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: <871s7bsmf1.fsf@oldenburg.str.redhat.com>
Date:   Fri, 23 Nov 2018 19:01:38 +0100
From:   Florian Weimer <fweimer@...hat.com>
To:     Rich Felker <dalias@...c.org>
Cc:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        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

* Rich Felker:

> On Fri, Nov 23, 2018 at 06:39:04PM +0100, Florian Weimer wrote:
>> * Rich Felker:
>> 
>> > On Fri, Nov 23, 2018 at 12:05:20PM -0500, Mathieu Desnoyers wrote:
>> >> There has been presumptions about signals being blocked when the thread
>> >> exits throughout this email thread. Out of curiosity, what code is
>> >> responsible for disabling signals in this situation ? Related to this,
>> >> is it valid to access a IE model TLS variable from a signal handler at
>> >> _any_ point where the signal handler nests over thread's execution ?
>> >> This includes early start and just before invoking the exit system call.
>> >
>> > It should be valid to access *any* TLS object like this, but the
>> > standards don't cover it well.
>> 
>> C++ makes it undefined:
>> 
>>   <http://eel.is/c++draft/support.signal#def:evaluation,signal-safe>
>
> C also leaves access to pretty much anything from a signal handler
> undefined, but that makes signals basically useless.

Access to atomic variables of thread storage duration is defined,
though.

Thanks,
Florian