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

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Message-ID: <20200624212737.GV4817@hirez.programming.kicks-ass.net>
Date:   Wed, 24 Jun 2020 23:27:37 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Sami Tolvanen <samitolvanen@...gle.com>
Cc:     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 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.