netdev - Re: [PATCH v3 bpf-next 1/3] bpf: switch BPF UAPI #define constants used from BPF program side to enums

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Date:   Wed, 4 Mar 2020 16:57:46 +0100
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Toke Høiland-Jørgensen <toke@...hat.com>,
        Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc:     Andrii Nakryiko <andriin@...com>, bpf <bpf@...r.kernel.org>,
        Networking <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...com>,
        Kernel Team <kernel-team@...com>, Yonghong Song <yhs@...com>
Subject: Re: [PATCH v3 bpf-next 1/3] bpf: switch BPF UAPI #define constants
 used from BPF program side to enums

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@...il.com> writes:
>>> On Tue, Mar 3, 2020 at 3:01 PM Daniel Borkmann <daniel@...earbox.net> 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@...com>
>>>>
>>>> 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

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