On Sun, Oct 29, 2023 at 4:22 PM Daniel Xu dxu@dxuuu.xyz wrote:
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 2It 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?
Ah, I missed that this is a write into a struct. I think we can support BPF_CORE_WRITE_BITFIELD() (not the PROBED version, though) using all the same CO-RE relocations. It's probably a very niche case, but BPF_CORE_READ_BITFIELD() is niche as well (though an absolute necessity when the need does come up).
Thanks, Daniel