linux-kernel - Re: [PATCH 1/5] glibc: Perform rseq(2) registration at C startup and thread creation (v10)

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Date:   Fri, 14 Jun 2019 09:59:04 -0400 (EDT)
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     Florian Weimer <fweimer@...hat.com>
Cc:     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>,
        Rich Felker <dalias@...c.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-api <linux-api@...r.kernel.org>
Subject: Re: [PATCH 1/5] glibc: Perform rseq(2) registration at C startup
 and thread creation (v10)

----- On Jun 14, 2019, at 3:53 PM, Florian Weimer fweimer@...hat.com wrote:

> * Mathieu Desnoyers:
> 
>> ----- On Jun 14, 2019, at 3:42 PM, Florian Weimer fweimer@...hat.com wrote:
>>
>>> * Mathieu Desnoyers:
>>> 
>>>> +  /* Publicize rseq registration ownership.  This must be performed
>>>> +     after rtld re-relocation, before invoking constructors of
>>>> +     preloaded libraries.  */
>>>> +  rseq_init ();
>>> 
>>> Please add a comment that IFUNC resolvers do not see the initialized
>>> value.  I think this is okay because we currently do not support access
>>> to extern variables in IFUNC resolvers.
>>
>> Do IFUNC resolvers happen to observe the __rseq_handled address that
>> was internal to ld.so ?
> 
> They should observe the correct address, but they can access the
> variable before initialization.  An initializer in ld.so will not have
> an effect if an interposed definition initalized the variable to
> something else.
> 
>> If so, we could simply initialize __rseq_handled twice: early before calling
>> IFUNC resolvers, and after ld.so re-relocation.
> 
> No, I don't think this will make a difference.

So comment it is:

  /* Publicize rseq registration ownership.  This must be performed
     after rtld re-relocation, before invoking constructors of
     preloaded libraries. IFUNC resolvers are called before this
     initialization, so they may not observe the initialized state.  */
  rseq_init ();

Thanks,

Mathieu


-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com