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Re: [RFC PATCH v4 1/5] glibc: Perform rseq(2) registration at nptl init and thread creation
- From: Mathieu Desnoyers <mathieu dot desnoyers at efficios dot com>
- To: Florian Weimer <fweimer at redhat dot com>
- Cc: Rich Felker <dalias at libc dot org>, 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 10:33:19 -0500 (EST)
- Subject: Re: [RFC PATCH v4 1/5] glibc: Perform rseq(2) registration at nptl init and thread creation
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----- On Nov 22, 2018, at 10:21 AM, Florian Weimer firstname.lastname@example.org 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.