Re: [PATCH] ipc/msg.c: Fix lost wakeup in msgsnd().

[Manfred Spraul]

Hi Sedat,

On 09/03/2013 06:13 PM, Sedat Dilek wrote:
> On Tue, Sep 3, 2013 at 4:00 PM, Manfred Spraul <manfred@xxxxxxxxxxxxxxxx> wrote:
> > The check if the queue is full and adding current to the wait queue of pending
> > msgsnd() operations (ss_add()) must be atomic.
> >
> > Otherwise:
> > - the thread that performs msgsnd() finds a full queue and decides to sleep.
> > - the thread that performs msgrcv() calls first reads all messages from the
> >    queue and then sleep, because the queue is empty.
> reads -> sleeps
Correct.
> > - the msgrcv() calls do not perform any wakeups, because the msgsnd() task
> >    has not yet called ss_add().
> > - then the msgsnd()-thread first calls ss_add() and then sleeps.
> > Net result: msgsnd() and msgrcv() both sleep forever.
> I don't know what and why "net result" - net in sense of networking?
http://en.wiktionary.org/wiki/net#Adjective
I.e.: Ignore/remove the "Net".

> > Observed with msgctl08 from ltp with a preemptible kernel.
> ...on ARC arch (that sounds funny somehow).
>
> > Fix: Call ipc_lock_object() before performing the check.
> >
> > The patch also moves security_msg_queue_msgsnd() under ipc_lock_object:
> > - msgctl(IPC_SET) explicitely mentions that it tries to expunge any pending
> >    operations that are not allowed anymore with the new permissions.
> >    If security_msg_queue_msgsnd() is called without locks, then there might be
> >    races.
> > - it makes the patch much simpler.
> >
> > Reported-by: Vineet Gupta <Vineet.Gupta1@xxxxxxxxxxxx>
> > Signed-off-by: Manfred Spraul <manfred@xxxxxxxxxxxxxxxx>
> I guess this is missing a "CC: stable" as Vineet reported against
> Linux v3.11-rc7 (and should enter v3.11.1)?
Yes. I didn't notice that Linus already released 3.11.

--
    Manfred
> - Sedat -
>
> > ---
> >   ipc/msg.c | 12 +++++-------
> >   1 file changed, 5 insertions(+), 7 deletions(-)
> >
> > diff --git a/ipc/msg.c b/ipc/msg.c
> > index 9f29d9e..b65fdf1 100644
> > --- a/ipc/msg.c
> > +++ b/ipc/msg.c
> > @@ -680,16 +680,18 @@ long do_msgsnd(int msqid, long mtype, void __user *mtext,
> >                  goto out_unlock1;
> >          }
> >
> > +       ipc_lock_object(&msq->q_perm);
> > +
> >          for (;;) {
> >                  struct msg_sender s;
> >
> >                  err = -EACCES;
> >                  if (ipcperms(ns, &msq->q_perm, S_IWUGO))
> > -                       goto out_unlock1;
> > +                       goto out_unlock0;
> >
> >                  err = security_msg_queue_msgsnd(msq, msg, msgflg);
> >                  if (err)
> > -                       goto out_unlock1;
> > +                       goto out_unlock0;
> >
> >                  if (msgsz + msq->q_cbytes <= msq->q_qbytes &&
> >                                  1 + msq->q_qnum <= msq->q_qbytes) {
> > @@ -699,10 +701,9 @@ long do_msgsnd(int msqid, long mtype, void __user *mtext,
> >                  /* queue full, wait: */
> >                  if (msgflg & IPC_NOWAIT) {
> >                          err = -EAGAIN;
> > -                       goto out_unlock1;
> > +                       goto out_unlock0;
> >                  }
> >
> > -               ipc_lock_object(&msq->q_perm);
> >                  ss_add(msq, &s);
> >
> >                  if (!ipc_rcu_getref(msq)) {
> > @@ -730,10 +731,7 @@ long do_msgsnd(int msqid, long mtype, void __user *mtext,
> >                          goto out_unlock0;
> >                  }
> >
> > -               ipc_unlock_object(&msq->q_perm);
> >          }
> > -
> > -       ipc_lock_object(&msq->q_perm);
> >          msq->q_lspid = task_tgid_vnr(current);
> >          msq->q_stime = get_seconds();
> >
> > --
> > 1.8.3.1

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