xfs: ATTR_REPLACE algorithm with LARP enabled needs rework

We can't use the same algorithm for replacing an existing attribute
when logging attributes. The existing algorithm is essentially:

1. create new attr w/ INCOMPLETE
2. atomically flip INCOMPLETE flags between old + new attribute
3. remove old attr which is marked w/ INCOMPLETE

This algorithm guarantees that we see either the old or new
attribute, and if we fail after the atomic flag flip, we don't have
to recover the removal of the old attr because we never see
INCOMPLETE attributes in lookups.

For logged attributes, however, this does not work. The logged
attribute intents do not track the work that has been done as the
transaction rolls, and hence the only recovery mechanism we have is
"run the replace operation from scratch".

This is further exacerbated by the attempt to avoid needing the
INCOMPLETE flag to create an atomic swap. This means we can create
a second active attribute of the same name before we remove the
original. If we fail at any point after the create but before the
removal has completed, we end up with duplicate attributes in
the attr btree and recovery only tries to replace one of them.

There are several other failure modes where we can leave partially
allocated remote attributes that expose stale data, partially free
remote attributes that enable UAF based stale data exposure, etc.

TO fix this, we need a different algorithm for replace operations
when LARP is enabled. Luckily, it's not that complex if we take the
right first step. That is, the first thing we log is the attri
intent with the new name/value pair and mark the old attr as
INCOMPLETE in the same transaction.

From there, we then remove the old attr and keep relogging the
new name/value in the intent, such that we always know that we have
to create the new attr in recovery. Once the old attr is removed,
we then run a normal ATTR_CREATE operation relogging the intent as
we go. If the new attr is local, then it gets created in a single
atomic transaction that also logs the final intent done. If the new
attr is remote, the we set INCOMPLETE on the new attr while we
allocate and set the remote value, and then we clear the INCOMPLETE
flag at in the last transaction taht logs the final intent done.

If we fail at any point in this algorithm, log recovery will always
see the same state on disk: the new name/value in the intent, and
either an INCOMPLETE attr or no attr in the attr btree. If we find
an INCOMPLETE attr, we run the full replace starting with removing
the INCOMPLETE attr. If we don't find it, then we simply create the
new attr.

Notably, recovery of a failed create that has an INCOMPLETE flag set
is now the same - we start with the lookup of the INCOMPLETE attr,
and if that exists then we do the full replace recovery process,
otherwise we just create the new attr.

Hence changing the way we do the replace operation when LARP is
enabled allows us to use the same log recovery algorithm for both
the ATTR_CREATE and ATTR_REPLACE operations. This is also the same
algorithm we use for runtime ATTR_REPLACE operations (except for the
step setting up the initial conditions).

The result is that:

- ATTR_CREATE uses the same algorithm regardless of whether LARP is
  enabled or not
- ATTR_REPLACE with larp=0 is identical to the old algorithm
- ATTR_REPLACE with larp=1 runs an unmodified attr removal algorithm
  from the larp=0 code and then runs the unmodified ATTR_CREATE
  code.
- log recovery when larp=1 runs the same ATTR_REPLACE algorithm as
  it uses at runtime.

Because the state machine is now quite clean, changing the algorithm
is really just a case of changing the initial state and how the
states link together for the ATTR_REPLACE case. Hence it's not a
huge amount of code for what is a fairly substantial rework
of the attr logging and recovery algorithm....

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Allison Henderson <allison.henderson@oracle.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Dave Chinner <david@fromorbit.com>

