linux-kernel - Re: [PATCH 04/22] kbuild: lto: fix recordmcount

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Message-ID: <20200625074530.GW4817@hirez.programming.kicks-ass.net>
Date:   Thu, 25 Jun 2020 09:45:30 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Sami Tolvanen <samitolvanen@...gle.com>
Cc:     Steven Rostedt <rostedt@...dmis.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@...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 04/22] kbuild: lto: fix recordmcount

On Wed, Jun 24, 2020 at 02:45:30PM -0700, Sami Tolvanen wrote:
> On Wed, Jun 24, 2020 at 11:27:37PM +0200, Peter Zijlstra wrote:
> > On Wed, Jun 24, 2020 at 01:31:42PM -0700, Sami Tolvanen wrote:
> > > With LTO, LLVM bitcode won't be compiled into native code until
> > > modpost_link. This change postpones calls to recordmcount until after
> > > this step.
> > > 
> > > In order to exclude specific functions from inspection, we add a new
> > > code section .text..nomcount, which we tell recordmcount to ignore, and
> > > a __nomcount attribute for moving functions to this section.
> > 
> > I'm confused, you only add this to functions in ftrace itself, which is
> > compiled with:
> > 
> >  KBUILD_CFLAGS = $(subst $(CC_FLAGS_FTRACE),,$(ORIG_CFLAGS))
> > 
> > and so should not have mcount/fentry sites anyway. So what's the point
> > of ignoring them further?
> > 
> > This Changelog does not explain.
> 
> Normally, recordmcount ignores each ftrace.o file, but since we are
> running it on vmlinux.o, we need another way to stop it from looking
> at references to mcount/fentry that are not calls. Here's a comment
> from recordmcount.c:
> 
>   /*
>    * The file kernel/trace/ftrace.o references the mcount
>    * function but does not call it. Since ftrace.o should
>    * not be traced anyway, we just skip it.
>    */
> 
> But I agree, the commit message could use more defails. Also +Steven
> for thoughts about this approach.

Ah, is thi because recordmcount isn't smart enough to know the
difference between "CALL $mcount" and any other RELA that has mcount?

At least for x86_64 I can do a really quick take for a recordmcount pass
in objtool, but I suppose you also need this for ARM64 ?