linux-kernel - Re: [PATCH ipsec-next v1 6/7] bpf: selftests: test

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Date:   Tue, 28 Nov 2023 18:19:08 +0200
From:   Eduard Zingerman <eddyz87@...il.com>
To:     Daniel Xu <dxu@...uu.xyz>, Yonghong Song <yonghong.song@...ux.dev>
Cc:     Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Shuah Khan <shuah@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Steffen Klassert <steffen.klassert@...unet.com>,
        antony.antony@...unet.com, Mykola Lysenko <mykolal@...com>,
        Martin KaFai Lau <martin.lau@...ux.dev>,
        Song Liu <song@...nel.org>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>,
        Stanislav Fomichev <sdf@...gle.com>,
        Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>,
        bpf <bpf@...r.kernel.org>,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>, devel@...ux-ipsec.org,
        Network Development <netdev@...r.kernel.org>
Subject: Re: [PATCH ipsec-next v1 6/7] bpf: selftests: test_tunnel: Disable
 CO-RE relocations

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.