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

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Message-ID: <202104011433.48B4149E@keescook>
Date:   Thu, 1 Apr 2021 14:33:21 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Sami Tolvanen <samitolvanen@...gle.com>,
        Fangrui Song <maskray@...gle.com>,
        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>
Subject: Re: [PATCH] kbuild: Merge module sections if and only if
 CONFIG_LTO_CLANG is enabled

On Wed, Mar 31, 2021 at 08:07:18PM +0000, Sean Christopherson wrote:
> On Wed, Mar 31, 2021, Kees Cook wrote:
> > On Wed, Mar 24, 2021 at 10:45:36PM +0000, Sean Christopherson wrote:
> > > 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.
> > 
> > Are you able to open a bug against bfd for this?
> 
> Yes?  I'll ping you when I run into trouble ;-)

Hm, I can't tell if any of these are duplicates:

https://sourceware.org/bugzilla/buglist.cgi?bug_status=UNCONFIRMED&bug_status=NEW&bug_status=ASSIGNED&bug_status=SUSPENDED&bug_status=WAITING&bug_status=REOPENED&component=ld&list_id=59962&product=binutils&query_format=advanced&short_desc=symbol&short_desc_type=allwordssubstr

-- 
Kees Cook