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xfs: XFS_IS_REALTIME_INODE() should be false if no rt device present

If using a kernel with CONFIG_XFS_RT=y and we set the RHINHERIT flag on
a directory in a filesystem that does not have a realtime device and
create a new file in that directory, it gets marked as a real time file.
When data is written and a fsync is issued, the filesystem attempts to
flush a non-existent rt device during the fsync process.

This results in a crash dereferencing a null buftarg pointer in

  BUG: unable to handle kernel NULL pointer dereference at 0000000000000008
  IP: xfs_blkdev_issue_flush+0xd/0x20
  Call Trace:

Setting RT inode flags does not require special privileges so any
unprivileged user can cause this oops to occur.  To reproduce, confirm
kernel is compiled with CONFIG_XFS_RT=y and run:

  # mkfs.xfs -f /dev/pmem0
  # mount /dev/pmem0 /mnt/test
  # mkdir /mnt/test/foo
  # xfs_io -c 'chattr +t' /mnt/test/foo
  # xfs_io -f -c 'pwrite 0 5m' -c fsync /mnt/test/foo/bar

Or just run xfstests with MKFS_OPTIONS="-d rtinherit=1" and wait.

Kernels built with CONFIG_XFS_RT=n are not exposed to this bug.

Fixes: f538d4d ("[XFS] write barrier support")
Cc: <>
Signed-off-by: Richard Wareing <>
Signed-off-by: Dave Chinner <>
Signed-off-by: Linus Torvalds <>
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RichWpg authored and torvalds committed Sep 12, 2017
1 parent 5226971 commit b31ff3cdf540110da4572e3e29bd172087af65cc
Showing with 8 additions and 1 deletion.
  1. +8 −1 fs/xfs/xfs_linux.h
@@ -270,7 +270,14 @@ static inline uint64_t howmany_64(uint64_t x, uint32_t y)
#endif /* DEBUG */

#define XFS_IS_REALTIME_INODE(ip) ((ip)->i_d.di_flags & XFS_DIFLAG_REALTIME)

* make sure we ignore the inode flag if the filesystem doesn't have a
* configured realtime device.
(((ip)->i_d.di_flags & XFS_DIFLAG_REALTIME) && \
#define XFS_IS_REALTIME_INODE(ip) (0)

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