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Subject[PATCH AUTOSEL for 4.15 021/124] VFS: close race between getcwd() and d_move()
From: NeilBrown <>

[ Upstream commit 61647823aa920e395afcce4b57c32afb51456cab ]

d_move() will call __d_drop() and then __d_rehash()
on the dentry being moved. This creates a small window
when the dentry appears to be unhashed. Many tests
of d_unhashed() are made under ->d_lock and so are safe
from racing with this window, but some aren't.
In particular, getcwd() calls d_unlinked() (which calls
d_unhashed()) without d_lock protection, so it can race.

This races has been seen in practice with lustre, which uses d_move() as
part of name lookup. See:
It could race with a regular rename(), and result in ENOENT instead
of either the 'before' or 'after' name.

The race can be demonstrated with a simple program which
has two threads, one renaming a directory back and forth
while another calls getcwd() within that directory: it should never
fail, but does. See:

We could fix this race by taking d_lock and rechecking when
d_unhashed() reports true. Alternately when can remove the window,
which is the approach this patch takes.

___d_drop() is introduce which does *not* clear d_hash.pprev
so the dentry still appears to be hashed. __d_drop() calls
___d_drop(), then clears d_hash.pprev.
__d_move() now uses ___d_drop() and only clears d_hash.pprev
when not rehashing.

Signed-off-by: NeilBrown <>
Signed-off-by: Al Viro <>
Signed-off-by: Sasha Levin <>
fs/dcache.c | 23 ++++++++++++++++-------
1 file changed, 16 insertions(+), 7 deletions(-)

diff --git a/fs/dcache.c b/fs/dcache.c
index 5c7df1df81ff..10b47d16a8a4 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -468,9 +468,11 @@ static void dentry_lru_add(struct dentry *dentry)
* d_drop() is used mainly for stuff that wants to invalidate a dentry for some
* reason (NFS timeouts or autofs deletes).
- * __d_drop requires dentry->d_lock.
+ * __d_drop requires dentry->d_lock
+ * ___d_drop doesn't mark dentry as "unhashed"
+ * (dentry->d_hash.pprev will be LIST_POISON2, not NULL).
-void __d_drop(struct dentry *dentry)
+static void ___d_drop(struct dentry *dentry)
if (!d_unhashed(dentry)) {
struct hlist_bl_head *b;
@@ -486,12 +488,17 @@ void __d_drop(struct dentry *dentry)

- dentry->d_hash.pprev = NULL;
/* After this call, in-progress rcu-walk path lookup will fail. */
+void __d_drop(struct dentry *dentry)
+ ___d_drop(dentry);
+ dentry->d_hash.pprev = NULL;

void d_drop(struct dentry *dentry)
@@ -2381,7 +2388,7 @@ EXPORT_SYMBOL(d_delete);
static void __d_rehash(struct dentry *entry)
struct hlist_bl_head *b = d_hash(entry->d_name.hash);
- BUG_ON(!d_unhashed(entry));
hlist_bl_add_head_rcu(&entry->d_hash, b);
@@ -2816,9 +2823,9 @@ static void __d_move(struct dentry *dentry, struct dentry *target,
write_seqcount_begin_nested(&target->d_seq, DENTRY_D_LOCK_NESTED);

/* unhash both */
- /* __d_drop does write_seqcount_barrier, but they're OK to nest. */
- __d_drop(dentry);
- __d_drop(target);
+ /* ___d_drop does write_seqcount_barrier, but they're OK to nest. */
+ ___d_drop(dentry);
+ ___d_drop(target);

/* Switch the names.. */
if (exchange)
@@ -2830,6 +2837,8 @@ static void __d_move(struct dentry *dentry, struct dentry *target,
if (exchange)
+ else
+ target->d_hash.pprev = NULL;

/* ... and switch them in the tree */
if (IS_ROOT(dentry)) {
 \ /
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