linux-kernel - Re: [RFC patch 09/19] bpf: Use BPF_PROG_RUN_PIN_ON

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Date:   Wed, 19 Feb 2020 10:00:56 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Vinicius Costa Gomes <vinicius.gomes@...el.com>,
        LKML <linux-kernel@...r.kernel.org>
Cc:     David Miller <davem@...emloft.net>, bpf@...r.kernel.org,
        netdev@...r.kernel.org, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Sebastian Sewior <bigeasy@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Clark Williams <williams@...hat.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Ingo Molnar <mingo@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Will Drewry <wad@...omium.org>,
        Andy Lutomirski <luto@...nel.org>
Subject: Re: [RFC patch 09/19] bpf: Use BPF_PROG_RUN_PIN_ON_CPU() at simple call sites.

Vinicius Costa Gomes <vinicius.gomes@...el.com> writes:

Cc+: seccomp folks 

> Thomas Gleixner <tglx@...utronix.de> writes:
>
>> From: David Miller <davem@...emloft.net>

Leaving content for reference

>> All of these cases are strictly of the form:
>>
>> 	preempt_disable();
>> 	BPF_PROG_RUN(...);
>> 	preempt_enable();
>>
>> Replace this with BPF_PROG_RUN_PIN_ON_CPU() which wraps BPF_PROG_RUN()
>> with:
>>
>> 	migrate_disable();
>> 	BPF_PROG_RUN(...);
>> 	migrate_enable();
>>
>> On non RT enabled kernels this maps to preempt_disable/enable() and on RT
>> enabled kernels this solely prevents migration, which is sufficient as
>> there is no requirement to prevent reentrancy to any BPF program from a
>> preempting task. The only requirement is that the program stays on the same
>> CPU.
>>
>> Therefore, this is a trivially correct transformation.
>>
>> [ tglx: Converted to BPF_PROG_RUN_PIN_ON_CPU() ]
>>
>> Signed-off-by: David S. Miller <davem@...emloft.net>
>> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
>>
>> ---
>>  include/linux/filter.h    |    4 +---
>>  kernel/seccomp.c          |    4 +---
>>  net/core/flow_dissector.c |    4 +---
>>  net/core/skmsg.c          |    8 ++------
>>  net/kcm/kcmsock.c         |    4 +---
>>  5 files changed, 6 insertions(+), 18 deletions(-)
>>
>> --- a/include/linux/filter.h
>> +++ b/include/linux/filter.h
>> @@ -713,9 +713,7 @@ static inline u32 bpf_prog_run_clear_cb(
>>  	if (unlikely(prog->cb_access))
>>  		memset(cb_data, 0, BPF_SKB_CB_LEN);
>>  
>> -	preempt_disable();
>> -	res = BPF_PROG_RUN(prog, skb);
>> -	preempt_enable();
>> +	res = BPF_PROG_RUN_PIN_ON_CPU(prog, skb);
>>  	return res;
>>  }
>>  
>> --- a/kernel/seccomp.c
>> +++ b/kernel/seccomp.c
>> @@ -268,16 +268,14 @@ static u32 seccomp_run_filters(const str
>>  	 * All filters in the list are evaluated and the lowest BPF return
>>  	 * value always takes priority (ignoring the DATA).
>>  	 */
>> -	preempt_disable();
>>  	for (; f; f = f->prev) {
>> -		u32 cur_ret = BPF_PROG_RUN(f->prog, sd);
>> +		u32 cur_ret = BPF_PROG_RUN_PIN_ON_CPU(f->prog, sd);
>>
>
> More a question really, isn't the behavior changing here? i.e. shouldn't
> migrate_disable()/migrate_enable() be moved to outside the loop? Or is
> running seccomp filters on different cpus not a problem?

In my understanding this is a list of filters and they are independent
of each other.

Kees, Will. Andy?

Thanks,

        tglx