Re: [PATCH bpf-next 1/2] bpf: Remove bpf_jit_enable=2 debugging mode

[Jianlin Lv <iecedge@gmail.com>]

On Thu, Apr 15, 2021 at 10:38 PM Daniel Borkmann <daniel@iogearbox.net> wrote:
>
> On 4/15/21 11:32 AM, Jianlin Lv wrote:
> > For debugging JITs, dumping the JITed image to kernel log is discouraged,
> > "bpftool prog dump jited" is much better way to examine JITed dumps.
> > This patch get rid of the code related to bpf_jit_enable=2 mode and
> > update the proc handler of bpf_jit_enable, also added auxiliary
> > information to explain how to use bpf_jit_disasm tool after this change.
> >
> > Signed-off-by: Jianlin Lv <Jianlin.Lv@arm.com>
> [...]
> > diff --git a/arch/x86/net/bpf_jit_comp32.c b/arch/x86/net/bpf_jit_comp32.c
> > index 0a7a2870f111..8d36b4658076 100644
> > --- a/arch/x86/net/bpf_jit_comp32.c
> > +++ b/arch/x86/net/bpf_jit_comp32.c
> > @@ -2566,9 +2566,6 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *prog)
> >               cond_resched();
> >       }
> >
> > -     if (bpf_jit_enable > 1)
> > -             bpf_jit_dump(prog->len, proglen, pass + 1, image);
> > -
> >       if (image) {
> >               bpf_jit_binary_lock_ro(header);
> >               prog->bpf_func = (void *)image;
> > diff --git a/net/core/sysctl_net_core.c b/net/core/sysctl_net_core.c
> > index c8496c1142c9..990b1720c7a4 100644
> > --- a/net/core/sysctl_net_core.c
> > +++ b/net/core/sysctl_net_core.c
> > @@ -273,16 +273,8 @@ static int proc_dointvec_minmax_bpf_enable(struct ctl_table *table, int write,
> >
> >       tmp.data = &jit_enable;
> >       ret = proc_dointvec_minmax(&tmp, write, buffer, lenp, ppos);
> > -     if (write && !ret) {
> > -             if (jit_enable < 2 ||
> > -                 (jit_enable == 2 && bpf_dump_raw_ok(current_cred()))) {
> > -                     *(int *)table->data = jit_enable;
> > -                     if (jit_enable == 2)
> > -                             pr_warn("bpf_jit_enable = 2 was set! NEVER use this in production, only for JIT debugging!\n");
> > -             } else {
> > -                     ret = -EPERM;
> > -             }
> > -     }
> > +     if (write && !ret)
> > +             *(int *)table->data = jit_enable;
> >       return ret;
> >   }
> >
> > @@ -389,7 +381,7 @@ static struct ctl_table net_core_table[] = {
> >               .extra2         = SYSCTL_ONE,
> >   # else
> >               .extra1         = SYSCTL_ZERO,
> > -             .extra2         = &two,
> > +             .extra2         = SYSCTL_ONE,
> >   # endif
> >       },
> >   # ifdef CONFIG_HAVE_EBPF_JIT
> > diff --git a/tools/bpf/bpf_jit_disasm.c b/tools/bpf/bpf_jit_disasm.c
> > index c8ae95804728..efa4b17ae016 100644
> > --- a/tools/bpf/bpf_jit_disasm.c
> > +++ b/tools/bpf/bpf_jit_disasm.c
> > @@ -7,7 +7,7 @@
> >    *
> >    * To get the disassembly of the JIT code, do the following:
> >    *
> > - *  1) `echo 2 > /proc/sys/net/core/bpf_jit_enable`
> > + *  1) Insert bpf_jit_dump() and recompile the kernel to output JITed image into log
>
> Hmm, if we remove bpf_jit_dump(), the next drive-by cleanup patch will be thrown
> at bpf@vger stating that bpf_jit_dump() has no in-tree users and should be removed.
> Maybe we should be removing bpf_jit_disasm.c along with it as well as bpf_jit_dump()
> itself ... I guess if it's ever needed in those rare occasions for JIT debugging we
> can resurrect it from old kernels just locally. But yeah, bpftool's jit dump should
> suffice for vast majority of use cases.
>
> There was a recent set for ppc32 jit which was merged into ppc tree which will create
> a merge conflict with this one [0]. So we would need a rebase and take it maybe during
> merge win once the ppc32 landed..
>
>    [0] https://lore.kernel.org/bpf/cover.1616430991.git.christophe.leroy@csgroup.eu/
>
> >    *  2) Load a BPF filter (e.g. `tcpdump -p -n -s 0 -i eth1 host 192.168.20.0/24`)
> >    *  3) Run e.g. `bpf_jit_disasm -o` to read out the last JIT code
> >    *
> > diff --git a/tools/bpf/bpftool/feature.c b/tools/bpf/bpftool/feature.c
> > index 40a88df275f9..98c7eec2923f 100644
> > --- a/tools/bpf/bpftool/feature.c
> > +++ b/tools/bpf/bpftool/feature.c
> > @@ -203,9 +203,6 @@ static void probe_jit_enable(void)
> >               case 1:
> >                       printf("JIT compiler is enabled\n");
> >                       break;
> > -             case 2:
> > -                     printf("JIT compiler is enabled with debugging traces in kernel logs\n");
> > -                     break;
>
> This would still need to be there for older kernels ...

I will submit another version after ppc32 landed to remove
bpf_jit_disasm.c and restore bpftool/feature.c

Jianlin


>
> >               case -1:
> >                       printf("Unable to retrieve JIT-compiler status\n");
> >                       break;
> >
>