fix races between __d_instantiate() and checks of dentry flags
in non-lazy walk we need to be careful about dentry switching from
negative to positive - both ->d_flags and ->d_inode are updated,
and in some places we might see only one store. The cases where
dentry has been obtained by dcache lookup with ->i_mutex held on
parent are safe - ->d_lock and ->i_mutex provide all the barriers
we need. However, there are several places where we run into
trouble:
* do_last() fetches ->d_inode, then checks ->d_flags and
assumes that inode won't be NULL unless d_is_negative() is true.
Race with e.g. creat() - we might have fetched the old value of
->d_inode (still NULL) and new value of ->d_flags (already not
DCACHE_MISS_TYPE). Lin Ming has observed and reported the resulting
oops.
* a bunch of places checks ->d_inode for being non-NULL,
then checks ->d_flags for "is it a symlink". Race with symlink(2)
in case if our CPU sees ->d_inode update first - we see non-NULL
there, but ->d_flags still contains DCACHE_MISS_TYPE instead of
DCACHE_SYMLINK_TYPE. Result: false negative on "should we follow
link here?", with subsequent unpleasantness.
Cc: stable@vger.kernel.org # 3.13 and 3.14 need that one
Reported-and-tested-by: Lin Ming <minggr@gmail.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
diff --git a/fs/dcache.c b/fs/dcache.c
index 40707d8..494a9def 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -1647,8 +1647,7 @@
unsigned add_flags = d_flags_for_inode(inode);
spin_lock(&dentry->d_lock);
- dentry->d_flags &= ~DCACHE_ENTRY_TYPE;
- dentry->d_flags |= add_flags;
+ __d_set_type(dentry, add_flags);
if (inode)
hlist_add_head(&dentry->d_alias, &inode->i_dentry);
dentry->d_inode = inode;