Re: [PATCH bpf] bpf: Add explicit cast to 'void *' for __BPF_DISPATCHER_UPDATE()

[Björn Töpel]

Nathan Chancellor <nathan@xxxxxxxxxx> writes:

> When building with clang:
>
>   kernel/bpf/dispatcher.c:126:33: error: pointer type mismatch ('void *' and 'unsigned int (*)(const void *, const struct bpf_insn *, bpf_func_t)' (aka 'unsigned int (*)(const void *, const struct bpf_insn *, unsigned int (*)(const void *, const struct bpf_insn *))')) [-Werror,-Wpointer-type-mismatch]
>           __BPF_DISPATCHER_UPDATE(d, new ?: &bpf_dispatcher_nop_func);
>                                      ~~~ ^  ~~~~~~~~~~~~~~~~~~~~~~~~
>   ./include/linux/bpf.h:1045:54: note: expanded from macro '__BPF_DISPATCHER_UPDATE'
>           __static_call_update((_d)->sc_key, (_d)->sc_tramp, (_new))
>                                                               ^~~~
>   1 error generated.
>
> The warning is pointing out that the type of new ('void *') and
> &bpf_dispatcher_nop_func are not compatible, which could have side
> effects coming out of a conditional operator due to promotion rules.
>
> Add the explicit cast to 'void *' to make it clear that this is
> expected, as __BPF_DISPATCHER_UPDATE() expands to a call to
> __static_call_update(), which expects a 'void *' as its final argument.
>
> Fixes: c86df29d11df ("bpf: Convert BPF_DISPATCHER to use static_call() (not ftrace)")
> Link: https://github.com/ClangBuiltLinux/linux/issues/1755
> Reported-by: kernel test robot <lkp@xxxxxxxxx>
> Reported-by: "kernelci.org bot" <bot@xxxxxxxxxxxx>
> Signed-off-by: Nathan Chancellor <nathan@xxxxxxxxxx>

Acked-by: Björn Töpel <bjorn@xxxxxxxxxx>