Re: [PATCH bpf-next v2 2/2] bpf: Implement formatted output helpers with bstr_printf

[Florent Revest <revest@chromium.org>]

On Wed, Apr 28, 2021 at 2:51 AM Alexei Starovoitov
<alexei.starovoitov@gmail.com> wrote:
>
> On Tue, Apr 27, 2021 at 5:20 PM Florent Revest <revest@chromium.org> wrote:
> >
> > On Wed, Apr 28, 2021 at 1:46 AM Alexei Starovoitov
> > <alexei.starovoitov@gmail.com> wrote:
> > >
> > > On Tue, Apr 27, 2021 at 10:43 AM Florent Revest <revest@chromium.org> wrote:
> > > > +                       if (fmt[i + 1] == 'B') {
> > > > +                               if (tmp_buf)  {
> > > > +                                       err = snprintf(tmp_buf,
> > > > +                                                      (tmp_buf_end - tmp_buf),
> > > > +                                                      "%pB",
> > > ...
> > > > +                       if ((tmp_buf_end - tmp_buf) < sizeof_cur_ip) {
> > >
> > > I removed a few redundant () like above
> >
> > Oh, sorry about that.
> >
> > > and applied.
> >
> > Nice! :)
> >
> > > >                 if (fmt[i] == 'l') {
> > > > -                       cur_mod = BPF_PRINTF_LONG;
> > > > +                       sizeof_cur_arg = sizeof(long);
> > > >                         i++;
> > > >                 }
> > > >                 if (fmt[i] == 'l') {
> > > > -                       cur_mod = BPF_PRINTF_LONG_LONG;
> > > > +                       sizeof_cur_arg = sizeof(long long);
> > > >                         i++;
> > > >                 }
> > >
> > > This bit got me thinking.
> > > I understand that this is how bpf_trace_printk behaved
> > > and the sprintf continued the tradition, but I think it will
> > > surprise bpf users.
> > > The bpf progs are always 64-bit. The sizeof(long) == 8
> > > inside any bpf program. So printf("%ld") matches that long.
> >
> > Yes, this also surprised me.
> >
> > > The clang could even do type checking to make sure the prog
> > > is passing the right type into printf() if we add
> > > __attribute__ ((format (printf))) to bpf_helper_defs.h
> > > But this sprintf() implementation will trim the value to 32-bit
> > > to satisfy 'fmt' string on 32-bit archs.
> > > So bpf program behavior would be different on 32 and 64-bit archs.
> > > I think that would be confusing, since the rest of bpf prog is
> > > portable. The progs work the same way on all archs
> > > (except endianess, of course).
> > > I'm not sure how to fix it though.
> > > The sprintf cannot just pass 64-bit unconditionally, since
> > > bstr_printf on 32-bit archs will process %ld incorrectly.
> > > The verifier could replace %ld with %Ld.
> > > The fmt string is a read only string for bpf_snprintf,
> > > but for bpf_trace_printk it's not and messing with it at run-time
> > > is not good. Copying the fmt string is not great either.
> > > Messing with internals of bstr_printf is ugly too.
> >
> > Indeed, none of these solutions are satisfying.
>
> Maybe Daniel has other ideas?
>
> > > Maybe we just have to live with this quirk ?
> >
> > If we were starting from scratch, maybe just banning %ld could have
> > been an option, but now that bpf_trace_printk has been behaving like
> > this for a while, I think it might be best to just keep the behavior
> > as it is.
> >
> > > Just add a doc to uapi/bpf.h to discourage %ld and be done?
> >
> > More doc is always good. Something like "Note: %ld behaves differently
> > depending on the host architecture, it is recommended to avoid it and
> > use %d or %lld instead" in the helper description of the three
> > helpers? If you don't have the time to do it today, I can send a patch
> > tomorrow.
>
> bpf_trace_printk was like this for a long time, so there is no rush.
> Pls wait until everything comes back to bpf tree and send a patch against it.
> bpf_trace_printk comment in uapi/bpf.h is outdated too. Would be good
> to document the latest behavior for them all.

Ok :)