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Message-ID: <CAK8P3a04nQwJkK-CPWNzvfKavZnHQYzKX5OGB7Rm3Ee_62oXhA@mail.gmail.com>
Date: Wed, 9 Dec 2020 18:51:34 +0100
From: Arnd Bergmann <arnd@...nel.org>
To: Sami Tolvanen <samitolvanen@...gle.com>
Cc: Masahiro Yamada <masahiroy@...nel.org>, Steven Rostedt <rostedt@...dmis.org>,
Will Deacon <will@...nel.org>, Josh Poimboeuf <jpoimboe@...hat.com>,
Peter Zijlstra <peterz@...radead.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 <clang-built-linux@...glegroups.com>,
Kernel Hardening <kernel-hardening@...ts.openwall.com>,
linux-arch <linux-arch@...r.kernel.org>,
Linux ARM <linux-arm-kernel@...ts.infradead.org>,
Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, linux-pci <linux-pci@...r.kernel.org>
Subject: Re: [PATCH v8 00/16] Add support for Clang LTO
On Wed, Dec 9, 2020 at 5:25 PM 'Sami Tolvanen' via Clang Built Linux
<clang-built-linux@...glegroups.com> wrote:
>
> On Wed, Dec 9, 2020 at 4:36 AM Arnd Bergmann <arnd@...nel.org> wrote:
> >
> > On Tue, Dec 8, 2020 at 1:15 PM Arnd Bergmann <arnd@...nel.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
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