linux-kernel - Re: [PATCH] kbuild: Merge module sections if and only if CONFIG_LTO

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Date:   Tue, 23 Mar 2021 11:14:05 -0700
From:   Sami Tolvanen <samitolvanen@...gle.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Masahiro Yamada <masahiroy@...nel.org>,
        Michal Marek <michal.lkml@...kovi.net>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        linux-kbuild <linux-kbuild@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Kees Cook <keescook@...omium.org>
Subject: Re: [PATCH] kbuild: Merge module sections if and only if
 CONFIG_LTO_CLANG is enabled

On Tue, Mar 23, 2021 at 9:36 AM Sean Christopherson <seanjc@...gle.com> wrote:
>
> On Tue, Mar 23, 2021, Sami Tolvanen wrote:
> > On Mon, Mar 22, 2021 at 4:44 PM Sean Christopherson <seanjc@...gle.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@...gle.com>
Tested-by: Sami Tolvanen <samitolvanen@...gle.com>

Sami