Re: [PATCH v2 08/28] x86, build: use objtool mcount

Sami Tolvanen
2020-09-03T22:12:27Z

On Thu, Sep 3, 2020 at 2:58 PM Kees Cook <keescook@chromium.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@google.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