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

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Date:   Fri, 23 Nov 2018 14:29:18 +0100
From:   Florian Weimer <fweimer@...hat.com>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     Rich Felker <dalias@...c.org>, carlos <carlos@...hat.com>,
        Joseph Myers <joseph@...esourcery.com>,
        Szabolcs Nagy <szabolcs.nagy@....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

* Mathieu Desnoyers:

>>>> I don't think you need unregistering if the memory is initial-exec TLS
>>>> memory.  Initial-exec TLS memory is tied directly to the TCB and cannot
>>>> be freed while the thread is running, so it should be safe to put the
>>>> rseq area there even if glibc knows nothing about it.
>>>
>>> Is it true for user-supplied stacks as well ?
>> 
>> I'm not entirely sure because the glibc terminology is confusing, but I
>> think it places intial-exec TLS into the static TLS area (so that it has
>> a fixed offset from the TCB).  The static TLS area is placed on the
>> user-supplied stack.
>
> You said earlier in the email thread that user-supplied stack can be
> reclaimed by __free_tcb () while the thread still runs, am I correct ?
> If so, then we really want to unregister the rseq TLS before that.

No, dynamic TLS can be reclaimed.  Static TLS (which I assume includes
initial-exec TLS) is not deallocated.

> I notice that __free_tcb () calls __deallocate_stack (), which invokes
> _dl_deallocate_tls (). Accessing the TLS from the kernel upon preemption
> would appear fragile after this call.

_dl_deallocate_tls only covers dynamic TLS.

Thanks,
Florian