linux-kernel - Re: [PATCH v2 5/7] bpf: do not create bpf_non_sleepable_error

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Date: Wed, 19 Jun 2024 18:18:48 -0700
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Vlastimil Babka <vbabka@...e.cz>
Cc: Akinobu Mita <akinobu.mita@...il.com>, Christoph Lameter <cl@...ux.com>, 
	David Rientjes <rientjes@...gle.com>, Alexei Starovoitov <ast@...nel.org>, 
	Daniel Borkmann <daniel@...earbox.net>, Andrii Nakryiko <andrii@...nel.org>, 
	"Naveen N. Rao" <naveen.n.rao@...ux.ibm.com>, 
	Anil S Keshavamurthy <anil.s.keshavamurthy@...el.com>, "David S. Miller" <davem@...emloft.net>, 
	Masami Hiramatsu <mhiramat@...nel.org>, Steven Rostedt <rostedt@...dmis.org>, 
	Mark Rutland <mark.rutland@....com>, Jiri Olsa <jolsa@...nel.org>, 
	Roman Gushchin <roman.gushchin@...ux.dev>, Hyeonggon Yoo <42.hyeyoo@...il.com>, 
	LKML <linux-kernel@...r.kernel.org>, linux-mm <linux-mm@...ck.org>, 
	bpf <bpf@...r.kernel.org>, 
	linux-trace-kernel <linux-trace-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 5/7] bpf: do not create bpf_non_sleepable_error_inject
 list when unnecessary

On Wed, Jun 19, 2024 at 3:49 PM Vlastimil Babka <vbabka@...e.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@...e.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.