Re: [PATCH v1 net-next 2/3] af_unix: Remove io_uring code for GC.

[Pengfei Xu <pengfei.xu@intel.com>]

Hi Jens Axboe,

On 2024-02-12 at 10:47:20 -0700, Jens Axboe wrote:
> On 2/11/24 7:17 PM, Pengfei Xu wrote:
> > Hi,
> > 
> > On 2024-01-29 at 11:04:34 -0800, Kuniyuki Iwashima wrote:
> >> Since commit 705318a99a13 ("io_uring/af_unix: disable sending
> >> io_uring over sockets"), io_uring's unix socket cannot be passed
> >> via SCM_RIGHTS, so it does not contribute to cyclic reference and
> >> no longer be candidate for garbage collection.
> >>
> >> Also, commit 6e5e6d274956 ("io_uring: drop any code related to
> >> SCM_RIGHTS") cleaned up SCM_RIGHTS code in io_uring.
> >>
> >> Let's do it in AF_UNIX as well by reverting commit 0091bfc81741
> >> ("io_uring/af_unix: defer registered files gc to io_uring release")
> >> and commit 10369080454d ("net: reclaim skb->scm_io_uring bit").
> >>
> >> Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
> >> ---
> >>  include/net/af_unix.h |  1 -
> >>  net/unix/garbage.c    | 25 ++-----------------------
> >>  net/unix/scm.c        |  6 ------
> >>  3 files changed, 2 insertions(+), 30 deletions(-)
> >>
> >> diff --git a/include/net/af_unix.h b/include/net/af_unix.h
> >> index f045bbd9017d..9e39b2ec4524 100644
> >> --- a/include/net/af_unix.h
> >> +++ b/include/net/af_unix.h
> >> @@ -20,7 +20,6 @@ static inline struct unix_sock *unix_get_socket(struct file *filp)
> >>  void unix_inflight(struct user_struct *user, struct file *fp);
> >>  void unix_notinflight(struct user_struct *user, struct file *fp);
> >>  void unix_destruct_scm(struct sk_buff *skb);
> >> -void io_uring_destruct_scm(struct sk_buff *skb);
> >>  void unix_gc(void);
> >>  void wait_for_unix_gc(struct scm_fp_list *fpl);
> >>  struct sock *unix_peer_get(struct sock *sk);
> >> diff --git a/net/unix/garbage.c b/net/unix/garbage.c
> >> index af676bb8fb67..ce5b5f87b16e 100644
> >> --- a/net/unix/garbage.c
> >> +++ b/net/unix/garbage.c
> >> @@ -184,12 +184,10 @@ static bool gc_in_progress;
> >>  
> >>  static void __unix_gc(struct work_struct *work)
> >>  {
> >> -	struct sk_buff *next_skb, *skb;
> >> -	struct unix_sock *u;
> >> -	struct unix_sock *next;
> >>  	struct sk_buff_head hitlist;
> >> -	struct list_head cursor;
> >> +	struct unix_sock *u, *next;
> >>  	LIST_HEAD(not_cycle_list);
> >> +	struct list_head cursor;
> >>  
> >>  	spin_lock(&unix_gc_lock);
> >>  
> >> @@ -269,30 +267,11 @@ static void __unix_gc(struct work_struct *work)
> >>  
> >>  	spin_unlock(&unix_gc_lock);
> >>  
> >> -	/* We need io_uring to clean its registered files, ignore all io_uring
> >> -	 * originated skbs. It's fine as io_uring doesn't keep references to
> >> -	 * other io_uring instances and so killing all other files in the cycle
> >> -	 * will put all io_uring references forcing it to go through normal
> >> -	 * release.path eventually putting registered files.
> >> -	 */
> >> -	skb_queue_walk_safe(&hitlist, skb, next_skb) {
> >> -		if (skb->destructor == io_uring_destruct_scm) {
> >> -			__skb_unlink(skb, &hitlist);
> >> -			skb_queue_tail(&skb->sk->sk_receive_queue, skb);
> >> -		}
> >> -	}
> >> -
> >>  	/* Here we are. Hitlist is filled. Die. */
> >>  	__skb_queue_purge(&hitlist);
> >>  
> >>  	spin_lock(&unix_gc_lock);
> >>  
> >> -	/* There could be io_uring registered files, just push them back to
> >> -	 * the inflight list
> >> -	 */
> >> -	list_for_each_entry_safe(u, next, &gc_candidates, link)
> >> -		list_move_tail(&u->link, &gc_inflight_list);
> >> -
> >>  	/* All candidates should have been detached by now. */
> >>  	WARN_ON_ONCE(!list_empty(&gc_candidates));
> >>  
> >> diff --git a/net/unix/scm.c b/net/unix/scm.c
> >> index 505e56cf02a2..db65b0ab5947 100644
> >> --- a/net/unix/scm.c
> >> +++ b/net/unix/scm.c
> >> @@ -148,9 +148,3 @@ void unix_destruct_scm(struct sk_buff *skb)
> >>  	sock_wfree(skb);
> >>  }
> >>  EXPORT_SYMBOL(unix_destruct_scm);
> >> -
> >> -void io_uring_destruct_scm(struct sk_buff *skb)
> >> -{
> >> -	unix_destruct_scm(skb);
> >> -}
> >> -EXPORT_SYMBOL(io_uring_destruct_scm);
> > 
> > Syzkaller found below issue.
> > There is WARNING in __unix_gc in v6.8-rc3_internal-devel_hourly-20240205-094544,
> > the kernel contains kernel-next patches.
> > 
> > Bisected and found first bad commit:
> > "
> > 11498715f266 af_unix: Remove io_uring code for GC.
> > "
> > It's the same patch as above.
> 
> It should be fixed by:
> 
> commit 1279f9d9dec2d7462823a18c29ad61359e0a007d
> Author: Kuniyuki Iwashima <kuniyu@amazon.com>
> Date:   Sat Feb 3 10:31:49 2024 -0800
> 
>     af_unix: Call kfree_skb() for dead unix_(sk)->oob_skb in GC.
> 
> which is in Linus's tree.

Thank you for the commit tip for the fix. This is indeed the same problem and
has been fixed.
I will check the community mail carefully next time to avoid reporting
duplicate problems.

Best Regards,
Thanks!

> 
> -- 
> Jens Axboe
>