1 files changed, 31 insertions, 18 deletions

diff --git a/fs/xfs/libxfs/xfs_attr.h b/fs/xfs/libxfs/xfs_attr.h
index 689a96689f1a..1af7abe29eef 100644
--- a/fs/xfs/libxfs/xfs_attr.h
+++ b/fs/xfs/libxfs/xfs_attr.h
@@ -444,18 +444,23 @@ struct xfs_attr_list_context {
  */
 enum xfs_delattr_state {
 	XFS_DAS_UNINIT		= 0,	/* No state has been set yet */
-	XFS_DAS_SF_ADD,			/* Initial shortform set iter state */
-	XFS_DAS_LEAF_ADD,		/* Initial leaf form set iter state */
-	XFS_DAS_NODE_ADD,		/* Initial node form set iter state */
-	XFS_DAS_RMTBLK,			/* Removing remote blks */
-	XFS_DAS_RM_NAME,		/* Remove attr name */
-	XFS_DAS_RM_SHRINK,		/* We are shrinking the tree */
-
-	XFS_DAS_SF_REMOVE,		/* Initial shortform set iter state */
-	XFS_DAS_LEAF_REMOVE,		/* Initial leaf form set iter state */
-	XFS_DAS_NODE_REMOVE,		/* Initial node form set iter state */
-
-	/* Leaf state set/replace sequence */
+
+	/*
+	 * Initial sequence states. The replace setup code relies on the
+	 * ADD and REMOVE states for a specific format to be sequential so
+	 * that we can transform the initial operation to be performed
+	 * according to the xfs_has_larp() state easily.
+	 */
+	XFS_DAS_SF_ADD,			/* Initial sf add state */
+	XFS_DAS_SF_REMOVE,		/* Initial sf replace/remove state */
+
+	XFS_DAS_LEAF_ADD,		/* Initial leaf add state */
+	XFS_DAS_LEAF_REMOVE,		/* Initial leaf replace/remove state */
+
+	XFS_DAS_NODE_ADD,		/* Initial node add state */
+	XFS_DAS_NODE_REMOVE,		/* Initial node replace/remove state */
+
+	/* Leaf state set/replace/remove sequence */
 	XFS_DAS_LEAF_SET_RMT,		/* set a remote xattr from a leaf */
 	XFS_DAS_LEAF_ALLOC_RMT,		/* We are allocating remote blocks */
 	XFS_DAS_LEAF_REPLACE,		/* Perform replace ops on a leaf */
@@ -463,7 +468,7 @@ enum xfs_delattr_state {
 	XFS_DAS_LEAF_REMOVE_RMT,	/* A rename is removing remote blocks */
 	XFS_DAS_LEAF_REMOVE_ATTR,	/* Remove the old attr from a leaf */
 
-	/* Node state set/replace sequence, must match leaf state above */
+	/* Node state sequence, must match leaf state above */
 	XFS_DAS_NODE_SET_RMT,		/* set a remote xattr from a node */
 	XFS_DAS_NODE_ALLOC_RMT,		/* We are allocating remote blocks */
 	XFS_DAS_NODE_REPLACE,		/* Perform replace ops on a node */
@@ -477,11 +482,11 @@ enum xfs_delattr_state {
 #define XFS_DAS_STRINGS	\
 	{ XFS_DAS_UNINIT,		"XFS_DAS_UNINIT" }, \
 	{ XFS_DAS_SF_ADD,		"XFS_DAS_SF_ADD" }, \
+	{ XFS_DAS_SF_REMOVE,		"XFS_DAS_SF_REMOVE" }, \
 	{ XFS_DAS_LEAF_ADD,		"XFS_DAS_LEAF_ADD" }, \
+	{ XFS_DAS_LEAF_REMOVE,		"XFS_DAS_LEAF_REMOVE" }, \
 	{ XFS_DAS_NODE_ADD,		"XFS_DAS_NODE_ADD" }, \
-	{ XFS_DAS_RMTBLK,		"XFS_DAS_RMTBLK" }, \
-	{ XFS_DAS_RM_NAME,		"XFS_DAS_RM_NAME" }, \
-	{ XFS_DAS_RM_SHRINK,		"XFS_DAS_RM_SHRINK" }, \
+	{ XFS_DAS_NODE_REMOVE,		"XFS_DAS_NODE_REMOVE" }, \
 	{ XFS_DAS_LEAF_SET_RMT,		"XFS_DAS_LEAF_SET_RMT" }, \
 	{ XFS_DAS_LEAF_ALLOC_RMT,	"XFS_DAS_LEAF_ALLOC_RMT" }, \
 	{ XFS_DAS_LEAF_REPLACE,		"XFS_DAS_LEAF_REPLACE" }, \
@@ -525,8 +530,7 @@ struct xfs_attr_item {
 	enum xfs_delattr_state		xattri_dela_state;
 
 	/*
-	 * Indicates if the attr operation is a set or a remove
-	 * XFS_ATTR_OP_FLAGS_{SET,REMOVE}
+	 * Attr operation being performed - XFS_ATTR_OP_FLAGS_*
 	 */
 	unsigned int			xattri_op_flags;
 
@@ -614,10 +618,19 @@ xfs_attr_init_remove_state(struct xfs_da_args *args)
 	return XFS_DAS_NODE_REMOVE;
 }
 
+/*
+ * If we are logging the attributes, then we have to start with removal of the
+ * old attribute so that there is always consistent state that we can recover
+ * from if the system goes down part way through. We always log the new attr
+ * value, so even when we remove the attr first we still have the information in
+ * the log to finish the replace operation atomically.
+ */
 static inline enum xfs_delattr_state
 xfs_attr_init_replace_state(struct xfs_da_args *args)
 {
 	args->op_flags |= XFS_DA_OP_ADDNAME | XFS_DA_OP_REPLACE;
+	if (xfs_has_larp(args->dp->i_mount))
+		return xfs_attr_init_remove_state(args);
 	return xfs_attr_init_add_state(args);
 }