On Wed, Mar 4, 2020 at 7:57 AM Daniel Borkmann <daniel@xxxxxxxxxxxxx> wrote:
>
> On 3/4/20 4:38 PM, Daniel Borkmann wrote:
> > On 3/4/20 10:37 AM, Toke Høiland-Jørgensen wrote:
> >> Andrii Nakryiko <andrii.nakryiko@xxxxxxxxx> writes:
> >>> On Tue, Mar 3, 2020 at 3:01 PM Daniel Borkmann <daniel@xxxxxxxxxxxxx> wrote:
> >>>>
> >>>> On 3/3/20 1:32 AM, Andrii Nakryiko wrote:
> >>>>> Switch BPF UAPI constants, previously defined as #define macro, to anonymous
> >>>>> enum values. This preserves constants values and behavior in expressions, but
> >>>>> has added advantaged of being captured as part of DWARF and, subsequently, BTF
> >>>>> type info. Which, in turn, greatly improves usefulness of generated vmlinux.h
> >>>>> for BPF applications, as it will not require BPF users to copy/paste various
> >>>>> flags and constants, which are frequently used with BPF helpers. Only those
> >>>>> constants that are used/useful from BPF program side are converted.
> >>>>>
> >>>>> Signed-off-by: Andrii Nakryiko <andriin@xxxxxx>
> >>>>
> >>>> Just thinking out loud, is there some way this could be resolved generically
> >>>> either from compiler side or via additional tooling where this ends up as BTF
> >>>> data and thus inside vmlinux.h as anon enum eventually? bpf.h is one single
> >>>> header and worst case libbpf could also ship a copy of it (?), but what about
> >>>> all the other things one would need to redefine e.g. for tracing? Small example
> >>>> that comes to mind are all these TASK_* defines in sched.h etc, and there's
> >>>> probably dozens of other similar stuff needed too depending on the particular
> >>>> case; would be nice to have some generic catch-all, hmm.
> >>>
> >>> Enum convertion seems to be the simplest and cleanest way,
> >>> unfortunately (as far as I know). DWARF has some extensions capturing
> >>> #defines, but values are strings (and need to be parsed, which is pain
> >>> already for "1 << 1ULL"), and it's some obscure extension, not a
> >>> standard thing. I agree would be nice not to have and change all UAPI
> >>> headers for this, but I'm not aware of the solution like that.
> >>
> >> Since this is a UAPI header, are we sure that no userspace programs are
> >> using these defines in #ifdefs or something like that?
> >
> > Hm, yes, anyone doing #ifdefs on them would get build issues. Simple example:
> >
> > enum {
> > FOO = 42,
> > //#define FOO FOO
> > };
> >
> > #ifndef FOO
> > # warning "bar"
> > #endif
> >
> > int main(int argc, char **argv)
> > {
> > return FOO;
> > }
> >
> > $ gcc -Wall -O2 foo.c
> > foo.c:7:3: warning: #warning "bar" [-Wcpp]
> > 7 | # warning "bar"
> > | ^~~~~~~
> >
> > Commenting #define FOO FOO back in fixes it as we discussed in v2:
> >
> > $ gcc -Wall -O2 foo.c
> > $
> >
> > There's also a flag_enum attribute, but with the experiments I tried yesterday
> > night I couldn't get a warning to trigger for anonymous enums at least, so that
> > part should be ok.
> >
> > I was about to push the series out, but agree that there may be a risk for #ifndefs
> > in the BPF C code. If we want to be on safe side, #define FOO FOO would be needed.
>
> I checked Cilium, LLVM, bcc, bpftrace code, and various others at least there it
> seems okay with the current approach, meaning no such if{,n}def seen that would
> cause a build warning. Also suricata seems to ship the BPF header itself. But
> iproute2 had the following in include/bpf_util.h:
>
> #ifndef BPF_PSEUDO_MAP_FD
> # define BPF_PSEUDO_MAP_FD 1
> #endif
This actually would still work, even if BPF_PSEUDO_MAP_FD was
converted to enum (it would just ignore enum value and hard-code it to
1).
>
> It's still not what was converted though. I would expect risk might be rather low.
> Toke, is there anything on your side affected?
>
> Thanks,
> Daniel
