Commit c95356ca authored by Darrick J. Wong's avatar Darrick J. Wong Committed by Dave Chinner
Browse files

xfs: _{attr,data}_map_shared should take ILOCK_EXCL until iread_extents is completely done 



While fuzzing the data fork extent count on a btree-format directory
with xfs/375, I observed the following (excerpted) splat:

XFS: Assertion failed: xfs_isilocked(ip, XFS_ILOCK_EXCL), file: fs/xfs/libxfs/xfs_bmap.c, line: 1208
------------[ cut here ]------------
WARNING: CPU: 0 PID: 43192 at fs/xfs/xfs_message.c:104 assfail+0x46/0x4a [xfs]
Call Trace:
 <TASK>
 xfs_iread_extents+0x1af/0x210 [xfs 09f66509ece4938760fac7de64732a0cbd3e39cd]
 xchk_dir_walk+0xb8/0x190 [xfs 09f66509ece4938760fac7de64732a0cbd3e39cd]
 xchk_parent_count_parent_dentries+0x41/0x80 [xfs 09f66509ece4938760fac7de64732a0cbd3e39cd]
 xchk_parent_validate+0x199/0x2e0 [xfs 09f66509ece4938760fac7de64732a0cbd3e39cd]
 xchk_parent+0xdf/0x130 [xfs 09f66509ece4938760fac7de64732a0cbd3e39cd]
 xfs_scrub_metadata+0x2b8/0x730 [xfs 09f66509ece4938760fac7de64732a0cbd3e39cd]
 xfs_scrubv_metadata+0x38b/0x4d0 [xfs 09f66509ece4938760fac7de64732a0cbd3e39cd]
 xfs_ioc_scrubv_metadata+0x111/0x160 [xfs 09f66509ece4938760fac7de64732a0cbd3e39cd]
 xfs_file_ioctl+0x367/0xf50 [xfs 09f66509ece4938760fac7de64732a0cbd3e39cd]
 __x64_sys_ioctl+0x82/0xa0
 do_syscall_64+0x2b/0x80
 entry_SYSCALL_64_after_hwframe+0x46/0xb0

The cause of this is a race condition in xfs_ilock_data_map_shared,
which performs an unlocked access to the data fork to guess which lock
mode it needs:

Thread 0                          Thread 1

xfs_need_iread_extents
<observe no iext tree>
xfs_ilock(..., ILOCK_EXCL)
xfs_iread_extents
<observe no iext tree>
<check ILOCK_EXCL>
<load bmbt extents into iext>
<notice iext size doesn't
 match nextents>
                                  xfs_need_iread_extents
                                  <observe iext tree>
                                  xfs_ilock(..., ILOCK_SHARED)
<tear down iext tree>
xfs_iunlock(..., ILOCK_EXCL)
                                  xfs_iread_extents
                                  <observe no iext tree>
                                  <check ILOCK_EXCL>
                                  *BOOM*

Fix this race by adding a flag to the xfs_ifork structure to indicate
that we have not yet read in the extent records and changing the
predicate to look at the flag state, not if_height.  The memory barrier
ensures that the flag will not be set until the very end of the
function.

Signed-off-by: default avatarDarrick J. Wong <djwong@kernel.org>
Reviewed-by: default avatarDave Chinner <dchinner@redhat.com>
Signed-off-by: default avatarDave Chinner <david@fromorbit.com>
parent 4b827b3f

fs/xfs/libxfs/xfs_bmap.c

+6 −0
Original line number Diff line number Diff line
@@ -1171,6 +1171,12 @@ xfs_iread_extents( 
		goto out;

	}

	ASSERT(ir.loaded == xfs_iext_count(ifp));

	/*

	 * Use release semantics so that we can use acquire semantics in

	 * xfs_need_iread_extents and be guaranteed to see a valid mapping tree

	 * after that load.

	 */

	smp_store_release(&ifp->if_needextents, 0);

	return 0;

out:

	xfs_iext_destroy(ifp);

fs/xfs/libxfs/xfs_inode_fork.c

+15 −1
Original line number Diff line number Diff line
@@ -226,10 +226,15 @@ xfs_iformat_data_fork( 


	/*

	 * Initialize the extent count early, as the per-format routines may

	 * depend on it.

	 * depend on it.  Use release semantics to set needextents /after/ we

	 * set the format. This ensures that we can use acquire semantics on

	 * needextents in xfs_need_iread_extents() and be guaranteed to see a

	 * valid format value after that load.

	 */

	ip->i_df.if_format = dip->di_format;

	ip->i_df.if_nextents = xfs_dfork_data_extents(dip);

	smp_store_release(&ip->i_df.if_needextents,

			   ip->i_df.if_format == XFS_DINODE_FMT_BTREE ? 1 : 0);



	switch (inode->i_mode & S_IFMT) {

	case S_IFIFO:
@@ -282,8 +287,17 @@ xfs_ifork_init_attr( 
	enum xfs_dinode_fmt	format,

	xfs_extnum_t		nextents)

{

	/*

	 * Initialize the extent count early, as the per-format routines may

	 * depend on it.  Use release semantics to set needextents /after/ we

	 * set the format. This ensures that we can use acquire semantics on

	 * needextents in xfs_need_iread_extents() and be guaranteed to see a

	 * valid format value after that load.

	 */

	ip->i_af.if_format = format;

	ip->i_af.if_nextents = nextents;

	smp_store_release(&ip->i_af.if_needextents,

			   ip->i_af.if_format == XFS_DINODE_FMT_BTREE ? 1 : 0);

}



void

fs/xfs/libxfs/xfs_inode_fork.h

+4 −2
Original line number Diff line number Diff line
@@ -24,6 +24,7 @@ struct xfs_ifork { 
	xfs_extnum_t		if_nextents;	/* # of extents in this fork */

	short			if_broot_bytes;	/* bytes allocated for root */

	int8_t			if_format;	/* format of this fork */

	uint8_t			if_needextents;	/* extents have not been read */

};



/*
@@ -260,9 +261,10 @@ int xfs_iext_count_upgrade(struct xfs_trans *tp, struct xfs_inode *ip, 
		uint nr_to_add);



/* returns true if the fork has extents but they are not read in yet. */

static inline bool xfs_need_iread_extents(struct xfs_ifork *ifp)

static inline bool xfs_need_iread_extents(const struct xfs_ifork *ifp)

{

	return ifp->if_format == XFS_DINODE_FMT_BTREE && ifp->if_height == 0;

	/* see xfs_iformat_{data,attr}_fork() for needextents semantics */

	return smp_load_acquire(&ifp->if_needextents) != 0;

}



#endif	/* __XFS_INODE_FORK_H__ */