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

Rich Felker
2018-11-26T17:08:45Z

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