Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Date:   Thu, 22 Nov 2018 10:17:10 -0500
From:   Rich Felker <dalias@libc.org>
To:     Florian Weimer <fweimer@redhat.com>
Cc:     Mathieu Desnoyers <mathieu.desnoyers@efficios.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
Message-ID: <20181122151710.GF23599@brightrain.aerifal.cx>
References: <20181121183936.8176-1-mathieu.desnoyers@efficios.com>
 <20181122143603.GD23599@brightrain.aerifal.cx>
 <782067422.9852.1542899056778.JavaMail.zimbra@efficios.com>
 <87a7m1ywni.fsf@oldenburg.str.redhat.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <87a7m1ywni.fsf@oldenburg.str.redhat.com>
User-Agent: Mutt/1.5.21 (2010-09-15)
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

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.

Rich