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

[Sean Christopherson]

On Tue, Mar 23, 2021, Sami Tolvanen wrote:
> On Tue, Mar 23, 2021 at 9:36 AM Sean Christopherson <seanjc@xxxxxxxxxx> wrote:
> >
> > On Tue, Mar 23, 2021, Sami Tolvanen wrote:
> > > On Mon, Mar 22, 2021 at 4:44 PM Sean Christopherson <seanjc@xxxxxxxxxx> 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.

FWIW, clang + bfd also produces mangled tables, so it does appear to be bfd
specific.

> 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@xxxxxxxxxx>
> Tested-by: Sami Tolvanen <samitolvanen@xxxxxxxxxx>
> 
> Sami