Re: [PATCH dwarves] btf_encoder: Keep the source prototype of kfuncs

[Breno Leitao <leitao@debian.org>]

On Wed, Nov 11, 2026 at 02:24:47PM -0800, Yonghong Song wrote:
> The location based checks can drop kfuncs from BTF even though their ABI
> did not change. With the arm64 KCSAN setup in the report, resolve_btfids
> fails with:
> 
>   WARN: resolve_btfids: no BTF func for kfunc bpf_tcp_ops_set_rcvlowat in bpf_tcp_ops_set_rcvlowat_kfunc_set
>   WARN: resolve_btfids: unresolved symbol bpf_tcp_ops_set_rcvlowat
> 
> The location list of 'rcvlowat' starts after the function entry, in a
> callee-saved register, so the function is marked unexpected_reg and
> skipped. scx_bpf_tid_to_task() hits the same skip through a
> different DWARF pattern.
> 
> Kfuncs are defined with __bpf_kfunc ("__used __retain __noclone
> noinline"), so the compiler keeps the calling convention of the C
> prototype. Odd parameter locations of a kfunc are debug info artifacts,
> not ABI changes.
> 
> For kfuncs, clear the location derived flags when saving the state,
> keep all source parameters, and skip the location based checks, also in
> btf_encoder__add_saved_funcs() where saved_functions_combine() can merge
> the flags back in. The inconsistent prototype and ambiguous address
> checks still apply.
> 
> Kfuncs were only collected with decl_tag_kfuncs, which the kernel no
> longer passes since commit 692393104909 ("resolve_btfids: Emit bpf_kfunc
> and bpf_fastcall decl tags"). Collect them whenever BTF is encoded; the
> decl tags are still only emitted with decl_tag_kfuncs.
> 
> With this change, bpf_tcp_ops_set_rcvlowat() and scx_bpf_tid_to_task()
> are encoded again, and no other function in vmlinux BTF changes.
> 
> Reported-by: Breno Leitao <leitao@debian.org>
> Closes: https://lore.kernel.org/all/au27Y9m78_3EH2v1@gmail.com/
> Signed-off-by: Yonghong Song <yonghong.song@linux.dev>

Thanks, this fixes my build. I tested it on top of pahole master
(9eb594d) with next-20261111 and clang 22.1.3:

 - arm64 KCSAN, CONFIG_WERROR=y: vmlinux links again,
   bpf_tcp_ops_set_rcvlowat() and scx_bpf_tid_to_task() are back in BTF,
   __bpf_tcp_ops_init() and scx_init() return 0 instead of -22, and the
   prog_tests/tcp_autolowat.c cases pass (run standalone).

 - x86_64 KCSAN drops bpf_tcp_ops_set_rcvlowat() in the same way, and
   this fixes it there too.

 - Running pahole with and without this patch on the same vmlinux, the
   only BTF difference on both architectures is the kfuncs coming back.

Tested-by: Breno Leitao <leitao@debian.org>

--breno