make sure that __dentry_kill() always invalidates d_seq, unhashed or not

RCU pathwalk relies upon the assumption that anything that changes
->d_inode of a dentry will invalidate its ->d_seq.  That's almost
true - the one exception is that the final dput() of already unhashed
dentry does *not* touch ->d_seq at all.  Unhashing does, though,
so for anything we'd found by RCU dcache lookup we are fine.
Unfortunately, we can *start* with an unhashed dentry or jump into
it.

We could try and be careful in the (few) places where that could
happen.  Or we could just make the final dput() invalidate the damn
thing, unhashed or not.  The latter is much simpler and easier to
backport, so let's do it that way.

Reported-by: "Dae R. Jeong" <threeearcat@gmail.com>
Cc: stable@vger.kernel.org
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

diff --git a/fs/dcache.c b/fs/dcache.c
index d677290b0acae084149bd7d6a6a4e1bff0bd37d0..ceb7b491d1b9c32844b40caeab21207eabeddc68 100644 (file)
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -358,14 +358,11 @@ static void dentry_unlink_inode(struct dentry * dentry)
        __releases(dentry->d_inode->i_lock)
 {
        struct inode *inode = dentry->d_inode;
-       bool hashed = !d_unhashed(dentry);
 
-       if (hashed)
-               raw_write_seqcount_begin(&dentry->d_seq);
+       raw_write_seqcount_begin(&dentry->d_seq);
        __d_clear_type_and_inode(dentry);
        hlist_del_init(&dentry->d_u.d_alias);
-       if (hashed)
-               raw_write_seqcount_end(&dentry->d_seq);
+       raw_write_seqcount_end(&dentry->d_seq);
        spin_unlock(&dentry->d_lock);
        spin_unlock(&inode->i_lock);
        if (!inode->i_nlink)