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Re: [RFC PATCH v4 1/5] glibc: Perform rseq(2) registration at nptl init and thread creation
- From: Florian Weimer <fweimer at redhat dot com>
- To: Rich Felker <dalias at libc dot org>
- Cc: Mathieu Desnoyers <mathieu dot desnoyers at efficios dot com>, carlos <carlos at redhat dot com>, Joseph Myers <joseph at codesourcery dot com>, Szabolcs Nagy <szabolcs dot nagy at arm dot com>, libc-alpha <libc-alpha at sourceware dot org>, Thomas Gleixner <tglx at linutronix dot de>, Ben Maurer <bmaurer at fb dot com>, Peter Zijlstra <peterz at infradead dot org>, "Paul E. McKenney" <paulmck at linux dot vnet dot ibm dot com>, Boqun Feng <boqun dot feng at gmail dot com>, Will Deacon <will dot deacon at arm dot com>, Dave Watson <davejwatson at fb dot com>, Paul Turner <pjt at google dot com>, linux-kernel <linux-kernel at vger dot kernel dot org>, linux-api <linux-api at vger dot kernel dot org>
- Date: Thu, 22 Nov 2018 16:21:02 +0100
- Subject: Re: [RFC PATCH v4 1/5] glibc: Perform rseq(2) registration at nptl init and thread creation
- 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> <20181122151710.GF23599@brightrain.aerifal.cx>
* 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.
Thanks,
Florian