Re: [PATCH] kbuild: Merge module sections if and only if CONFIG_LTO_CLANG is enabled

Sami Tolvanen
2021-03-23T18:15:02Z

On Tue, Mar 23, 2021 at 9:36 AM Sean Christopherson <seanjc@google.com> wrote:
>
> On Tue, Mar 23, 2021, Sami Tolvanen wrote:
> > On Mon, Mar 22, 2021 at 4:44 PM Sean Christopherson <seanjc@google.com> wrote:
> > >
> > > Merge module sections only when using Clang LTO. With gcc-10, merging
> > > sections does not appear to update the symbol tables for the module,
> > > e.g. 'readelf -s' shows the value that a symbol would have had, if
> > > sections were not merged.
> >
> > I'm fine with limiting this to LTO only, but it would be helpful to
> > understand which sections are actually getting merged here.
>
> It doesn't appear to matter which sections get merged, the tables only show the
> correct data if there is no merging whatsoever, e.g. allowing merging for any
> one of the four types (.bss, .data, .rodata and .text) results in breakage.
> AFAICT, merging any sections causes the layout to change and throw off the
> symbol tables.

Thanks for the clarification. I can reproduce this issue with gcc +
bfd if any of the sections are merged, but gcc + lld produces valid
symbol tables.

Perhaps someone more familiar with bfd can comment on whether this is
a bug or a feature, and if there's a flag we can pass to bfd that
would fix the issue. In the meanwhile, this patch looks like a
reasonable workaround to me.

Reviewed-by: Sami Tolvanen <samitolvanen@google.com>
Tested-by: Sami Tolvanen <samitolvanen@google.com>

Sami