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SubjectRe: [PATCH 3/3] VFS: close race between getcwd() and d_move()
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On 9.11.2017 05:22, NeilBrown wrote:
> 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:
> https://jira.hpdd.intel.com/browse/LU-9735
> 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:
> https://patchwork.kernel.org/patch/9455345/
>
> 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.
>
> When __d_drop and __d_rehash are used to move a dentry, an extra
> flag is passed which causes d_hash.pprev to not be cleared, and
> to not be tested.
>
> Signed-off-by: NeilBrown <neilb@suse.com>
> ---
> fs/dcache.c | 31 ++++++++++++++++++++-----------
> 1 file changed, 20 insertions(+), 11 deletions(-)
>
> diff --git a/fs/dcache.c b/fs/dcache.c
> index d5952306206b..3130d62f29c9 100644
> --- a/fs/dcache.c
> +++ b/fs/dcache.c
> @@ -471,8 +471,11 @@ static void dentry_lru_add(struct dentry *dentry)
> * reason (NFS timeouts or autofs deletes).
> *
> * __d_drop requires dentry->d_lock.
> + * ___d_drop takes an extra @moving argument.
> + * If true, d_hash.pprev is not cleared, so there is no transient d_unhashed()
> + * state.
> */
> -void __d_drop(struct dentry *dentry)
> +static void inline ___d_drop(struct dentry *dentry, bool moving)
> {
> if (!d_unhashed(dentry)) {
> struct hlist_bl_head *b;
> @@ -493,12 +496,18 @@ void __d_drop(struct dentry *dentry)
> } else
> hlist_bl_lock(b);
> __hlist_bl_del(&dentry->d_hash);
> - dentry->d_hash.pprev = NULL;
> + if (likely(!moving))
> + dentry->d_hash.pprev = NULL;

nit: isn't a bit more explicit if (unlikely(moving)). I suspect the end
result is the same, however it's easy to miss the !. It's not a big deal
but just wondering.

> hlist_bl_unlock(b);
> /* After this call, in-progress rcu-walk path lookup will fail. */
> write_seqcount_invalidate(&dentry->d_seq);
> }
> }
> +
> +void __d_drop(struct dentry *dentry)
> +{
> + ___d_drop(dentry, false);
> +}
> EXPORT_SYMBOL(__d_drop);
>
> void d_drop(struct dentry *dentry)
> @@ -2387,10 +2396,10 @@ void d_delete(struct dentry * dentry)
> }
> EXPORT_SYMBOL(d_delete);
>
> -static void __d_rehash(struct dentry *entry)
> +static void __d_rehash(struct dentry *entry, bool moving)
> {
> struct hlist_bl_head *b = d_hash(entry->d_name.hash);
> - BUG_ON(!d_unhashed(entry));
> + BUG_ON(!moving && !d_unhashed(entry));
> hlist_bl_lock(b);
> hlist_bl_add_head_rcu(&entry->d_hash, b);
> hlist_bl_unlock(b);
> @@ -2406,7 +2415,7 @@ static void __d_rehash(struct dentry *entry)
> void d_rehash(struct dentry * entry)
> {
> spin_lock(&entry->d_lock);
> - __d_rehash(entry);
> + __d_rehash(entry, false);
> spin_unlock(&entry->d_lock);
> }
> EXPORT_SYMBOL(d_rehash);
> @@ -2580,7 +2589,7 @@ static inline void __d_add(struct dentry *dentry, struct inode *inode)
> raw_write_seqcount_end(&dentry->d_seq);
> fsnotify_update_flags(dentry);
> }
> - __d_rehash(dentry);
> + __d_rehash(dentry, false);
> if (dir)
> end_dir_add(dir, n);
> spin_unlock(&dentry->d_lock);
> @@ -2642,7 +2651,7 @@ struct dentry *d_exact_alias(struct dentry *entry, struct inode *inode)
> alias = NULL;
> } else {
> __dget_dlock(alias);
> - __d_rehash(alias);
> + __d_rehash(alias, false);
> spin_unlock(&alias->d_lock);
> }
> spin_unlock(&inode->i_lock);
> @@ -2828,8 +2837,8 @@ static void __d_move(struct dentry *dentry, struct dentry *target,
>
> /* unhash both */
> /* __d_drop does write_seqcount_barrier, but they're OK to nest. */
> - __d_drop(dentry);
> - __d_drop(target);
> + ___d_drop(dentry, true);
> + ___d_drop(target, exchange);
>
> /* Switch the names.. */
> if (exchange)
> @@ -2838,9 +2847,9 @@ static void __d_move(struct dentry *dentry, struct dentry *target,
> copy_name(dentry, target);
>
> /* rehash in new place(s) */
> - __d_rehash(dentry);
> + __d_rehash(dentry, true);
> if (exchange)
> - __d_rehash(target);
> + __d_rehash(target, true);
>
> /* ... and switch them in the tree */
> if (IS_ROOT(dentry)) {
>
>
>

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