Subject: Re: [PATCH v9 tip 6/9] samples: bpf: simple non-portable kprobe filter example

From: Ingo Molnar <[email-masked]>
Date: Mon, 23 Mar 2015 08:29:29 +0100

* Alexei Starovoitov <ast-uqk4Ao+rVK5Wk0Htik3J/[email-masked]> 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