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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <202103311228.D42822B@keescook>
Date:   Wed, 31 Mar 2021 12:30:24 -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 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?

> > 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>

Thanks, I'll get this sent to Linus.

-- 
Kees Cook