Re: [PATCH v8 00/16] Add support for Clang LTO

Arnd Bergmann
2020-12-09T17:53:15Z

On Wed, Dec 9, 2020 at 5:25 PM 'Sami Tolvanen' via Clang Built Linux
<clang-built-linux@googlegroups.com> wrote:
>
> On Wed, Dec 9, 2020 at 4:36 AM Arnd Bergmann <arnd@kernel.org> wrote:
> >
> > On Tue, Dec 8, 2020 at 1:15 PM Arnd Bergmann <arnd@kernel.org> wrote:
> >
> >
> > It seems to happen because of CONFIG_TRIM_UNUSED_KSYMS,
> > which is a shame, since I think that is an option we'd always want to
> > have enabled with LTO, to allow more dead code to be eliminated.
>
> Ah yes, this is a known issue. We use TRIM_UNUSED_KSYMS with LTO in
> Android's Generic Kernel Image and the problem is that bitcode doesn't
> yet contain calls to these functions, so autoksyms won't see them. The
> solution is to use a symbol whitelist with LTO to prevent these from
> being trimmed. I suspect we would need a default whitelist for LTO
> builds.

A built-in allowlist sounds good to me. FWIW, in the randconfigs so far, I only
saw five symbols that would need to be on it:

memcpy(), memmove(), memset(), __stack_chk_fail() and __stack_chk_guard

Arnd