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rename(): avoid a deadlock in the case of parents having no common ancestor

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... and fix the directory locking documentation and proof of correctness.
Holding ->s_vfs_rename_mutex *almost* prevents ->d_parent changes; the
case where we really don't want it is splicing the root of disconnected
tree to somewhere.

In other words, ->s_vfs_rename_mutex is sufficient to stabilize "X is an
ancestor of Y" only if X and Y are already in the same tree.  Otherwise
it can go from false to true, and one can construct a deadlock on that.

Make lock_two_directories() report an error in such case and update the
callers of lock_rename()/lock_rename_child() to handle such errors.

And yes, such conditions are not impossible to create ;-/

Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
This commit is contained in:
Al Viro 2023-11-20 20:02:11 -05:00
parent dbd4540df2
commit a8b0026847
11 changed files with 301 additions and 106 deletions
repo.diff.show_diff_stats Download patch file Download diff file Expand all files Collapse all files

322
Documentation/filesystems/directory-locking.rst
View file

@ -11,130 +11,268 @@ When taking the i_rwsem on multiple non-directory objects, we
always acquire the locks in order by increasing address. We'll call
that "inode pointer" order in the following.
For our purposes all operations fall in 5 classes:
1) read access. Locking rules: caller locks directory we are accessing.
The lock is taken shared.
Primitives
==========
2) object creation. Locking rules: same as above, but the lock is taken
exclusive.
For our purposes all operations fall in 6 classes:
3) object removal. Locking rules: caller locks parent, finds victim,
locks victim and calls the method. Locks are exclusive.
1. read access. Locking rules:
4) rename() that is _not_ cross-directory. Locking rules: caller locks
the parent and finds source and target. Then we decide which of the
source and target need to be locked. Source needs to be locked if it's a
non-directory; target - if it's a non-directory or about to be removed.
Take the locks that need to be taken, in inode pointer order if need
to take both (that can happen only when both source and target are
non-directories - the source because it wouldn't be locked otherwise
and the target because mixing directory and non-directory is allowed
only with RENAME_EXCHANGE, and that won't be removing the target).
After the locks had been taken, call the method. All locks are exclusive.
* lock the directory we are accessing (shared)
5) link creation. Locking rules:
2. object creation. Locking rules:
* lock parent
* check that source is not a directory
* lock source
* call the method.
* lock the directory we are accessing (exclusive)
All locks are exclusive.
3. object removal. Locking rules:
6) cross-directory rename. The trickiest in the whole bunch. Locking
rules:
* lock the parent (exclusive)
* find the victim
* lock the victim (exclusive)
4. link creation. Locking rules:
* lock the parent (exclusive)
* check that the source is not a directory
* lock the source (exclusive; probably could be weakened to shared)
5. rename that is _not_ cross-directory. Locking rules:
* lock the parent (exclusive)
* find the source and target
* decide which of the source and target need to be locked.
The source needs to be locked if it's a non-directory, target - if it's
a non-directory or about to be removed.
* take the locks that need to be taken (exlusive), in inode pointer order
if need to take both (that can happen only when both source and target
are non-directories - the source because it wouldn't need to be locked
otherwise and the target because mixing directory and non-directory is
allowed only with RENAME_EXCHANGE, and that won't be removing the target).
6. cross-directory rename. The trickiest in the whole bunch. Locking rules:
* lock the filesystem
* lock parents in "ancestors first" order. If one is not ancestor of
the other, lock the parent of source first.
* find source and target.
* if old parent is equal to or is a descendent of target
fail with -ENOTEMPTY
* if new parent is equal to or is a descendent of source
fail with -ELOOP
* Lock subdirectories involved (source before target).
* Lock non-directories involved, in inode pointer order.
* call the method.
* if the parents don't have a common ancestor, fail the operation.
* lock the parents in "ancestors first" order (exclusive). If neither is an
ancestor of the other, lock the parent of source first.
* find the source and target.
* verify that the source is not a descendent of the target and
target is not a descendent of source; fail the operation otherwise.
