linux-kernel - [PATCH v3] xfs: Honor FALLOC_FL_KEEP

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Date:   Thu, 30 Mar 2017 21:18:00 -0700
From:   Calvin Owens <calvinowens@...com>
To:     Brian Foster <bfoster@...hat.com>
CC:     "Darrick J. Wong" <darrick.wong@...cle.com>,
        <linux-xfs@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        Christoph Hellwig <hch@....de>,
        Dave Chinner <david@...morbit.com>, <kernel-team@...com>,
        <stable@...r.kernel.org>
Subject: [PATCH v3] xfs: Honor FALLOC_FL_KEEP_SIZE when punching ends of files

When punching past EOF on XFS, fallocate(mode=PUNCH_HOLE|KEEP_SIZE) will
round the file size up to the nearest multiple of PAGE_SIZE:

  calvinow@...disks/generic-xfs-1 ~$ dd if=/dev/urandom of=test bs=2048 count=1
  calvinow@...disks/generic-xfs-1 ~$ stat test
    Size: 2048            Blocks: 8          IO Block: 4096   regular file
  calvinow@...disks/generic-xfs-1 ~$ fallocate -n -l 2048 -o 2048 -p test
  calvinow@...disks/generic-xfs-1 ~$ stat test
    Size: 4096            Blocks: 8          IO Block: 4096   regular file

Commit 3c2bdc912a1cc050 ("xfs: kill xfs_zero_remaining_bytes") replaced
xfs_zero_remaining_bytes() with calls to iomap helpers. The new helpers
don't enforce that [pos,offset) lies strictly on [0,i_size) when being
called from xfs_free_file_space(), so by "leaking" these ranges into
xfs_zero_range() we get this buggy behavior.

Fix this by reintroducing the checks xfs_zero_remaining_bytes() did
against i_size at the bottom of xfs_free_file_space().

Reported-by: Aaron Gao <gzh@...com>
Fixes: 3c2bdc912a1cc050 ("xfs: kill xfs_zero_remaining_bytes")
Cc: Christoph Hellwig <hch@....de>
Cc: Brian Foster <bfoster@...hat.com>
Cc: <stable@...r.kernel.org> # 4.8+
Signed-off-by: Calvin Owens <calvinowens@...com>
---
 fs/xfs/xfs_bmap_util.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/fs/xfs/xfs_bmap_util.c b/fs/xfs/xfs_bmap_util.c
index 8b75dce..828532c 100644
--- a/fs/xfs/xfs_bmap_util.c
+++ b/fs/xfs/xfs_bmap_util.c
@@ -1311,8 +1311,16 @@ xfs_free_file_space(
 	/*
 	 * Now that we've unmap all full blocks we'll have to zero out any
 	 * partial block at the beginning and/or end.  xfs_zero_range is
-	 * smart enough to skip any holes, including those we just created.
+	 * smart enough to skip any holes, including those we just created,
+	 * but we must take care not to zero beyond EOF and enlarge i_size.
 	 */
+
+	if (offset >= XFS_ISIZE(ip))
+		return 0;
+
+	if (offset + len > XFS_ISIZE(ip))
+		len = XFS_ISIZE(ip) - offset;
+
 	return xfs_zero_range(ip, offset, len, NULL);
 }
 
-- 
2.9.3