linux-kernel - Re: [PATCH bpf-next] bpf: trace jit code when enable BPF_JIT_ALWAYS

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Message-ID: <CAADnVQJsG-c+AdpwS6xTCwZq4-uVoPH7FZ8CV_XCA-+QaCKA8g@mail.gmail.com>
Date:   Sat, 27 Mar 2021 08:19:07 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Jianlin Lv <Jianlin.Lv@....com>
Cc:     bpf <bpf@...r.kernel.org>, "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Andrey Ignatov <rdna@...com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Nicolas Dichtel <nicolas.dichtel@...nd.com>,
        Kees Cook <keescook@...omium.org>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Mahesh Bandewar <maheshb@...gle.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Network Development <netdev@...r.kernel.org>,
        "iecedge@...il.com" <iecedge@...il.com>, nd <nd@....com>
Subject: Re: [PATCH bpf-next] bpf: trace jit code when enable BPF_JIT_ALWAYS_ON

On Sat, Mar 27, 2021 at 1:19 AM Jianlin Lv <Jianlin.Lv@....com> wrote:
>
> > On Fri, Mar 26, 2021 at 5:40 AM Jianlin Lv <Jianlin.Lv@....com> wrote:
> > >
> > > When CONFIG_BPF_JIT_ALWAYS_ON is enabled, the value of
> > bpf_jit_enable
> > > in /proc/sys is limited to SYSCTL_ONE. This is not convenient for debugging.
> > > This patch modifies the value of extra2 (max) to 2 that support
> > > developers to emit traces on kernel log.
> > >
> > > Signed-off-by: Jianlin Lv <Jianlin.Lv@....com>
> > > ---
> > >  net/core/sysctl_net_core.c | 2 +-
> > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > >
> > > diff --git a/net/core/sysctl_net_core.c b/net/core/sysctl_net_core.c
> > > index d84c8a1b280e..aa16883ac445 100644
> > > --- a/net/core/sysctl_net_core.c
> > > +++ b/net/core/sysctl_net_core.c
> > > @@ -386,7 +386,7 @@ static struct ctl_table net_core_table[] = {
> > >                 .proc_handler   = proc_dointvec_minmax_bpf_enable,
> > >  # ifdef CONFIG_BPF_JIT_ALWAYS_ON
> > >                 .extra1         = SYSCTL_ONE,
> > > -               .extra2         = SYSCTL_ONE,
> > > +               .extra2         = &two,
> >
> > "bpftool prog dump jited" is much better way to examine JITed dumps.
> > I'd rather remove bpf_jit_enable=2 altogether.
>
> In my case, I introduced a bug when I made some adjustments to the arm64
> jit macro A64_MOV(), which caused the SP register to be replaced by the
> XZR register when building prologue, and the wrong value was stored in fp,
> which triggered a crash.
>
> This bug is likely to cause the instruction to access the BPF stack in
> jited prog to trigger a crash.
> I tried to use bpftool to debug, but bpftool crashed when I executed the
> "bpftool prog show" command.
> The syslog shown that bpftool is loading and running some bpf prog.
> because of the bug in the JIT compiler, the bpftool execution failed.

Right 'bpftool prog show' command is loading a bpf iterator prog,
but you didn't need to use it to dump JITed code.
"bpftool prog dump jited name my_prog"
would have dumped it even when JIT is all buggy.

> bpf_jit_disasm saved me, it helped me dump the jited image:
>
> echo 2> /proc/sys/net/core/bpf_jit_enable
> modprobe test_bpf test_name="SPILL_FILL"
> ./bpf_jit_disasm -o
>
> So keeping bpf_jit_enable=2 is still very meaningful for developers who
> try to modify the JIT compiler.

sure and such JIT developers can compile the kernel
without BPF_JIT_ALWAYS_ON just like you did.
They can also insert printk, etc.
bpf_jit_enable=2 was done long ago when there was no other way
to see JITed code. Now we have proper apis.
That =2 mode can and should be removed.

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please fix your email server/client/whatever. No patches will ever be
accepted with
such disclaimer.