Re: [PATCH bpf-next] bpf: trace jit code when enable BPF_JIT_ALWAYS_ON

[Alexei Starovoitov]

On Sat, Mar 27, 2021 at 1:19 AM Jianlin Lv <Jianlin.Lv@xxxxxxx> wrote:
>
> > On Fri, Mar 26, 2021 at 5:40 AM Jianlin Lv <Jianlin.Lv@xxxxxxx> 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@xxxxxxx>
> > > ---
> > >  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.

> IMPORTANT NOTICE: The contents of this email and any attachments are confidential and may also be privileged. If you are not the intended recipient, please notify the sender immediately and do not disclose the contents to any other person, use it for any purpose, or store or copy the information in any medium. Thank you.

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