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

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Date: Thu, 20 Jun 2024 10:15:23 +0200
From: Vlastimil Babka <vbabka@...e.cz>
To: Alexei Starovoitov <alexei.starovoitov@...il.com>
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 6/20/24 3:18 AM, Alexei Starovoitov wrote:
> 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.

The comment just tried to explain why the return value is 0 and not 1 (which
would be also somewhat logical) but ok, will make it more clear.