The patch titled
knfsd: close a race-opportunity in d_splice_alias
has been removed from the -mm tree. Its filename is
knfsd-close-a-race-opportunity-in-d_splice_alias.patch
This patch was dropped because it was merged into mainline or a subsystem tree
------------------------------------------------------
Subject: knfsd: close a race-opportunity in d_splice_alias
From: NeilBrown <neilb@xxxxxxx>
There is a possible race in d_splice_alias. Though __d_find_alias(inode, 1)
will only return a dentry with DCACHE_DISCONNECTED set, it is possible for it
to get cleared before the BUG_ON, and it is is not possible to lock against
that.
There are a couple of problems here. Firstly, the code doesn't match the
comment. The comment describes a 'disconnected' dentry as being IS_ROOT as
well as DCACHE_DISCONNECTED, however there is not testing of IS_ROOT anythere.
A dentry is marked DCACHE_DISCONNECTED when allocated with d_alloc_anon, and
remains DCACHE_DISCONNECTED while a path is built up towards the root. So a
dentry can have a valid name and a valid parent and even grandparent, but will
still be DCACHE_DISCONNECTED until a path to the root is created. Once the
path to the root is complete, everything in the path gets DCACHE_DISCONNECTED
cleared. So the fact that DCACHE_DISCONNECTED isn't enough to say that a
dentry is free to be spliced in with a given name. This can only be allowed
if the dentry does not yet have a name, so the IS_ROOT test is needed too.
However even adding that test to __d_find_alias isn't enough. As
d_splice_alias drops dcache_lock before calling d_move to perform the splice,
it could race with another thread calling d_splice_alias to splice the inode
in with a different name in a different part of the tree (in the case where a
file has hard links). So that splicing code is only really safe for
directories (as we know that directories only have one link). For
directories, the caller of d_splice_alias will be holding i_mutex on the
(unique) parent so there is no room for a race.
A consequence of this is that a non-directory will never benefit from being
spliced into a pre-exisiting dentry, but that isn't a problem. It is
perfectly OK for a non-directory to have multiple dentries, some anonymous,
some not. And the comment for d_splice_alias says that it only happens for
directories anyway.
Signed-off-by: Neil Brown <neilb@xxxxxxx>
Cc: Christoph Hellwig <hch@xxxxxx>
Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
Cc: Dipankar Sarma <dipankar@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxx>
---
fs/dcache.c | 9 +++++----
1 file changed, 5 insertions(+), 4 deletions(-)
diff -puN fs/dcache.c~knfsd-close-a-race-opportunity-in-d_splice_alias fs/dcache.c
--- a/fs/dcache.c~knfsd-close-a-race-opportunity-in-d_splice_alias
+++ a/fs/dcache.c
@@ -291,9 +291,9 @@ struct dentry * dget_locked(struct dentr
* it can be unhashed only if it has no children, or if it is the root
* of a filesystem.
*
- * If the inode has a DCACHE_DISCONNECTED alias, then prefer
+ * If the inode has an IS_ROOT, DCACHE_DISCONNECTED alias, then prefer
* any other hashed alias over that one unless @want_discon is set,
- * in which case only return a DCACHE_DISCONNECTED alias.
+ * in which case only return an IS_ROOT, DCACHE_DISCONNECTED alias.
*/
static struct dentry * __d_find_alias(struct inode *inode, int want_discon)
@@ -309,7 +309,8 @@ static struct dentry * __d_find_alias(st
prefetch(next);
alias = list_entry(tmp, struct dentry, d_alias);
if (S_ISDIR(inode->i_mode) || !d_unhashed(alias)) {
- if (alias->d_flags & DCACHE_DISCONNECTED)
+ if (IS_ROOT(alias) &&
+ (alias->d_flags & DCACHE_DISCONNECTED))
discon_alias = alias;
else if (!want_discon) {
__dget_locked(alias);
@@ -1004,7 +1005,7 @@ struct dentry *d_splice_alias(struct ino
{
struct dentry *new = NULL;
- if (inode) {
+ if (inode && S_ISDIR(inode->i_mode)) {
spin_lock(&dcache_lock);
new = __d_find_alias(inode, 1);
if (new) {
_
Patches currently in -mm which might be from neilb@xxxxxxx are
origin.patch
vfs-destroy-the-dentries-contributed-by-a-superblock-on-unmounting.patch
knfsd-add-nfs-export-support-to-tmpfs.patch
knfsd-add-nfs-export-support-to-tmpfs-fixes.patch
md-dm-reduce-stack-usage-with-stacked-block-devices.patch
-
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