* lock the subdirectories involved (exclusive), source before target.
* lock the non-directories involved (exclusive), in inode pointer order.
All ->i_rwsem are taken exclusive.
The rules above obviously guarantee that all directories that are going
to be read, modified or removed by method will be locked by the caller.
The rules above obviously guarantee that all directories that are going to be
read, modified or removed by method will be locked by caller.
Splicing
========
There is one more thing to consider - splicing. It's not an operation
in its own right; it may happen as part of lookup. We speak of the
operations on directory trees, but we obviously do not have the full
picture of those - especially for network filesystems. What we have
is a bunch of subtrees visible in dcache and locking happens on those.
Trees grow as we do operations; memory pressure prunes them. Normally
that's not a problem, but there is a nasty twist - what should we do
when one growing tree reaches the root of another? That can happen in
several scenarios, starting from "somebody mounted two nested subtrees
from the same NFS4 server and doing lookups in one of them has reached
the root of another"; there's also open-by-fhandle stuff, and there's a
possibility that directory we see in one place gets moved by the server
to another and we run into it when we do a lookup.
For a lot of reasons we want to have the same directory present in dcache
only once. Multiple aliases are not allowed. So when lookup runs into
a subdirectory that already has an alias, something needs to be done with
dcache trees. Lookup is already holding the parent locked. If alias is
a root of separate tree, it gets attached to the directory we are doing a
lookup in, under the name we'd been looking for. If the alias is already
a child of the directory we are looking in, it changes name to the one
we'd been looking for. No extra locking is involved in these two cases.
However, if it's a child of some other directory, the things get trickier.
First of all, we verify that it is *not* an ancestor of our directory
and fail the lookup if it is. Then we try to lock the filesystem and the
current parent of the alias. If either trylock fails, we fail the lookup.
If trylocks succeed, we detach the alias from its current parent and
attach to our directory, under the name we are looking for.
Note that splicing does *not* involve any modification of the filesystem;
all we change is the view in dcache. Moreover, holding a directory locked
exclusive prevents such changes involving its children and holding the
filesystem lock prevents any changes of tree topology, other than having a
root of one tree becoming a child of directory in another. In particular,
if two dentries have been found to have a common ancestor after taking
the filesystem lock, their relationship will remain unchanged until
the lock is dropped. So from the directory operations' point of view
splicing is almost irrelevant - the only place where it matters is one
step in cross-directory renames; we need to be careful when checking if
parents have a common ancestor.
Multiple-filesystem stuff
=========================
For some filesystems a method can involve a directory operation on
another filesystem; it may be ecryptfs doing operation in the underlying
filesystem, overlayfs doing something to the layers, network filesystem
using a local one as a cache, etc. In all such cases the operations
on other filesystems must follow the same locking rules. Moreover, "a
directory operation on this filesystem might involve directory operations
on that filesystem" should be an asymmetric relation (or, if you will,
it should be possible to rank the filesystems so that directory operation
on a filesystem could trigger directory operations only on higher-ranked
ones - in these terms overlayfs ranks lower than its layers, network
filesystem ranks lower than whatever it caches on, etc.)
Deadlock avoidance
==================
If no directory is its own ancestor, the scheme above is deadlock-free.
Proof:
[XXX: will be updated once we are done massaging the lock_rename()]
First of all, at any moment we have a linear ordering of the
objects - A < B iff (A is an ancestor of B) or (B is not an ancestor
of A and ptr(A) < ptr(B)).
There is a ranking on the locks, such that all primitives take
them in order of non-decreasing rank. Namely,
That ordering can change. However, the following is true:
* rank ->i_rwsem of non-directories on given filesystem in inode pointer
order.
* put ->i_rwsem of all directories on a filesystem at the same rank,
lower than ->i_rwsem of any non-directory on the same filesystem.
* put ->s_vfs_rename_mutex at rank lower than that of any ->i_rwsem
on the same filesystem.
