Re: [PATCH] bpf: hide cgroup functions for configs without cgroups

[Alexei Starovoitov]

On Fri, Oct 20, 2023 at 6:27 AM Arnd Bergmann <arnd@xxxxxxxxxx> wrote:
>
> From: Arnd Bergmann <arnd@xxxxxxxx>
>
> When cgroups are disabled, the newly added functions don't build:
>
> kernel/bpf/task_iter.c: In function 'bpf_iter_css_task_new':
> kernel/bpf/task_iter.c:917:14: error: 'CSS_TASK_ITER_PROCS' undeclared (first use in this function)
>   917 |         case CSS_TASK_ITER_PROCS | CSS_TASK_ITER_THREADED:
>       |              ^~~~~~~~~~~~~~~~~~~
> kernel/bpf/task_iter.c:925:60: error: invalid application of 'sizeof' to incomplete type 'struct css_task_iter'
>   925 |         kit->css_it = bpf_mem_alloc(&bpf_global_ma, sizeof(struct css_task_iter));
>       |                                                            ^~~~~~
>
> Hide them in an #ifdef section.
>
> Fixes: 9c66dc94b62ae ("bpf: Introduce css_task open-coded iterator kfuncs")
> Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx>
> ---
>  kernel/bpf/task_iter.c | 2 ++
>  1 file changed, 2 insertions(+)
>
> diff --git a/kernel/bpf/task_iter.c b/kernel/bpf/task_iter.c
> index 654601dd6b493..15a184f4f954d 100644
> --- a/kernel/bpf/task_iter.c
> +++ b/kernel/bpf/task_iter.c
> @@ -904,6 +904,7 @@ __diag_push();
>  __diag_ignore_all("-Wmissing-prototypes",
>                   "Global functions as their definitions will be in vmlinux BTF");
>
> +#ifdef CONFIG_CGROUPS
>  __bpf_kfunc int bpf_iter_css_task_new(struct bpf_iter_css_task *it,
>                 struct cgroup_subsys_state *css, unsigned int flags)
>  {
> @@ -947,6 +948,7 @@ __bpf_kfunc void bpf_iter_css_task_destroy(struct bpf_iter_css_task *it)
>         css_task_iter_end(kit->css_it);
>         bpf_mem_free(&bpf_global_ma, kit->css_it);
>  }
> +#endif

Did you actually test build it without cgroups and with bpf+btf?
I suspect the resolve_btfid step should be failing the build.
It needs
#ifdef CONFIG_CGROUPS
around BTF_ID_FLAGS(func, bpf_iter_css_task*