Re: [PATCH bpf-next v2 1/2] bpf: Introduce bpf_in_interrupt kfunc

["Leon Hwang" <leon.hwang@linux.dev>]

On Wed Aug 27, 2025 at 6:18 AM +08, Alexei Starovoitov wrote:
> On Mon, Aug 25, 2025 at 8:00 PM Leon Hwang <leon.hwang@linux.dev> wrote:
>>
>>
>>
>> On 25/8/25 23:17, Alexei Starovoitov wrote:
>> > On Mon, Aug 25, 2025 at 6:15 AM Leon Hwang <leon.hwang@linux.dev> wrote:

[...]

>> >
>> > It doesn't scale. Next thing people will ask for hard vs soft irq.
>> >
>>
>> How about adding a 'flags'?
>>
>> Here are the values for 'flags':
>>
>> * 0: return in_interrupt();
>> * 1(NMI): return in_nmi();
>> * 2(HARDIRQ): return in_hardirq();
>> * 3(SOFTIRQ): return in_softirq();
>
> That's an option, but before we argue whether to do as one kfunc with enum
> vs N kfuncs let's explore bpf only option that doesn't involve changing
> the kernel.
>
>> >> +#if defined(CONFIG_X86_64) && !defined(CONFIG_UML)
>> >> +               insn_buf[0] = BPF_MOV64_IMM(BPF_REG_0, (u32)(unsigned long)&__preempt_count);
>> >
>> > I think bpf_per_cpu_ptr() should already be able to read that per cpu var.
>> >
>>
>> Correct. bpf_per_cpu_ptr() and bpf_this_cpu_ptr() are helpful to read it.
>
> Can you add them as static inline functions to bpf_experimental.h
> and a selftest to make sure it's all working?
> At least for x86 and !PREEMPT_RT.
> Like:
> bool bpf_in_interrupt()
> {
>   bpf_this_cpu_ptr(...preempt_count..) &  (NMI_MASK | HARDIRQ_MASK |
> SOFTIRQ_MASK);
> }
>
> Of course, there is a danger that kernel implementation might
> diverge from bpf-only bit, but it's a risk we're taking all the time.

I do a PoC of adding bpf_in_interrupt() to bpf_experimental.h.

It works:

extern bool CONFIG_PREEMPT_RT __kconfig __weak;
#ifdef bpf_target_x86
extern const int __preempt_count __ksym;
#endif

struct task_struct__preempt_rt {
	int softirq_disable_cnt;
} __attribute__((preserve_access_index));

/* Description
 *      Report whether it is in interrupt context. Only works on x86.
 */
static inline int bpf_in_interrupt(void)
{
#ifdef bpf_target_x86
	int pcnt;

	pcnt = *(int *) bpf_this_cpu_ptr(&__preempt_count);
	if (!CONFIG_PREEMPT_RT) {
		return pcnt & (NMI_MASK | HARDIRQ_MASK | SOFTIRQ_MASK);
	} else {
		struct task_struct__preempt_rt *tsk;

		tsk = (void *) bpf_get_current_task_btf();
		return (pcnt & (NMI_MASK | HARDIRQ_MASK)) |
		       (tsk->softirq_disable_cnt | SOFTIRQ_MASK);
	}
#else
	return 0;
#endif
}

However, I only test it for !PREEMPT_RT on x86.

I'd like to respin the patchset by moving bpf_in_interrupt() to
bpf_experimental.h.

Thanks,
Leon