On Mon, May 24, 2021 at 8:05 AM Lorenz Bauer <lmb@xxxxxxxxxxxxxx> wrote:
>
> Hi Andrii,
>
> A user of bpf2go [1] recently ran into the problem of PT_REGS not
> being defined after including bpf_tracing.h. It turns out this is
> because we by default compile for bpfel / bpfeb so the logic in the
> header file doesn't kick in. I originally filed [2] as a quick fix,
> but looking at the issue some more made me wonder how to fit this into
> bpf2go better.
>
> Basically, the convention of bpf2go is that the compiled BPF is
> checked into the source code repository to facilitate distributing BPF
> as part of Go packages (as opposed to libbpf tooling which doesn't
> include generated source). To make this portable, bpf2go by default
> generates both bpfel and bpfeb variants of the C.
>
> However, code using bpf_tracing.h is inherently non-portable since the
> fields of struct pt_regs differ in name between platforms. It seems
> like this forces one to compile to each possible __TARGET_ARCH
> separately. If that is correct, could we extend CO-RE somehow to cover
> this as well?
If there are enums/types/fields that we can use to reliably detect the
platform, then yes, we can have a new set of helpers that would do
this with CO-RE. Someone will need to investigate how to do that for
all the platforms we have. It's all about finding something that's
already in the kernel and can server as a reliably indicator of a
target architecture.
>
> If that isn't possible, I want to avoid compiling and shipping BPF for
> each possible __TARGET_ARCH_xxx by default. Instead I would like to
> achieve:
> * Code that doesn't use bpf_tracing.h is distributed in bpfel and bpfeb variants
> * Code that uses bpf_tracing.h has to explicitly opt into the
> supported platforms via a flag to bpf2go
>
> The latter point is because the go tooling has to know the target arch
> to be able to generate the correct Go wrappers. How would you feel
> about adding something like the following to bpf_tracing.h:
Well, obviously I'm not a fan of even more magic #defines. But I think
we can achieve a similar effect with a more "lazy" approach. I.e., if
user tries to use PT_REGS_xxx macros but doesn't specify the platform
-- only then it gets compilation errors. There is stuff in
bpf_tracing.h that doesn't need pt_regs, so we can't just outright do
#error unconditinally. But we can do something like this:
#else /* !bpf_target_defined */
#define PT_REGS_PARM1(x) _Pragma("GCC error \"blah blah something
user-facing\"")
... and so on for all macros
#endif
Thoughts?
While on the topic, I've noticed that we added BPF_SEQ_PRINTF and
BPF_SNPRINTF into bpf_tracing.h, which seems like not the best fit
(definitely not for BPF_SNPRINTF). Florent, can you please help moving
them into bpf_helpers.h, as it's really more generic functionality
rather than low-level tracing primitives. I think it was put here
because we needed ___bpf_narg macros, but I'd rather copy/paste them
into bpf_helpers.h (they won't change at all) and put
BPF_SNPRINTF/BPF_SEQ_PRINTF into bpf_helpers.h.
>
> --- a/tools/lib/bpf/bpf_tracing.h
> +++ b/tools/lib/bpf/bpf_tracing.h
> @@ -25,6 +25,9 @@
> #define bpf_target_sparc
> #define bpf_target_defined
> #else
> + #if defined(BPF_REQUIRE_EXPLICIT_TARGET_ARCH)
> + #error BPF_REQUIRE_EXPLICIT_TARGET_ARCH set and no
> target arch defined
> + #endif
> #undef bpf_target_defined
> #endif
>
> bpf2go would always define BPF_REQUIRE_EXPLICIT_TARGET_ARCH. If the
> user included bpf_tracing.h they get this error. They can then add
> -target amd64, etc. and the tooling then defines __TARGET_ARCH_x86_64
>
> 1: https://pkg.go.dev/github.com/cilium/ebpf/cmd/bpf2go
> 2: https://github.com/cilium/ebpf/issues/305
>
> --
> Lorenz Bauer | Systems Engineer
> 6th Floor, County Hall/The Riverside Building, SE1 7PB, UK
>
> www.cloudflare.com
