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

Mathieu Desnoyers
2018-11-26T15:51:13Z

----- 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 ?

Thanks,

Mathieu

--
Mathieu Desnoyers
EfficiOS Inc.