* among the locks on different filesystems use the relative
rank of those filesystems.
(1) if object removal or non-cross-directory rename holds lock on A and
attempts to acquire lock on B, A will remain the parent of B until we
acquire the lock on B. (Proof: only cross-directory rename can change
the parent of object and it would have to lock the parent).
For example, if we have NFS filesystem caching on a local one, we have
(2) if cross-directory rename holds the lock on filesystem, order will not
change until rename acquires all locks. (Proof: other cross-directory
renames will be blocked on filesystem lock and we don't start changing
the order until we had acquired all locks).
1. ->s_vfs_rename_mutex of NFS filesystem
2. ->i_rwsem of directories on that NFS filesystem, same rank for all
3. ->i_rwsem of non-directories on that filesystem, in order of
increasing address of inode
4. ->s_vfs_rename_mutex of local filesystem
5. ->i_rwsem of directories on the local filesystem, same rank for all
6. ->i_rwsem of non-directories on local filesystem, in order of
increasing address of inode.
(3) locks on non-directory objects are acquired only after locks on
directory objects, and are acquired in inode pointer order.
(Proof: all operations but renames take lock on at most one
non-directory object, except renames, which take locks on source and
target in inode pointer order in the case they are not directories.)
It's easy to verify that operations never take a lock with rank
lower than that of an already held lock.
Now consider the minimal deadlock. Each process is blocked on
attempt to acquire some lock and already holds at least one lock. Let's
consider the set of contended locks. First of all, filesystem lock is
not contended, since any process blocked on it is not holding any locks.
Thus all processes are blocked on ->i_rwsem.
Suppose deadlocks are possible. Consider the minimal deadlocked
set of threads. It is a cycle of several threads, each blocked on a lock
held by the next thread in the cycle.
By (3), any process holding a non-directory lock can only be
waiting on another non-directory lock with a larger address. Therefore
the process holding the "largest" such lock can always make progress, and
non-directory objects are not included in the set of contended locks.
Since the locking order is consistent with the ranking, all
contended locks in the minimal deadlock will be of the same rank,
i.e. they all will be ->i_rwsem of directories on the same filesystem.
Moreover, without loss of generality we can assume that all operations
are done directly to that filesystem and none of them has actually
reached the method call.
Thus link creation can't be a part of deadlock - it can't be
blocked on source and it means that it doesn't hold any locks.
In other words, we have a cycle of threads, T1,..., Tn,
and the same number of directories (D1,...,Dn) such that
Any contended object is either held by cross-directory rename or
has a child that is also contended. Indeed, suppose that it is held by
operation other than cross-directory rename. Then the lock this operation
is blocked on belongs to child of that object due to (1).
T1 is blocked on D1 which is held by T2
It means that one of the operations is cross-directory rename.
Otherwise the set of contended objects would be infinite - each of them
would have a contended child and we had assumed that no object is its
own descendent. Moreover, there is exactly one cross-directory rename
(see above).
T2 is blocked on D2 which is held by T3
Consider the object blocking the cross-directory rename. One
of its descendents is locked by cross-directory rename (otherwise we
would again have an infinite set of contended objects). But that
means that cross-directory rename is taking locks out of order. Due
to (2) the order hadn't changed since we had acquired filesystem lock.
But locking rules for cross-directory rename guarantee that we do not
try to acquire lock on descendent before the lock on ancestor.
Contradiction. I.e. deadlock is impossible. Q.E.D.
...
Tn is blocked on Dn which is held by T1.
Each operation in the minimal cycle must have locked at least
one directory and blocked on attempt to lock another. That leaves
only 3 possible operations: directory removal (locks parent, then
child), same-directory rename killing a subdirectory (ditto) and
cross-directory rename of some sort.
There must be a cross-directory rename in the set; indeed,
if all operations had been of the "lock parent, then child" sort
we would have Dn a parent of D1, which is a parent of D2, which is
a parent of D3, ..., which is a parent of Dn. Relationships couldn't
have changed since the moment directory locks had been acquired,
so they would all hold simultaneously at the deadlock time and
