Re: [RFC bpf-next 5/6] bpf: selftests: test_tunnel: Disable CO-RE relocations

On Fri, Oct 27, 2023 at 01:33:09PM -0700, Andrii Nakryiko wrote:

On Fri, Oct 27, 2023 at 11:46 AM Daniel Xu dxu@dxuuu.xyz wrote:

...

Switching to vmlinux.h definitions seems to make the verifier very unhappy with bitfield accesses. The error is:

; md.u.md2.dir = direction;
33: (69) r1 = *(u16 *)(r2 +11)
misaligned stack access off (0x0; 0x0)+-64+11 size 2

It looks like disabling CO-RE relocations seem to make the error go away.

for accessing bitfields libbpf provides BPF_CORE_READ_BITFIELD_PROBED() and BPF_CORE_READ_BITFIELD() macros

In this case the code in question is:

__u8 direction = 0; md.u.md2.dir = direction;

IOW the problem is assigning to bitfields, not reading from them.

Is that something that libbpf needs to support as well?

Thanks, Daniel