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