On Thu, Apr 17, 2014 at 8:17 PM, Al Viro <viro@xxxxxxxxxxxxxxxxxx> wrote:
> On Fri, Apr 18, 2014 at 03:48:37AM +0100, Al Viro wrote:
>> Crap... No, it's a bit trickier - we start with clearing all flags,
>> so if we see the _intermediate_ d_flags and new d_inode, we'll sail
>> past the check. Which would leave us with correct inode, but might
>> give us a false negative in should_follow_link().
>
> Note that most of the places calling d_is_...() are protected by
> the following: if we'd obtained dentry by __lookup_hash() and friends
> *after* grabbing ->i_mutex on parent, we are fine - both positive-to-negative
> and negative-to-positive are possible only with ->i_mutex held, so it
> gives us a barrier.
>
> AFAICS, there are two more tricky places: walk_component() doing
> if (!inode)
> goto out_path_put;
>
> if (should_follow_link(path->dentry, follow)) {
> with unpleasant consequences if the second test gives a false negative,
> and similar for mountpoint_last().
>
> Basically, we'd concentrated on RCU races back then, and missed the possibility
> of non-RCU-but-without-i_mutex ones. Proposed fix follows:
Should be OK. I'll test it tomorrow.
Thanks.
>
> Signed-off-by: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
> ---
> 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 @@ static void __d_instantiate(struct dentry *dentry, struct inode *inode)
> 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;
> diff --git a/fs/namei.c b/fs/namei.c
> index c6157c8..8016827 100644
> --- a/fs/namei.c
> +++ b/fs/namei.c
> @@ -1542,7 +1542,7 @@ static inline int walk_component(struct nameidata *nd, struct path *path,
> inode = path->dentry->d_inode;
> }
> err = -ENOENT;
> - if (!inode)
> + if (!inode || d_is_negative(path->dentry))
> goto out_path_put;
>
> if (should_follow_link(path->dentry, follow)) {
> @@ -2249,7 +2249,7 @@ mountpoint_last(struct nameidata *nd, struct path *path)
> mutex_unlock(&dir->d_inode->i_mutex);
>
> done:
> - if (!dentry->d_inode) {
> + if (!dentry->d_inode || d_is_negative(dentry)) {
> error = -ENOENT;
> dput(dentry);
> goto out;
> @@ -2994,7 +2994,7 @@ retry_lookup:
> finish_lookup:
> /* we _can_ be in RCU mode here */
> error = -ENOENT;
> - if (d_is_negative(path->dentry)) {
> + if (!inode || d_is_negative(path->dentry)) {
> path_to_nameidata(path, nd);
> goto out;
> }
--
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