Re: [PATCH bpf-next v1 5/8] bpftool: Emit __bpf_core_pso on verifier-rewritten context types

[Ihor Solodrai <ihor.solodrai@linux.dev>]

On 2026-10-15 5:00 p.m., Ihor Solodrai wrote:
> The verifier translates accesses to some UAPI context and socket types
> into accesses to kernel structures. These accesses require an
> unmodified base and an immediate field offset.
> 
> Leaving preserve_access_index off does not stop the compiler from
> merging loads at different field offsets into one load through an
> adjusted pointer. The verifier rejects the resulting program.
> 
> preserve_static_offset, available since Clang 18, preserves static
> field offsets in load and store instruction immediates. Apply it to
> the 13 types that need static offsets through __bpf_core_pso.
> 
> Eduard introduced PSO in Clang and proposed annotating BPF context
> types in 2023 [1]. His v3 RFC [2] left pt_regs annotations in
> architecture headers unresolved. However since commit de6c7d99f898
> ("bpf: Relax fixed offset check for PTR_TO_CTX"), program types without
> convert_ctx_access() accept fixed offsets from modified context
> pointers. pt_regs can now keep PAI, while PSO on bpf_perf_event_data
> covers its regs member.
> 
> PSO creates no CO-RE relocations, so BPF_NO_PRESERVE_ACCESS_INDEX does
> not disable it.
> 
> Clang 18 through 23 crash on __builtin_preserve_field_info over a PSO
> record, including bpf_core_field_*() and the CO-RE bitfield helpers.
> -D__bpf_core_pso= can disable PSO for affected programs.

Opened a PR for clang with a fix:
https://github.com/llvm/llvm-project/pull/234280

> 
> [1]: https://lore.kernel.org/bpf/20231208000531.19179-1-eddyz87@gmail.com/
> [2]: https://lore.kernel.org/bpf/20231220133411.22978-1-eddyz87@gmail.com/
> 
> Signed-off-by: Ihor Solodrai <ihor.solodrai@linux.dev>
> ---
>   tools/bpf/bpftool/btf.c                          | 16 ++++++++++++++--
>   .../bpf/bpftool_btf_dump_sorted.expected         |  8 ++++++++
>   .../bpf/bpftool_btf_dump_unsorted.expected       |  8 ++++++++
>   3 files changed, 30 insertions(+), 2 deletions(-)
> 
> [...]