we would have a loop.
Since all operations are on the same filesystem, there can't be
more than one cross-directory rename among them. Without loss of
generality we can assume that T1 is the one doing a cross-directory
rename and everything else is of the "lock parent, then child" sort.
In other words, we have a cross-directory rename that locked
Dn and blocked on attempt to lock D1, which is a parent of D2, which is
a parent of D3, ..., which is a parent of Dn. Relationships between
D1,...,Dn all hold simultaneously at the deadlock time. Moreover,
cross-directory rename does not get to locking any directories until it
has acquired filesystem lock and verified that directories involved have
a common ancestor, which guarantees that ancestry relationships between
all of them had been stable.
Consider the order in which directories are locked by the
cross-directory rename; parents first, then possibly their children.
Dn and D1 would have to be among those, with Dn locked before D1.
Which pair could it be?
It can't be the parents - indeed, since D1 is an ancestor of Dn,
it would be the first parent to be locked. Therefore at least one of the
children must be involved and thus neither of them could be a descendent
of another - otherwise the operation would not have progressed past
locking the parents.
It can't be a parent and its child; otherwise we would've had
a loop, since the parents are locked before the children, so the parent
would have to be a descendent of its child.
It can't be a parent and a child of another parent either.
Otherwise the child of the parent in question would've been a descendent
of another child.
That leaves only one possibility - namely, both Dn and D1 are
among the children, in some order. But that is also impossible, since
neither of the children is a descendent of another.
That concludes the proof, since the set of operations with the
properties requiered for a minimal deadlock can not exist.
Note that the check for having a common ancestor in cross-directory
rename is crucial - without it a deadlock would be possible. Indeed,
suppose the parents are initially in different trees; we would lock the
parent of source, then try to lock the parent of target, only to have
an unrelated lookup splice a distant ancestor of source to some distant
descendent of the parent of target. At that point we have cross-directory
rename holding the lock on parent of source and trying to lock its
distant ancestor. Add a bunch of rmdir() attempts on all directories
in between (all of those would fail with -ENOTEMPTY, had they ever gotten
the locks) and voila - we have a deadlock.
Loop avoidance
==============
These operations are guaranteed to avoid loop creation. Indeed,
the only operation that could introduce loops is cross-directory rename.
Since the only new (parent, child) pair added by rename() is (new parent,
source), such loop would have to contain these objects and the rest of it
would have to exist before rename(). I.e. at the moment of loop creation
rename() responsible for that would be holding filesystem lock and new parent
would have to be equal to or a descendent of source. But that means that
new parent had been equal to or a descendent of source since the moment when
we had acquired filesystem lock and rename() would fail with -ELOOP in that
case.
Suppose after the operation there is a loop; since there hadn't been such
loops before the operation, at least on of the nodes in that loop must've
had its parent changed. In other words, the loop must be passing through
the source or, in case of exchange, possibly the target.
Since the operation has succeeded, neither source nor target could have
been ancestors of each other. Therefore the chain of ancestors starting
in the parent of source could not have passed through the target and
vice versa. On the other hand, the chain of ancestors of any node could
not have passed through the node itself, or we would've had a loop before
the operation. But everything other than source and target has kept
the parent after the operation, so the operation does not change the
chains of ancestors of (ex-)parents of source and target. In particular,
those chains must end after a finite number of steps.
Now consider the loop created by the operation. It passes through either
source or target; the next node in the loop would be the ex-parent of
target or source resp. After that the loop would follow the chain of
ancestors of that parent. But as we have just shown, that chain must
end after a finite number of steps, which means that it can't be a part
of any loop. Q.E.D.
While this locking scheme works for arbitrary DAGs, it relies on
ability to check that directory is a descendent of another object. Current

