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From: Sami Tolvanen <samitolvanen@google.com>
To: Arnd Bergmann <arnd@kernel.org>
Cc: Masahiro Yamada <masahiroy@kernel.org>,
	Steven Rostedt <rostedt@goodmis.org>,
	Will Deacon <will@kernel.org>,
	Josh Poimboeuf <jpoimboe@redhat.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	"Paul E. McKenney" <paulmck@kernel.org>,
	Kees Cook <keescook@chromium.org>,
	Nick Desaulniers <ndesaulniers@google.com>,
	clang-built-linux <clang-built-linux@googlegroups.com>,
	Kernel Hardening <kernel-hardening@lists.openwall.com>,
	linux-arch <linux-arch@vger.kernel.org>,
	Linux ARM <linux-arm-kernel@lists.infradead.org>,
	Linux Kbuild mailing list <linux-kbuild@vger.kernel.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	linux-pci <linux-pci@vger.kernel.org>
Subject: Re: [PATCH v8 00/16] Add support for Clang LTO
Date: Wed, 9 Dec 2020 08:25:13 -0800	[thread overview]
Message-ID: <CABCJKud7ZC7_rXVmrF5PnDOMZTJX9iB7uYAa03YF-dkEojnBxg@mail.gmail.com> (raw)
In-Reply-To: <CAK8P3a2DYDCjkqf7oqWFfBT_=rjyJGgnh6kBzUkR8GyvxsB6uQ@mail.gmail.com>

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:
> >
> > - one build seems to have dropped all symbols the string operations
> > from vmlinux,
> >   so while the link goes through, modules cannot be loaded:
> >  ERROR: modpost: "memmove" [drivers/media/rc/rc-core.ko] undefined!
> >  ERROR: modpost: "memcpy" [net/wireless/cfg80211.ko] undefined!
> >  ERROR: modpost: "memcpy" [net/8021q/8021q.ko] undefined!
> >  ERROR: modpost: "memset" [net/8021q/8021q.ko] undefined!
> >  ERROR: modpost: "memcpy" [net/unix/unix.ko] undefined!
> >  ERROR: modpost: "memset" [net/sched/cls_u32.ko] undefined!
> >  ERROR: modpost: "memcpy" [net/sched/cls_u32.ko] undefined!
> >  ERROR: modpost: "memset" [net/sched/sch_skbprio.ko] undefined!
> >  ERROR: modpost: "memcpy" [net/802/garp.ko] undefined!
> >  I first thought this was related to a clang-12 bug I saw the other day, but
> >  this also happens with clang-11
>
> 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.

Sami

  reply	other threads:[~2020-12-09 16:26 UTC|newest]

Thread overview: 48+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-12-01 21:36 [PATCH v8 00/16] Add support for Clang LTO Sami Tolvanen
2020-12-01 21:36 ` [PATCH v8 01/16] tracing: move function tracer options to Kconfig Sami Tolvanen
2020-12-01 21:47   ` Steven Rostedt
2020-12-01 21:36 ` [PATCH v8 02/16] kbuild: add support for Clang LTO Sami Tolvanen
2020-12-02  2:59   ` Masahiro Yamada
2020-12-03  0:07   ` Nick Desaulniers
2020-12-01 21:36 ` [PATCH v8 03/16] kbuild: lto: fix module versioning Sami Tolvanen
2020-12-01 21:36 ` [PATCH v8 04/16] kbuild: lto: limit inlining Sami Tolvanen
2020-12-01 21:36 ` [PATCH v8 05/16] kbuild: lto: merge module sections Sami Tolvanen
2020-12-01 21:36 ` [PATCH v8 06/16] kbuild: lto: remove duplicate dependencies from .mod files Sami Tolvanen
2020-12-01 21:36 ` [PATCH v8 07/16] init: lto: ensure initcall ordering Sami Tolvanen
2020-12-01 21:36 ` [PATCH v8 08/16] init: lto: fix PREL32 relocations Sami Tolvanen
2020-12-01 21:37 ` [PATCH v8 09/16] PCI: Fix PREL32 relocations for LTO Sami Tolvanen
2020-12-01 21:37 ` [PATCH v8 10/16] modpost: lto: strip .lto from module names Sami Tolvanen
2020-12-01 21:37 ` [PATCH v8 11/16] scripts/mod: disable LTO for empty.c Sami Tolvanen
2020-12-01 21:37 ` [PATCH v8 12/16] efi/libstub: disable LTO Sami Tolvanen
2020-12-01 21:37 ` [PATCH v8 13/16] drivers/misc/lkdtm: disable LTO for rodata.o Sami Tolvanen
2020-12-01 21:37 ` [PATCH v8 14/16] arm64: vdso: disable LTO Sami Tolvanen
2020-12-01 21:37 ` [PATCH v8 15/16] arm64: disable recordmcount with DYNAMIC_FTRACE_WITH_REGS Sami Tolvanen
2020-12-01 21:37 ` [PATCH v8 16/16] arm64: allow LTO to be selected Sami Tolvanen
2020-12-03  0:01 ` [PATCH v8 00/16] Add support for Clang LTO Nick Desaulniers
2020-12-03 11:26 ` Will Deacon
2020-12-03 17:07   ` Sami Tolvanen
2020-12-03 18:21     ` Nathan Chancellor
2020-12-03 18:22     ` Will Deacon
2020-12-03 22:32       ` Nick Desaulniers
2020-12-04  9:35         ` Will Deacon
2020-12-04 22:52         ` Sami Tolvanen
2020-12-06  6:50           ` Nathan Chancellor
2020-12-06 20:09             ` Sami Tolvanen
2020-12-08  0:46               ` Nathan Chancellor
2020-12-08 12:15 ` Arnd Bergmann
2020-12-08 13:54   ` Arnd Bergmann
2020-12-08 16:53     ` Sami Tolvanen
2020-12-08 16:56       ` Arnd Bergmann
2020-12-08 16:43   ` Sami Tolvanen
     [not found]     ` <CAK8P3a1Xfpt7QLkvxjtXKcgzcWkS8g9bmxD687+rqjTafTzKrg@mail.gmail.com>
2020-12-08 21:09       ` Nick Desaulniers
2020-12-08 22:20         ` Arnd Bergmann
2020-12-09 16:11           ` Sami Tolvanen
     [not found]       ` <CAK8P3a3O65m6Us=YvCP3QA+0kqAeEqfi-DLOJa+JYmBqs8-JcA@mail.gmail.com>
2020-12-09  5:23         ` Fāng-ruì Sòng
2020-12-09  9:07           ` Arnd Bergmann
2020-12-09 16:09         ` Sami Tolvanen
2020-12-09 19:24           ` Arnd Bergmann
2020-12-09  4:55     ` Fangrui Song
2020-12-09  9:19       ` Arnd Bergmann
2020-12-09 12:35   ` Arnd Bergmann
2020-12-09 16:25     ` Sami Tolvanen [this message]
2020-12-09 17:51       ` Arnd Bergmann

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