netdev - Re: [PATCH v4 bpf-next 2/4] bpf: fail PERF_EVENT_IOC_SET_BPF when bpf_get

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Message-ID: <20200723055518.onydx7uhmzomt7ud@ast-mbp.dhcp.thefacebook.com>
Date:   Wed, 22 Jul 2020 22:55:18 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Song Liu <songliubraving@...com>
Cc:     linux-kernel@...r.kernel.org, bpf@...r.kernel.org,
        netdev@...r.kernel.org, ast@...nel.org, daniel@...earbox.net,
        kernel-team@...com, john.fastabend@...il.com, kpsingh@...omium.org,
        brouer@...hat.com, peterz@...radead.org
Subject: Re: [PATCH v4 bpf-next 2/4] bpf: fail PERF_EVENT_IOC_SET_BPF when
 bpf_get_[stack|stackid] cannot work

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.