9
Documentation/filesystems/porting.rst
View file

@ -1079,3 +1079,12 @@ On same-directory ->rename() the (tautological) update of .. is not protected
by any locks; just don't do it if the old parent is the same as the new one.
We really can't lock two subdirectories in same-directory rename - not without
deadlocks.
---
**mandatory**
lock_rename() and lock_rename_child() may fail in cross-directory case, if
their arguments do not have a common ancestor. In that case ERR_PTR(-EXDEV)
is returned, with no locks taken. In-tree users updated; out-of-tree ones
would need to do so.

2
fs/cachefiles/namei.c
View file

@ -305,6 +305,8 @@ int cachefiles_bury_object(struct cachefiles_cache *cache,
/* do the multiway lock magic */
trap = lock_rename(cache->graveyard, dir);
if (IS_ERR(trap))
return PTR_ERR(trap);
/* do some checks before getting the grave dentry */
if (rep->d_parent != dir || IS_DEADDIR(d_inode(rep))) {

2
fs/ecryptfs/inode.c
View file

@ -599,6 +599,8 @@ ecryptfs_rename(struct mnt_idmap *idmap, struct inode *old_dir,
target_inode = d_inode(new_dentry);
trap = lock_rename(lower_old_dir_dentry, lower_new_dir_dentry);
if (IS_ERR(trap))
return PTR_ERR(trap);
dget(lower_new_dentry);
rc = -EINVAL;
if (lower_old_dentry->d_parent != lower_old_dir_dentry)

37
fs/namei.c
View file

@ -3014,21 +3014,37 @@ static inline int may_create(struct mnt_idmap *idmap,
return inode_permission(idmap, dir, MAY_WRITE | MAY_EXEC);
}
// p1 != p2, both are on the same filesystem, ->s_vfs_rename_mutex is held
static struct dentry *lock_two_directories(struct dentry *p1, struct dentry *p2)
{
struct dentry *p;
struct dentry *p = p1, *q = p2, *r;
p = d_ancestor(p2, p1);
if (p) {
while ((r = p->d_parent) != p2 && r != p)
p = r;
if (r == p2) {
// p is a child of p2 and an ancestor of p1 or p1 itself
inode_lock_nested(p2->d_inode, I_MUTEX_PARENT);
inode_lock_nested(p1->d_inode, I_MUTEX_PARENT2);
return p;
}
p = d_ancestor(p1, p2);
inode_lock_nested(p1->d_inode, I_MUTEX_PARENT);
inode_lock_nested(p2->d_inode, I_MUTEX_PARENT2);
return p;
// p is the root of connected component that contains p1
// p2 does not occur on the path from p to p1
while ((r = q->d_parent) != p1 && r != p && r != q)
q = r;
if (r == p1) {
// q is a child of p1 and an ancestor of p2 or p2 itself
inode_lock_nested(p1->d_inode, I_MUTEX_PARENT);
inode_lock_nested(p2->d_inode, I_MUTEX_PARENT2);
return q;
} else if (likely(r == p)) {
// both p2 and p1 are descendents of p
inode_lock_nested(p1->d_inode, I_MUTEX_PARENT);
inode_lock_nested(p2->d_inode, I_MUTEX_PARENT2);
return NULL;
} else { // no common ancestor at the time we'd been called
mutex_unlock(&p1->d_sb->s_vfs_rename_mutex);
return ERR_PTR(-EXDEV);
}
}
/*
@ -4947,6 +4963,10 @@ int do_renameat2(int olddfd, struct filename *from, int newdfd,
retry_deleg:
trap = lock_rename(new_path.dentry, old_path.dentry);
if (IS_ERR(trap)) {
error = PTR_ERR(trap);
goto exit_lock_rename;
}
old_dentry = lookup_one_qstr_excl(&old_last, old_path.dentry,
lookup_flags);
@ -5014,6 +5034,7 @@ int do_renameat2(int olddfd, struct filename *from, int newdfd,
dput(old_dentry);
exit3:
unlock_rename(new_path.dentry, old_path.dentry);
exit_lock_rename:
if (delegated_inode) {
error = break_deleg_wait(&delegated_inode);
if (!error)

