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Date: Mon, 23 Mar 2015 08:29:29 +0100
From: Ingo Molnar <mingo@kernel.org>
To: Alexei Starovoitov <ast@plumgrid.com>
Cc: Steven Rostedt <rostedt@goodmis.org>, Namhyung Kim <namhyung@kernel.org>,
        Arnaldo Carvalho de Melo <acme@infradead.org>,
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        "David S. Miller" <davem@davemloft.net>,
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        Peter Zijlstra <a.p.zijlstra@chello.nl>, linux-api@vger.kernel.org,
        netdev@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v9 tip 6/9] samples: bpf: simple non-portable kprobe
 filter example
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* Alexei Starovoitov <ast@plumgrid.com> wrote:

> tracex1_kern.c - C program compiled into BPF.
> It attaches to kprobe:netif_receive_skb
> When skb->dev->name == "lo", it prints sample debug message into trace_pipe
> via bpf_trace_printk() helper function.
> 
> tracex1_user.c - corresponding user space component that:
> - loads bpf program via bpf() syscall
> - opens kprobes:netif_receive_skb event via perf_event_open() syscall
> - attaches the program to event via ioctl(event_fd, PERF_EVENT_IOC_SET_BPF, prog_fd);
> - prints from trace_pipe
> 
> Note, this bpf program is completely non-portable. It must be recompiled
> with current kernel headers. kprobe is not a stable ABI and bpf+kprobe scripts
> may stop working any time.
> 
> bpf verifier will detect that it's using bpf_trace_printk() and kernel will
> print warning banner:
> ** trace_printk() being used. Allocating extra memory.  **

Printing this might be OK.

> **                                                      **
> ** This means that this is a DEBUG kernel and it is     **
> ** unsafe for production use.                           **

But I think printing that it's unsafe for production use is over the 
top: it's up to the admin whether it's safe or unsafe, just like 
inserting a kprobe can be safe or unsafe.

Informing that something happened is enough.

Thanks,

	Ingo