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

From: Kees Cook <keescook@chromium.org>
To: Sean Christopherson <seanjc@google.com>
Cc: Sami Tolvanen <samitolvanen@google.com>,
	Fangrui Song <maskray@google.com>,
	Masahiro Yamada <masahiroy@kernel.org>,
	Michal Marek <michal.lkml@markovi.net>,
	Nathan Chancellor <nathan@kernel.org>,
	Nick Desaulniers <ndesaulniers@google.com>,
	linux-kbuild <linux-kbuild@vger.kernel.org>,
	LKML <linux-kernel@vger.kernel.org>,
	clang-built-linux <clang-built-linux@googlegroups.com>
Subject: Re: [PATCH] kbuild: Merge module sections if and only if CONFIG_LTO_CLANG is enabled
Date: Wed, 31 Mar 2021 12:30:24 -0700	[thread overview]
Message-ID: <202103311228.D42822B@keescook> (raw)
In-Reply-To: <YFvBEIkuFY2ajNlG@google.com>

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

Thanks, I'll get this sent to Linus.

-- 
Kees Cook