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Date:   Thu, 22 Nov 2018 10:44:57 -0500
From:   Rich Felker <dalias@libc.org>
To:     Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc:     Florian Weimer <fweimer@redhat.com>, carlos <carlos@redhat.com>,
        Joseph Myers <joseph@codesourcery.com>,
        Szabolcs Nagy <szabolcs.nagy@arm.com>,
        libc-alpha <libc-alpha@sourceware.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ben Maurer <bmaurer@fb.com>,
        Peter Zijlstra <peterz@infradead.org>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Boqun Feng <boqun.feng@gmail.com>,
        Will Deacon <will.deacon@arm.com>,
        Dave Watson <davejwatson@fb.com>, Paul Turner <pjt@google.com>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        linux-api <linux-api@vger.kernel.org>
Subject: Re: [RFC PATCH v4 1/5] glibc: Perform rseq(2) registration at nptl
 init and thread creation
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On Thu, Nov 22, 2018 at 10:33:19AM -0500, Mathieu Desnoyers wrote:
> ----- On Nov 22, 2018, at 10:21 AM, Florian Weimer fweimer@redhat.com wrote:
> 
> > * Rich Felker:
> > 
> >> On Thu, Nov 22, 2018 at 04:11:45PM +0100, Florian Weimer wrote:
> >>> * Mathieu Desnoyers:
> >>> 
> >>> > Thoughts ?
> >>> >
> >>> >   /* Unregister rseq TLS from kernel. */
> >>> >   if (has_rseq && __rseq_unregister_current_thread ())
> >>> >     abort();
> >>> >
> >>> >   advise_stack_range (pd->stackblock, pd->stackblock_size, (uintptr_t) pd,
> >>> >                       pd->guardsize);
> >>> >
> >>> >   /* If the thread is detached free the TCB.  */
> >>> >   if (IS_DETACHED (pd))
> >>> >     /* Free the TCB.  */
> >>> >     __free_tcb (pd);
> >>> 
> >>> Considering that we proceed to free the TCB, I really hope that all
> >>> signals are blocked at this point.  (I have not checked this, though.)
> >>> 
> >>> Wouldn't this address your concern about access to the rseq area?
> >>
> >> I'm not familiar with glibc's logic here, but for other reasons, I
> >> don't think freeing it is safe until the kernel task exit futex (set
> >> via clone or set_tid_address) has fired. I would guess __free_tcb just
> >> sets up for it to be reclaimable when this happens rather than
> >> immediately freeing it for reuse.
> > 
> > Right, but in case of user-supplied stacks, we actually free TLS memory
> > at this point, so signals need to be blocked because the TCB is
> > (partially) gone after that.
> 
> Unfortuntately, disabling signals is not enough.
> 
> With rseq registered, the kernel accesses the rseq TLS area when returning to
> user-space after _preemption_ of user-space, which can be triggered at any
> point by an interrupt or a fault, even if signals are blocked.
> 
> So if there are cases where the TLS memory is freed while the thread is still
> running, we _need_ to explicitly unregister rseq beforehand.

OK, that makes sense. I was wrongly under the impression that the TLS
memory could not be reused until the task exit futex fired, but in
glibc that's not the case with caller-provided stacks.

I still don't understand the need for a reference count though.

Rich