESIZE directly and
aborting the mount with a warning if our assumptions ever break.  I have
no answer for why this seems to have been broken for years and nobody
noticed.

Signed-off-by: default avatarDarrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: default avatarChristoph Hellwig <hch@lst.de>
parent 4bbb04ab

fs/xfs/xfs_super.c

+21 −27
Original line number Diff line number Diff line
@@ -193,32 +193,6 @@ xfs_fs_show_options( 
	return 0;

}



static uint64_t

xfs_max_file_offset(

	unsigned int		blockshift)

{

	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).

	 */



#if BITS_PER_LONG == 32

	ASSERT(sizeof(sector_t) == 8);

	pagefactor = PAGE_SIZE;

	bitshift = BITS_PER_LONG;

#endif



	return (((uint64_t)pagefactor) << bitshift) - 1;

}



/*

 * Set parameters for inode allocation heuristics, taking into account

 * filesystem size and inode32/inode64 mount options; i.e. specifically
@@ -1424,6 +1398,26 @@ xfs_fc_fill_super( 
	if (error)

		goto out_free_sb;



	/*

	 * 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.

	 *

	 * Avoid integer overflow by comparing the maximum bmbt offset to the

	 * maximum pagecache offset in units of fs blocks.

	 */

	if (XFS_B_TO_FSBT(mp, MAX_LFS_FILESIZE) > XFS_MAX_FILEOFF) {

		xfs_warn(mp,

"MAX_LFS_FILESIZE block offset (%llu) exceeds extent map maximum (%llu)!",

			 XFS_B_TO_FSBT(mp, MAX_LFS_FILESIZE),

			 XFS_MAX_FILEOFF);

		error = -EINVAL;

		goto out_free_sb;

	}



	error = xfs_filestream_mount(mp);

	if (error)

		goto out_free_sb;
@@ -1435,7 +1429,7 @@ 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 = MAX_LFS_FILESIZE;

	sb->s_max_links = XFS_MAXLINK;

	sb->s_time_gran = 1;

	sb->s_time_min = S32_MIN;

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