linux-kernel - Re: [PATCH 00/22] add support for Clang LTO

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Message-ID: <CABCJKuedpxAqndgL=jHT22KtjnLkb1dsYaM6hQYyhqrWjkEe6A@mail.gmail.com>
Date:   Sun, 12 Jul 2020 16:34:39 -0700
From:   Sami Tolvanen <samitolvanen@...gle.com>
To:     Paul Menzel <pmenzel@...gen.mpg.de>
Cc:     Masahiro Yamada <masahiroy@...nel.org>,
        Will Deacon <will@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>,
        linux-arch <linux-arch@...r.kernel.org>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        linux-kbuild <linux-kbuild@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>, linux-pci@...r.kernel.org,
        X86 ML <x86@...nel.org>
Subject: Re: [PATCH 00/22] add support for Clang LTO

On Sat, Jul 11, 2020 at 9:32 AM Paul Menzel <pmenzel@...gen.mpg.de> wrote:
> Thank you very much for sending these changes.
>
> Do you have a branch, where your current work can be pulled from? Your
> branch on GitHub [1] seems 15 months old.

The clang-lto branch is rebased regularly on top of Linus' tree.
GitHub just looks at the commit date of the last commit in the tree,
which isn't all that informative.

> Out of curiosity, I applied the changes, allowed the selection for i386
> (x86), and with Clang 1:11~++20200701093119+ffee8040534-1~exp1 from
> Debian experimental, it failed with `Invalid absolute R_386_32
> relocation: KERNEL_PAGES`:

I haven't looked at getting this to work on i386, which is why we only
select ARCH_SUPPORTS_LTO for x86_64. I would expect there to be a few
issues to address.

> >   arch/x86/tools/relocs vmlinux > arch/x86/boot/compressed/vmlinux.relocs;arch/x86/tools/relocs --abs-relocs vmlinux
> > Invalid absolute R_386_32 relocation: KERNEL_PAGES

KERNEL_PAGES looks like a constant, so it's probably safe to ignore
the absolute relocation in tools/relocs.c.

Sami