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Message-ID: <CABCJKufxq2b0854MBA_Kkb0B1k5D1Z431a=m=w2zoOVUh2c==Q@mail.gmail.com>
Date: Thu, 3 Sep 2020 15:11:54 -0700
From: Sami Tolvanen <samitolvanen@...gle.com>
To: Kees Cook <keescook@...omium.org>
Cc: Masahiro Yamada <masahiroy@...nel.org>, Will Deacon <will@...nel.org>,
Peter Zijlstra <peterz@...radead.org>, Steven Rostedt <rostedt@...dmis.org>,
Greg Kroah-Hartman <gregkh@...uxfoundation.org>, "Paul E. McKenney" <paulmck@...nel.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>
Subject: Re: [PATCH v2 08/28] x86, build: use objtool mcount
On Thu, Sep 3, 2020 at 2:58 PM Kees Cook <keescook@...omium.org> wrote:
>
> On Thu, Sep 03, 2020 at 01:30:33PM -0700, Sami Tolvanen wrote:
> > Select HAVE_OBJTOOL_MCOUNT if STACK_VALIDATION is selected to use
> > objtool to generate __mcount_loc sections for dynamic ftrace with
> > Clang and gcc <5.
> >
> > Signed-off-by: Sami Tolvanen <samitolvanen@...gle.com>
>
> Am I right to understand that this fixes mcount for Clang generally
> (i.e. it's not _strictly_ related to LTO, though LTO depends on this
> change)?
No, this works fine with Clang when LTO is disabled, because
recordmcount ignores files named "ftrace.o". However, with LTO, we
process vmlinux.o instead, so we need a different method of ignoring
__fentry__ relocations that are not calls.
In v1, I used a function attribute to whitelist functions that refer
to __fentry__, but as Peter pointed out back then, objtool already
knows where the call sites are, so using it to generate __mcount_loc
is cleaner.
> And does this mean mcount was working for gcc < 5?
Yes. I should have been clearer in the commit message. The reason I
mentioned gcc <5 is that later gcc versions support -mrecord-mcount,
which means they don't need an external tool for generating
__mcount_loc anymore.
Sami
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