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

[Jianlin Lv]

On Sat, Mar 27, 2021 at 11:19 PM Alexei Starovoitov
<alexei.starovoitov@xxxxxxxxx> wrote:
>
> 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.

Thanks for your reply, I will prepare another patch to remove =2mode.

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

Apologize for this.
Jianlin