Re: [syzbot] WARNING in mntput_no_expire (2)

[Al Viro]

On Mon, Apr 05, 2021 at 06:23:49PM +0000, Al Viro wrote:
> On Mon, Apr 05, 2021 at 07:08:01PM +0200, Christian Brauner wrote:
> 
> > Ah dentry count of -127 looks... odd.
> 
> dead + 1...
> 
> void lockref_mark_dead(struct lockref *lockref)
> {
>         assert_spin_locked(&lockref->lock);
>       lockref->count = -128;
> }
> 
> IOW, a leaked (uncounted) reference to dentry, that got dget() called on
> it after dentry had been freed.
> 
>       IOW, current->fs->pwd.dentry happens to point to an already freed
> struct dentry here.  Joy...
> 
>       Could you slap
> 
> spin_lock(&current->fs->lock);
> WARN_ON(d_count(current->fs->pwd.dentry) < 0);
> spin_unlock(&current->fs->lock);
> 
> before and after calls of io_issue_sqe() and see if it triggers?  We 
> definitely
> are seeing buggered dentry refcounting here.

Check if this helps, please.

diff --git a/fs/namei.c b/fs/namei.c
index 216f16e74351..82344f1139ff 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -2289,6 +2289,9 @@ static const char *path_init(struct nameidata *nd, 
unsigned flags)
        int error;
        const char *s = nd->name->name;
 
+       nd->path.mnt = NULL;
+       nd->path.dentry = NULL;
+
        /* LOOKUP_CACHED requires RCU, ask caller to retry */
        if ((flags & (LOOKUP_RCU | LOOKUP_CACHED)) == LOOKUP_CACHED)
                return ERR_PTR(-EAGAIN);
@@ -2322,8 +2325,6 @@ static const char *path_init(struct nameidata *nd, 
unsigned flags)
        }
 
        nd->root.mnt = NULL;
-       nd->path.mnt = NULL;
-       nd->path.dentry = NULL;
 
        /* Absolute pathname -- fetch the root (LOOKUP_IN_ROOT uses nd->dfd). */
        if (*s == '/' && !(flags & LOOKUP_IN_ROOT)) {