Bcc: "Add build_id support for BPF stackmap" broke libbcc for ubuntu 18.04

[rjmccabe3701]

2098 broke our code including bcc/BPFTable.h

I'm running on a stock Ubuntu 18.04 system, with a 4.15 Linux kernel.

Example :

//BPFTable_failure.cpp
#include <bcc/BPFTable.h>
int main(void) { }
````

```bash
-> % g++ BPFTable_failure.cpp
In file included from BPFTable_failure.cpp:1:0:
/usr/include/bcc/BPFTable.h:320:54: error: field ‘trace’ has incomplete type ‘bpf_stack_build_id [127]’
   struct bpf_stack_build_id trace[BPF_MAX_STACK_DEPTH];
                                                      ^
In file included from /usr/include/bcc/BPFTable.h:30:0,
                 from BPFTable_failure.cpp:1:
/usr/include/bcc/bcc_syms.h:76:38: note: forward declaration of ‘struct bpf_stack_build_id’
                               struct bpf_stack_build_id *trace,

There should be pre-processor guards in these files if the features aren't supported by the host's kernel.

[yonghong-song]

@vijunag could you take a look? probably need to a correct include path in cmake file to contain bpf.h from libbpf/include/uapi/linux directory.

[vijunag]

Yes, will take a look at it. There is no -I incantation to that directory already ?

[yonghong-song]

I think your patch is tested against code base before libbpf work. That is why the buildbot did not catch the issue.

[yonghong-song]

Or I could be wrong as later buildbot seems fine. In any case, please take a look. Thanks.

[vijunag]

@yonghong-song - This is a compile outside of bcc repository. I don't think we ship headers under libbpf/include/uapi/linux directory since it is taken from the system ? This was the reason why I had created an auxiliary structure(bcc_stack_build_id) so that there is no dependency with the system header.

The below compile fails if it lacks definition. Should we guard it around kernel version ? What is an elegant fix for this ?
g++ BPFTable_failure.cpp

[yonghong-song]

We do ship uapi headers at /usr/include/bcc/compat/linux directory. User codes should include the bpf header there with proper include patch.

[yonghong-song]

In bcc_syms.h, linux/bpf.h is already there. So user space does not need to change code, add an include path should work.

[vijunag]

@yonghong-song - Can we close this PR if nothing needs to be done from bcc end ?

[rjmccabe3701]

For completeness, this is what makes it work:

g++ -I /usr/include/bcc/compat BPFTable_failure.cpp

Thanks.