linux-kernel - Re: [PATCH v2 01/28] x86/boot/compressed: Disable relocation relaxation

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Message-ID: <20200904071411.GA1712031@ubuntu-n2-xlarge-x86>
Date:   Fri, 4 Sep 2020 00:14:11 -0700
From:   Nathan Chancellor <natechancellor@...il.com>
To:     Arvind Sankar <nivedita@...m.mit.edu>
Cc:     Kees Cook <keescook@...omium.org>,
        Sami Tolvanen <samitolvanen@...gle.com>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Will Deacon <will@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        clang-built-linux@...glegroups.com,
        kernel-hardening@...ts.openwall.com, linux-arch@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-kbuild@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-pci@...r.kernel.org,
        x86@...nel.org
Subject: Re: [PATCH v2 01/28] x86/boot/compressed: Disable relocation
 relaxation

On Thu, Sep 03, 2020 at 07:42:15PM -0400, Arvind Sankar wrote:
> On Thu, Sep 03, 2020 at 02:44:41PM -0700, Kees Cook wrote:
> > On Thu, Sep 03, 2020 at 01:30:26PM -0700, Sami Tolvanen wrote:
> > > From: Arvind Sankar <nivedita@...m.mit.edu>
> > > 
> > > Patch series [4] is a solution to allow the compressed kernel to be
> > > linked with -pie unconditionally, but even if merged is unlikely to be
> > > backported. As a simple solution that can be applied to stable as well,
> > > prevent the assembler from generating the relaxed relocation types using
> > > the -mrelax-relocations=no option. For ease of backporting, do this
> > > unconditionally.
> > > 
> > > [0] https://gitlab.com/x86-psABIs/x86-64-ABI/-/blob/master/x86-64-ABI/linker-optimization.tex#L65
> > > [1] https://lore.kernel.org/lkml/20200807194100.3570838-1-ndesaulniers@google.com/
> > > [2] https://github.com/ClangBuiltLinux/linux/issues/1121
> > > [3] https://reviews.llvm.org/rGc41a18cf61790fc898dcda1055c3efbf442c14c0
> > > [4] https://lore.kernel.org/lkml/20200731202738.2577854-1-nivedita@alum.mit.edu/
> > > 
> > > Reported-by: Nick Desaulniers <ndesaulniers@...gle.com>
> > > Signed-off-by: Arvind Sankar <nivedita@...m.mit.edu>
> > 
> > Reviewed-by: Kees Cook <keescook@...omium.org>
> > 
> > -- 
> > Kees Cook
> 
> Note that since [4] is now in tip, assuming it doesn't get dropped for
> some reason, this patch isn't necessary unless you need to backport this
> LTO series to 5.9 or below.
> 
> Thanks.

It is still necessary for tip of tree LLVM to work properly
(specifically clang and ld.lld) regardless of whether or not LTO is
used.

[4] also fixes it but I don't think it can be backported to stable so it
would still be nice to get it picked up so that it can be sent back
there. We have been carrying it in our CI for a decent amount of time...

Cheers,
Nathan