Re: [PATCH ipsec-next v1 6/7] bpf: selftests: test_tunnel: Disable CO-RE relocations

[Eduard Zingerman]

On Tue, 2023-11-28 at 10:13 -0600, Daniel Xu wrote:
[...]
> > One thing for sure is memory layout of bitfields should be the same
> > for both clang and gcc as it is determined by C standard. Register
> > representation and how to manipulate could be different for different
> > compilers.
> 
> I was reading this thread:
> https://github.com/Lora-net/LoRaMac-node/issues/697. It's obviously not
> authoritative, but they sure sound confident!
> 
> I think I've also heard it before a long time ago when I was working on
> adding bitfield support to bpftrace.
> 
> 
> [...]

Here is a citation from ISO/IEC 9899:201x (C11 standard) §6.7.2.1
(Structure and union specifiers), paragraph 11 (page 114 in my pdf):

> An implementation may allocate any addressable storage unit large
> enough to hold a bit- field. If enough space remains, a bit-field
> that immediately follows another bit-field in a structure shall be
> packed into adjacent bits of the same unit. If insufficient space
> remains, whether a bit-field that does not fit is put into the next
> unit or overlaps adjacent units is implementation-defined. The order
> of allocation of bit-fields within a unit (high-order to low-order
> or low-order to high-order) is implementation-defined. The alignment
> of the addressable storage unit is unspecified.