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

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Date:   Mon, 26 Nov 2018 12:07:58 -0500
From:   Rich Felker <dalias@...c.org>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     Florian Weimer <fweimer@...hat.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

On Mon, Nov 26, 2018 at 11:30:51AM -0500, Mathieu Desnoyers wrote:
> ----- On Nov 26, 2018, at 10:51 AM, Mathieu Desnoyers mathieu.desnoyers@...icios.com wrote:
> 
> > ----- 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 ?
> 
> Actually, looking into ld(1) --warn-common, it looks like "weak" would be cleaner
> after all, especially for __rseq_abi which we needs to be initialized to a specific
> value, which is therefore not a common symbol.
> 
> "      --warn-common
>            Warn when a common symbol is combined with another common symbol or with a symbol definition.  Unix
>            linkers allow this somewhat sloppy practice, but linkers on some other operating systems do not.
>            This option allows you to find potential problems from combining global symbols.  Unfortunately,
>            some C libraries use this practice, so you may get some warnings about symbols in the libraries as
>            well as in your programs."
> 
> Thoughts ?

AFAIK this has nothing to do with dynamic linking.

Rich