4
fs/nfsd/vfs.c
View file

@ -1813,6 +1813,10 @@ nfsd_rename(struct svc_rqst *rqstp, struct svc_fh *ffhp, char *fname, int flen,
}
trap = lock_rename(tdentry, fdentry);
if (IS_ERR(trap)) {
err = (rqstp->rq_vers == 2) ? nfserr_acces : nfserr_xdev;
goto out;
}
err = fh_fill_pre_attrs(ffhp);
if (err != nfs_ok)
goto out_unlock;

9
fs/overlayfs/copy_up.c
View file

@ -722,7 +722,7 @@ static int ovl_copy_up_workdir(struct ovl_copy_up_ctx *c)
struct inode *inode;
struct inode *udir = d_inode(c->destdir), *wdir = d_inode(c->workdir);
struct path path = { .mnt = ovl_upper_mnt(ofs) };
struct dentry *temp, *upper;
struct dentry *temp, *upper, *trap;
struct ovl_cu_creds cc;
int err;
struct ovl_cattr cattr = {
@ -760,9 +760,11 @@ static int ovl_copy_up_workdir(struct ovl_copy_up_ctx *c)
* If temp was moved, abort without the cleanup.
*/
ovl_start_write(c->dentry);
if (lock_rename(c->workdir, c->destdir) != NULL ||
temp->d_parent != c->workdir) {
trap = lock_rename(c->workdir, c->destdir);
if (trap || temp->d_parent != c->workdir) {
err = -EIO;
if (IS_ERR(trap))
goto out;
goto unlock;
} else if (err) {
goto cleanup;
@ -803,6 +805,7 @@ static int ovl_copy_up_workdir(struct ovl_copy_up_ctx *c)
ovl_set_flag(OVL_WHITEOUTS, inode);
unlock:
unlock_rename(c->workdir, c->destdir);
out:
ovl_end_write(c->dentry);
return err;

4
fs/overlayfs/dir.c
View file

@ -1180,6 +1180,10 @@ static int ovl_rename(struct mnt_idmap *idmap, struct inode *olddir,
}
trap = lock_rename(new_upperdir, old_upperdir);
if (IS_ERR(trap)) {
err = PTR_ERR(trap);
goto out_revert_creds;
}
olddentry = ovl_lookup_upper(ofs, old->d_name.name, old_upperdir,
old->d_name.len);

6
fs/overlayfs/super.c
View file

@ -439,8 +439,10 @@ static bool ovl_workdir_ok(struct dentry *workdir, struct dentry *upperdir)
bool ok = false;
if (workdir != upperdir) {
ok = (lock_rename(workdir, upperdir) == NULL);
unlock_rename(workdir, upperdir);
struct dentry *trap = lock_rename(workdir, upperdir);
if (!IS_ERR(trap))
unlock_rename(workdir, upperdir);
ok = (trap == NULL);
}
return ok;
}

7
fs/overlayfs/util.c
View file

@ -1198,12 +1198,17 @@ void ovl_nlink_end(struct dentry *dentry)
int ovl_lock_rename_workdir(struct dentry *workdir, struct dentry *upperdir)
{
struct dentry *trap;
/* Workdir should not be the same as upperdir */
if (workdir == upperdir)
goto err;
/* Workdir should not be subdir of upperdir and vice versa */
if (lock_rename(workdir, upperdir) != NULL)
trap = lock_rename(workdir, upperdir);
if (IS_ERR(trap))
goto err;
if (trap)
goto err_unlock;
return 0;

5
fs/smb/server/vfs.c
View file

@ -708,6 +708,10 @@ int ksmbd_vfs_rename(struct ksmbd_work *work, const struct path *old_path,
goto out2;
trap = lock_rename_child(old_child, new_path.dentry);
if (IS_ERR(trap)) {
err = PTR_ERR(trap);
goto out_drop_write;
}
old_parent = dget(old_child->d_parent);
if (d_unhashed(old_child)) {
@ -770,6 +774,7 @@ int ksmbd_vfs_rename(struct ksmbd_work *work, const struct path *old_path,
out3:
dput(old_parent);
unlock_rename(old_parent, new_path.dentry);
out_drop_write:
mnt_drop_write(old_path->mnt);
out2:
path_put(&new_path);