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: <688718071.12798.1543247469553.JavaMail.zimbra@efficios.com>
Date:   Mon, 26 Nov 2018 10:51:09 -0500 (EST)
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     Florian Weimer <fweimer@...hat.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

----- On Nov 26, 2018, at 3:28 AM, Florian Weimer fweimer@...hat.com wrote:

> * Mathieu Desnoyers:
> 
>> Using a "weak" symbol in early adopter libraries is important, so they
>> can be loaded together into the same process without causing loader
>> errors due to many definitions of the same strong symbol.
> 
> This is not how ELF dynamic linking works.  If the symbol name is the
> same, one definition interposes the others.
> 
> You need to ensure that the symbol has the same size everywhere, though.
> There are some tricky interactions with symbol versions, too.  (The
> interposing libraries must not use symbol versioning.)

I was under the impression that loading the same strong symbol into an
application multiple times would cause some kind of warning if non-weak. I did
some testing to figure out which case I remembered would cause this.

When compiling with "-fno-common", dynamic and static linking work fine, but
trying to add multiple instances of a given symbol into a single object fails
with:

/tmp/ccSakXZV.o:(.bss+0x0): multiple definition of `a'
/tmp/ccQBJBOo.o:(.bss+0x0): first defined here

Even if the symbol has the same size.

So considering that we don't care about compiling into a single object here,
and only care about static and dynamic linking of libraries, indeed the "weak"
symbol is not useful.

So let's make __rseq_abi and __rseq_refcount strong symbols then ?

Thanks,

Mathieu

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