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Re: [RFC PATCH v4 1/5] glibc: Perform rseq(2) registration at nptl init and thread creation
- From: Mathieu Desnoyers <mathieu dot desnoyers at efficios dot com>
- To: Florian Weimer <fweimer at redhat dot com>
- Cc: Rich Felker <dalias at libc dot org>, carlos <carlos at redhat dot com>, Joseph Myers <joseph at codesourcery dot com>, Szabolcs Nagy <szabolcs dot nagy at arm dot com>, libc-alpha <libc-alpha at sourceware dot org>, Thomas Gleixner <tglx at linutronix dot de>, Ben Maurer <bmaurer at fb dot com>, Peter Zijlstra <peterz at infradead dot org>, "Paul E. McKenney" <paulmck at linux dot vnet dot ibm dot com>, Boqun Feng <boqun dot feng at gmail dot com>, Will Deacon <will dot deacon at arm dot com>, Dave Watson <davejwatson at fb dot com>, Paul Turner <pjt at google dot com>, linux-kernel <linux-kernel at vger dot kernel dot org>, linux-api <linux-api at vger dot kernel dot org>
- Date: Mon, 26 Nov 2018 11:30:51 -0500 (EST)
- Subject: Re: [RFC PATCH v4 1/5] glibc: Perform rseq(2) registration at nptl init and thread creation
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----- On Nov 26, 2018, at 10:51 AM, Mathieu Desnoyers firstname.lastname@example.org wrote:
> ----- On Nov 26, 2018, at 3:28 AM, Florian Weimer email@example.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
> /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 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."
> Mathieu Desnoyers
> EfficiOS Inc.