Subject: Re: [RFC patch 09/19] bpf: Use BPF_PROG_RUN_PIN_ON_CPU() at simple call sites.

From: Vinicius Costa Gomes <[email-masked]>
Date: Tue, 18 Feb 2020 17:39:45 -0800

Hi,

Thomas Gleixner <[email-masked]> writes:

> From: David Miller <[email-masked]>
>
> 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 <[email-masked]>
> Signed-off-by: Thomas Gleixner <[email-masked]>
>
> ---
>  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?

-- 
Vinicius