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

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Date:   Wed, 24 Mar 2021 22:45:36 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Sami Tolvanen <samitolvanen@...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, Sami Tolvanen wrote:
> 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.

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