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Date:   Sun, 28 Jul 2019 13:09:03 +0200
From:   Sedat Dilek <sedat.dilek@...il.com>
To:     Yonghong Song <yhs@...com>
Cc:     Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Martin Lau <kafai@...com>, Song Liu <songliubraving@...com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "bpf@...r.kernel.org" <bpf@...r.kernel.org>,
        Clang-Built-Linux ML <clang-built-linux@...glegroups.com>,
        Kees Cook <keescook@...omium.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Nathan Chancellor <natechancellor@...il.com>
Subject: Re: next-20190723: bpf/seccomp - systemd/journald issue?

On Sat, Jul 27, 2019 at 7:08 PM Yonghong Song <yhs@...com> wrote:
>
>
>
> On 7/27/19 12:36 AM, Sedat Dilek wrote:
> > On Sat, Jul 27, 2019 at 4:24 AM Alexei Starovoitov
> > <alexei.starovoitov@...il.com> wrote:
> >>
> >> On Fri, Jul 26, 2019 at 2:19 PM Sedat Dilek <sedat.dilek@...il.com> wrote:
> >>>
> >>> On Fri, Jul 26, 2019 at 11:10 PM Yonghong Song <yhs@...com> wrote:
> >>>>
> >>>>
> >>>>
> >>>> On 7/26/19 2:02 PM, Sedat Dilek wrote:
> >>>>> On Fri, Jul 26, 2019 at 10:38 PM Sedat Dilek <sedat.dilek@...il.com> wrote:
> >>>>>>
> >>>>>> Hi Yonghong Song,
> >>>>>>
> >>>>>> On Fri, Jul 26, 2019 at 5:45 PM Yonghong Song <yhs@...com> wrote:
> >>>>>>>
> >>>>>>>
> >>>>>>>
> >>>>>>> On 7/26/19 1:26 AM, Sedat Dilek wrote:
> >>>>>>>> Hi,
> >>>>>>>>
> >>>>>>>> I have opened a new issue in the ClangBuiltLinux issue tracker.
> >>>>>>>
> >>>>>>> Glad to know clang 9 has asm goto support and now It can compile
> >>>>>>> kernel again.
> >>>>>>>
> >>>>>>
> >>>>>> Yupp.
> >>>>>>
> >>>>>>>>
> >>>>>>>> I am seeing a problem in the area bpf/seccomp causing
> >>>>>>>> systemd/journald/udevd services to fail.
> >>>>>>>>
> >>>>>>>> [Fri Jul 26 08:08:43 2019] systemd[453]: systemd-udevd.service: Failed
> >>>>>>>> to connect stdout to the journal socket, ignoring: Connection refused
> >>>>>>>>
> >>>>>>>> This happens when I use the (LLVM) LLD ld.lld-9 linker but not with
> >>>>>>>> BFD linker ld.bfd on Debian/buster AMD64.
> >>>>>>>> In both cases I use clang-9 (prerelease).
> >>>>>>>
> >>>>>>> Looks like it is a lld bug.
> >>>>>>>
> >>>>>>> I see the stack trace has __bpf_prog_run32() which is used by
> >>>>>>> kernel bpf interpreter. Could you try to enable bpf jit
> >>>>>>>      sysctl net.core.bpf_jit_enable = 1
> >>>>>>> If this passed, it will prove it is interpreter related.
> >>>>>>>
> >>>>>>
> >>>>>> After...
> >>>>>>
> >>>>>> sysctl -w net.core.bpf_jit_enable=1
> >>>>>>
> >>>>>> I can start all failed systemd services.
> >>>>>>
> >>>>>> systemd-journald.service
> >>>>>> systemd-udevd.service
> >>>>>> haveged.service
> >>>>>>
> >>>>>> This is in maintenance mode.
> >>>>>>
> >>>>>> What is next: Do set a permanent sysctl setting for net.core.bpf_jit_enable?
> >>>>>>
> >>>>>
> >>>>> This is what I did:
> >>>>
> >>>> I probably won't have cycles to debug this potential lld issue.
> >>>> Maybe you already did, I suggest you put enough reproducible
> >>>> details in the bug you filed against lld so they can take a look.
> >>>>
> >>>
> >>> I understand and will put the journalctl-log into the CBL issue
> >>> tracker and update informations.
> >>>
> >>> Thanks for your help understanding the BPF correlations.
> >>>
> >>> Is setting 'net.core.bpf_jit_enable = 2' helpful here?
> >>
> >> jit_enable=1 is enough.
> >> Or use CONFIG_BPF_JIT_ALWAYS_ON to workaround.
> >>
> >> It sounds like clang miscompiles interpreter.
> >> modprobe test_bpf
> >> should be able to point out which part of interpreter is broken.
> >
> > Maybe we need something like...
> >
> > "bpf: Disable GCC -fgcse optimization for ___bpf_prog_run()"
> >
> > ...for clang?
>
> Not sure how do you get conclusion it is gcse causing the problem.
> But anyway, adding such flag in the kernel is not a good idea.
> clang/llvm should be fixed instead. Esp. there is still time
> for 9.0.0 release to fix bugs.
>

To clarify: This is a snapshot release of clang-9 built with tc-build.

Building with -O0 is not possible as I see asm-goto failing.

- Sedat -

[1] https://github.com/ClangBuiltLinux/tc-build

> >
> > - Sedat -
> >
> > [1] https://git.kernel.org/linus/3193c0836f203a91bef96d88c64cccf0be090d9c
> >

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