kernel-hardening - Re: [RFC][PATCH] objtool,x86_64: Replace recordmcount with objtool

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Date: Wed, 22 Jul 2020 13:03:12 -0700
From: Sami Tolvanen <samitolvanen@...gle.com>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: Peter Zijlstra <peterz@...radead.org>, 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>, 
	Josh Poimboeuf <jpoimboe@...hat.com>
Subject: Re: [RFC][PATCH] objtool,x86_64: Replace recordmcount with objtool

On Wed, Jul 22, 2020 at 12:09 PM Steven Rostedt <rostedt@...dmis.org> wrote:
>
> On Wed, 22 Jul 2020 20:41:37 +0200
> Peter Zijlstra <peterz@...radead.org> wrote:
>
> > > That said, Andi Kleen added an option to gcc called -mnop-mcount which
> > > will have gcc do both create the mcount section and convert the calls
> > > into nops. When doing so, it defines CC_USING_NOP_MCOUNT which will
> > > tell ftrace to expect the calls to already be converted.
> >
> > That seems like the much easier solution, then we can forget about
> > recordmcount / objtool entirely for this.
>
> Of course that was only for some gcc compilers, and I'm not sure if
> clang can do this.
>
> Or do you just see all compilers doing this in the future, and not
> worrying about record-mcount at all, and bothering with objtool?

Clang appears to only support -mrecord-mcount and -mnop-mcount for
s390, so we still need recordmcount / objtool for x86.

Sami

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