linux-kernel - Re: [RFC PATCH v4 1/5] glibc: Perform rseq(2) registration at nptl init and thread creation

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Message-ID: <7032ab45-8314-b85d-5b4d-f83b41dab5b6@arm.com>
Date:   Thu, 22 Nov 2018 16:24:56 +0000
From:   Szabolcs Nagy <Szabolcs.Nagy@....com>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Florian Weimer <fweimer@...hat.com>
CC:     nd <nd@....com>, Rich Felker <dalias@...c.org>,
        carlos <carlos@...hat.com>,
        Joseph Myers <joseph@...esourcery.com>,
        libc-alpha <libc-alpha@...rceware.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ben Maurer <bmaurer@...com>,
        Peter Zijlstra <peterz@...radead.org>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Will Deacon <Will.Deacon@....com>,
        Dave Watson <davejwatson@...com>, Paul Turner <pjt@...gle.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-api <linux-api@...r.kernel.org>
Subject: Re: [RFC PATCH v4 1/5] glibc: Perform rseq(2) registration at nptl
 init and thread creation

On 22/11/18 15:33, Mathieu Desnoyers wrote:
> ----- On Nov 22, 2018, at 10:21 AM, Florian Weimer fweimer@...hat.com wrote:
>> 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.

i think the man page should point this out.

the memory of a registered rseq object must not be freed
before thread exit. (either unregister it or free later)

and ideally also point out that c language thread storage
duration does not provide this guarantee: it may be freed
by the implementation before thread exit (which is currently
not observable, but with the rseq syscall it is).