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

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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: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>
This commit is contained in:
Darrick J. Wong 2023-04-12 15:49:10 +10:00 committed by Dave Chinner
parent 4b827b3f30
commit c95356ca88
3 changed files with 25 additions and 3 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

6
fs/xfs/libxfs/xfs_bmap.c
View File

@ -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);

16
fs/xfs/libxfs/xfs_inode_fork.c
View File

@ -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

6
fs/xfs/libxfs/xfs_inode_fork.h
View File

@ -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__ */