Re: [PATCH v4 bpf-next 2/4] bpf: fail PERF_EVENT_IOC_SET_BPF when bpf_get_[stack|stackid] cannot work

[Song Liu <songliubraving@fb.com>]

> On Jul 22, 2020, at 10:55 PM, Alexei Starovoitov <alexei.starovoitov@gmail.com> wrote:
> 
> On Wed, Jul 22, 2020 at 11:42:08AM -0700, Song Liu wrote:
>> diff --git a/kernel/events/core.c b/kernel/events/core.c
>> index 856d98c36f562..f77d009fcce95 100644
>> --- a/kernel/events/core.c
>> +++ b/kernel/events/core.c
>> @@ -9544,6 +9544,24 @@ static int perf_event_set_bpf_handler(struct perf_event *event, u32 prog_fd)
>> 	if (IS_ERR(prog))
>> 		return PTR_ERR(prog);
>> 
>> +	if (event->attr.precise_ip &&
>> +	    prog->call_get_stack &&
>> +	    (!(event->attr.sample_type & __PERF_SAMPLE_CALLCHAIN_EARLY) ||
>> +	     event->attr.exclude_callchain_kernel ||
>> +	     event->attr.exclude_callchain_user)) {
>> +		/*
>> +		 * On perf_event with precise_ip, calling bpf_get_stack()
>> +		 * may trigger unwinder warnings and occasional crashes.
>> +		 * bpf_get_[stack|stackid] works around this issue by using
>> +		 * callchain attached to perf_sample_data. If the
>> +		 * perf_event does not full (kernel and user) callchain
>> +		 * attached to perf_sample_data, do not allow attaching BPF
>> +		 * program that calls bpf_get_[stack|stackid].
>> +		 */
>> +		bpf_prog_put(prog);
>> +		return -EINVAL;
> 
> I suspect this will be a common error. bpftrace and others will be hitting
> this issue and would need to fix how they do perf_event_open.
> But EINVAL is too ambiguous and sys_perf_event_open has no ability to
> return a string.
> So how about we pick some different errno here to make future debugging
> a bit less painful?
> May be EBADFD or EPROTO or EPROTOTYPE ?
> I think anything would be better than EINVAL.

I like EPROTO most. I will change it to EPROTO if we don't have better ideas.

Btw, this is not the error code on sys_perf_event_open(). It is the ioctl()
on the perf_event fd. So debugging this error will be less painful than 
debugging sys_perf_event_open() errors. 

Thanks,
Song