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Message-ID: <20200722184137.GP10769@hirez.programming.kicks-ass.net>
Date: Wed, 22 Jul 2020 20:41:37 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: Sami Tolvanen <samitolvanen@...gle.com>,
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@...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, Josh Poimboeuf <jpoimboe@...hat.com>,
mhelsley@...are.com
Subject: Re: [RFC][PATCH] objtool,x86_64: Replace recordmcount with objtool
On Wed, Jul 22, 2020 at 01:55:42PM -0400, Steven Rostedt wrote:
> > Ha! it is trying to convert the "CALL __fentry__" into a NOP and not
> > finding the CALL -- because objtool already made it a NOP...
> >
> > Weird, I thought recordmcount would also write NOPs, it certainly has
> > code for that. I suppose we can use CC_USING_NOP_MCOUNT to avoid those,
> > but I'd rather Steve explain this before I wreck things further.
>
> The reason for not having recordmcount insert all the nops, is because
> x86 has more than one optimal nop which is determined by the machine it
> runs on, and not at compile time. So we figured just updated it then.
>
> We can change it to be a nop on boot, and just modify it if it's not
> the optimal nop already.
Right, I throught that's what we'd be doing already, anyway:
> 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.
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