Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
MIME-Version: 1.0
References: <20200624203200.78870-1-samitolvanen@google.com>
 <20200624203200.78870-5-samitolvanen@google.com> <20200624212737.GV4817@hirez.programming.kicks-ass.net>
 <20200624214530.GA120457@google.com> <20200625074530.GW4817@hirez.programming.kicks-ass.net>
 <20200625161503.GB173089@google.com> <20200625200235.GQ4781@hirez.programming.kicks-ass.net>
 <20200625224042.GA169781@google.com> <20200626112931.GF4817@hirez.programming.kicks-ass.net>
In-Reply-To: <20200626112931.GF4817@hirez.programming.kicks-ass.net>
From:   Sami Tolvanen <samitolvanen@google.com>
Date:   Fri, 17 Jul 2020 10:28:13 -0700
Message-ID: <CABCJKucSM7gqWmUtiBPbr208wB0pc25afJXc6yBQzJDZf4LSWA@mail.gmail.com>
Subject: Re: [RFC][PATCH] objtool,x86_64: Replace recordmcount with objtool
To:     Steven Rostedt <rostedt@goodmis.org>,
        Peter Zijlstra <peterz@infradead.org>
Cc:     Masahiro Yamada <masahiroy@kernel.org>,
        Will Deacon <will@kernel.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        "Paul E. McKenney" <paulmck@kernel.org>,
        Kees Cook <keescook@chromium.org>,
        Nick Desaulniers <ndesaulniers@google.com>,
        clang-built-linux <clang-built-linux@googlegroups.com>,
        Kernel Hardening <kernel-hardening@lists.openwall.com>,
        linux-arch <linux-arch@vger.kernel.org>,
        linux-arm-kernel <linux-arm-kernel@lists.infradead.org>,
        linux-kbuild <linux-kbuild@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>, linux-pci@vger.kernel.org,
        X86 ML <x86@kernel.org>, Josh Poimboeuf <jpoimboe@redhat.com>,
        Matt Helsley <mhelsley@vmware.com>
Content-Type: text/plain; charset="UTF-8"
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Fri, Jun 26, 2020 at 4:29 AM Peter Zijlstra <peterz@infradead.org> wrote:
>
> On Thu, Jun 25, 2020 at 03:40:42PM -0700, Sami Tolvanen wrote:
>
> > > Not boot tested, but it generates the required sections and they look
> > > more or less as expected, ymmv.
>
> > > diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> > > index a291823f3f26..189575c12434 100644
> > > --- a/arch/x86/Kconfig
> > > +++ b/arch/x86/Kconfig
> > > @@ -174,7 +174,6 @@ config X86
> > >     select HAVE_EXIT_THREAD
> > >     select HAVE_FAST_GUP
> > >     select HAVE_FENTRY                      if X86_64 || DYNAMIC_FTRACE
> > > -   select HAVE_FTRACE_MCOUNT_RECORD
> > >     select HAVE_FUNCTION_GRAPH_TRACER
> > >     select HAVE_FUNCTION_TRACER
> > >     select HAVE_GCC_PLUGINS
> >
> > This breaks DYNAMIC_FTRACE according to kernel/trace/ftrace.c:
> >
> >   #ifndef CONFIG_FTRACE_MCOUNT_RECORD
> >   # error Dynamic ftrace depends on MCOUNT_RECORD
> >   #endif
> >
> > And the build errors after that seem to confirm this. It looks like we might
> > need another flag to skip recordmcount.
>
> Hurm, Steve, how you want to do that?

Steven, did you have any thoughts about this? Moving recordmcount to
an objtool pass that knows about call sites feels like a much cleaner
solution than annotating kernel code to avoid unwanted relocations.

Sami