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

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Date:   Tue, 23 Mar 2021 16:36:21 +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 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. 

> Are you compiling the kernel with -ffunction-sections and/or -fdata-sections?

I tried both.  Default off, and forcing those flags by hacking the Makefile had
no effect.

> Does this issue only happen with gcc 10?

gcc-7 shows the same behavior, I haven't checked anything older or anything in
between.