Messages in this thread | | | From | Sami Tolvanen <> | Date | Thu, 3 Sep 2020 15:11:54 -0700 | Subject | Re: [PATCH v2 08/28] x86, build: use objtool mcount |
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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
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