llvm dev - Mar 2017 - [lld] elf linker creates undefined empty symbol

Sean Silva <chisophugis at gmail.com> writes:
>> When __libc_start_main is hidden, it doesn't end up in the dynamic
symbol
>> table and so we use the default DynsymIndex of 0.
>>
>
>
> I think BFD is doing the right thing. The `extern_weak hidden` is resolved
> to null since by virtue of being hidden it doesn't refer to the
function in
> libc.so (and there is no definition in the executable). For some reason,
> LLD thinks that it needs to resolve __libc_start_main dynamically and
> things go horribly wrong. Rafael, what do you think?
If it is hidden it should really not end up in the dynamic symbol
table. I will try to take a look at your reproducible latter in the day.

Cheers,
Rafael

On Wed, Feb 22, 2017 at 6:14 AM, Rafael Avila de Espindola <
rafael.espindola at gmail.com> wrote:
> Sean Silva <chisophugis at gmail.com> writes:
> >> When __libc_start_main is hidden, it doesn't end up in the
dynamic
> symbol
> >> table and so we use the default DynsymIndex of 0.
> >>
> >
> >
> > I think BFD is doing the right thing. The `extern_weak hidden` is
> resolved
> > to null since by virtue of being hidden it doesn't refer to the
function
> in
> > libc.so (and there is no definition in the executable). For some
reason,
> > LLD thinks that it needs to resolve __libc_start_main dynamically and
> > things go horribly wrong. Rafael, what do you think?
>
> If it is hidden it should really not end up in the dynamic symbol
> table. I will try to take a look at your reproducible latter in the day.
>

FWIW, gold issues an error:

/usr/bin/ld.gold: error: hidden symbol '__libc_start_main' is not
defined
locally
/usr/bin/ld.gold: error: hidden symbol '__libc_start_main' is not
defined
locally

In theory `extern_weak hidden` could still fail to have a local definition
and then it would resolve to 0 due to being weak, which is what BFD does.
But `extern_weak hidden` is still pretty questionable. E.g. in the original
case that Carlo ran into, this was not the expected behavior (and this
error may have saved him some time actually).

-- Sean Silva

>
> Cheers,
> Rafael
>
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Rafael, did you ever get a chance to look at this?

-- Sean Silva

On Thu, Feb 23, 2017 at 1:14 AM, Sean Silva <chisophugis at gmail.com>
wrote:
>
>
> On Wed, Feb 22, 2017 at 6:14 AM, Rafael Avila de Espindola <
> rafael.espindola at gmail.com> wrote:
>
>> Sean Silva <chisophugis at gmail.com> writes:
>> >> When __libc_start_main is hidden, it doesn't end up in the
dynamic
>> symbol
>> >> table and so we use the default DynsymIndex of 0.
>> >>
>> >
>> >
>> > I think BFD is doing the right thing. The `extern_weak hidden` is
>> resolved
>> > to null since by virtue of being hidden it doesn't refer to
the
>> function in
>> > libc.so (and there is no definition in the executable). For some
reason,
>> > LLD thinks that it needs to resolve __libc_start_main dynamically
and
>> > things go horribly wrong. Rafael, what do you think?
>>
>> If it is hidden it should really not end up in the dynamic symbol
>> table. I will try to take a look at your reproducible latter in the
day.
>>
>
>
> FWIW, gold issues an error:
>
> /usr/bin/ld.gold: error: hidden symbol '__libc_start_main' is not
defined
> locally
> /usr/bin/ld.gold: error: hidden symbol '__libc_start_main' is not
defined
> locally
>
> In theory `extern_weak hidden` could still fail to have a local definition
> and then it would resolve to 0 due to being weak, which is what BFD does.
> But `extern_weak hidden` is still pretty questionable. E.g. in the original
> case that Carlo ran into, this was not the expected behavior (and this
> error may have saved him some time actually).
>
> -- Sean Silva
>
>
>>
>> Cheers,
>> Rafael
>>
>
>
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