kernel-hardening - Re: [PATCH v5 00/29] Add support for Clang LTO

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Message-ID: <CABCJKueGW5UeH1++ES7ZRDcAnZ6hV-tFVwt6usjcZUnR95YQPQ@mail.gmail.com>
Date: Fri, 9 Oct 2020 17:00:51 -0700
From: Sami Tolvanen <samitolvanen@...gle.com>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: Masahiro Yamada <masahiroy@...nel.org>, Will Deacon <will@...nel.org>, 
	Peter Zijlstra <peterz@...radead.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 v5 00/29] Add support for Clang LTO

On Fri, Oct 9, 2020 at 4:38 PM Steven Rostedt <rostedt@...dmis.org> wrote:
>
> On Fri, 9 Oct 2020 14:05:48 -0700
> Sami Tolvanen <samitolvanen@...gle.com> wrote:
>
> > Ah yes, X86_DECODER_SELFTEST seems to be broken in tip/master. If you
> > prefer, I have these patches on top of mainline here:
> >
> >   https://github.com/samitolvanen/linux/tree/clang-lto
> >
> > Testing your config with LTO on this tree, it does build and boot for
> > me, although I saw a couple of new objtool warnings, and with LLVM=1,
> > one warning from llvm-objdump.
>
> Thanks, I disabled X86_DECODER_SELFTEST and it now builds.
>
> I forced the objdump mcount logic with the below patch, which produces:
>
> CONFIG_FTRACE_MCOUNT_RECORD=y
> CONFIG_FTRACE_MCOUNT_USE_OBJTOOL=y
>
> But I don't see the __mcount_loc sections being created.
>
> I applied patches 1 - 6.

Patch 6 is missing the part where we actually pass --mcount to
objtool, it's in patch 11 ("kbuild: lto: postpone objtool"). I'll fix
this in v6. In the meanwhile, please apply patches 1-11 to test the
objtool change. Do you have any thoughts about the approach otherwise?

Sami

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