Re: [PATCH v2 5/7] bpf: do not create bpf_non_sleepable_error_inject list when unnecessary

[Alexei Starovoitov <alexei.starovoitov@gmail.com>]

On Wed, Jun 19, 2024 at 3:49 PM Vlastimil Babka <vbabka@suse.cz> wrote:
>
> When CONFIG_FUNCTION_ERROR_INJECTION is disabled,
> within_error_injection_list() will return false for any address and the
> result of check_non_sleepable_error_inject() denylist is thus redundant.
> The bpf_non_sleepable_error_inject list thus does not need to be
> constructed at all, so #ifdef it out.
>
> This will allow to inline functions on the list when
> CONFIG_FUNCTION_ERROR_INJECTION is disabled as there will be no BTF_ID()
> reference for them.
>
> Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
> ---
>  kernel/bpf/verifier.c | 15 +++++++++++++++
>  1 file changed, 15 insertions(+)
>
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 77da1f438bec..5cd93de37d68 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -21044,6 +21044,8 @@ static int check_attach_modify_return(unsigned long addr, const char *func_name)
>         return -EINVAL;
>  }
>
> +#ifdef CONFIG_FUNCTION_ERROR_INJECTION
> +
>  /* list of non-sleepable functions that are otherwise on
>   * ALLOW_ERROR_INJECTION list
>   */
> @@ -21061,6 +21063,19 @@ static int check_non_sleepable_error_inject(u32 btf_id)
>         return btf_id_set_contains(&btf_non_sleepable_error_inject, btf_id);
>  }
>
> +#else /* CONFIG_FUNCTION_ERROR_INJECTION */
> +
> +/*
> + * Pretend the denylist is empty, within_error_injection_list() will return
> + * false anyway.
> + */
> +static int check_non_sleepable_error_inject(u32 btf_id)
> +{
> +       return 0;
> +}
> +
> +#endif

The comment reads like this is an optimization, but it's a mandatory
ifdef since should_failslab() might not be found by resolve_btfid
during the build.
Please make it clear in the comment.