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Date: Mon, 29 Jun 2020 16:20:59 -0700
From: Sami Tolvanen <samitolvanen@...gle.com>
To: Masahiro Yamada <masahiroy@...nel.org>
Cc: Will Deacon <will@...nel.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-kernel <linux-arm-kernel@...ts.infradead.org>,
Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
linux-pci@...r.kernel.org, X86 ML <x86@...nel.org>
Subject: Re: [PATCH 00/22] add support for Clang LTO
Hi Masahiro,
On Mon, Jun 29, 2020 at 01:56:19AM +0900, Masahiro Yamada wrote:
> On Thu, Jun 25, 2020 at 5:32 AM 'Sami Tolvanen' via Clang Built Linux
> <clang-built-linux@...glegroups.com> wrote:
> >
> > This patch series adds support for building x86_64 and arm64 kernels
> > with Clang's Link Time Optimization (LTO).
> >
> > In addition to performance, the primary motivation for LTO is to allow
> > Clang's Control-Flow Integrity (CFI) to be used in the kernel. Google's
> > Pixel devices have shipped with LTO+CFI kernels since 2018.
> >
> > Most of the patches are build system changes for handling LLVM bitcode,
> > which Clang produces with LTO instead of ELF object files, postponing
> > ELF processing until a later stage, and ensuring initcall ordering.
> >
> > Note that first objtool patch in the series is already in linux-next,
> > but as it's needed with LTO, I'm including it also here to make testing
> > easier.
>
>
> I put this series on a testing branch,
> and 0-day bot started reporting some issues.
Yes, I'll fix those issues in v2.
> (but 0-day bot is quieter than I expected.
> Perhaps, 0-day bot does not turn on LLVM=1 ?)
In order for it to test an LTO build, it would need to enable LTO_CLANG
explicitly though, in addition to LLVM=1.
> I also got an error for
> ARCH=arm64 allyesconfig + CONFIG_LTO_CLANG=y
>
>
>
> $ make ARCH=arm64 LLVM=1 LLVM_IAS=1
> CROSS_COMPILE=~/tools/aarch64-linaro-7.5/bin/aarch64-linux-gnu-
> -j24
>
> ...
>
> GEN .version
> CHK include/generated/compile.h
> UPD include/generated/compile.h
> CC init/version.o
> AR init/built-in.a
> GEN .tmp_initcalls.lds
> GEN .tmp_symversions.lds
> LTO vmlinux.o
> MODPOST vmlinux.symvers
> MODINFO modules.builtin.modinfo
> GEN modules.builtin
> LD .tmp_vmlinux.kallsyms1
> ld.lld: error: undefined symbol: __compiletime_assert_905
> >>> referenced by irqbypass.c
> >>> vmlinux.o:(jeq_imm)
> make: *** [Makefile:1161: vmlinux] Error 1
I can reproduce this with ToT LLVM and it's BUILD_BUG_ON_MSG(..., "value
too large for the field") in drivers/net/ethernet/netronome/nfp/bpf/jit.c.
Specifically, the FIELD_FIT / __BF_FIELD_CHECK macro in ur_load_imm_any.
This compiles just fine with an earlier LLVM revision, so it could be a
relatively recent regression. I'll take a look. Thanks for catching this!
Sami
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