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Date: Sat, 28 Feb 2015 11:20:43 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Alexei Starovoitov <ast@plumgrid.com>
Cc: Ingo Molnar <mingo@kernel.org>, Steven Rostedt <rostedt@goodmis.org>,
        Namhyung Kim <namhyung@kernel.org>,
        Arnaldo Carvalho de Melo <acme@infradead.org>,
        Jiri Olsa <jolsa@redhat.com>,
        Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>,
        "David S. Miller" <davem@davemloft.net>,
        Daniel Borkmann <dborkman@redhat.com>, linux-api@vger.kernel.org,
        netdev@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v4 tip 3/7] tracing: allow BPF programs to call
 ktime_get_ns()
Message-ID: <20150228102043.GX5029@twins.programming.kicks-ass.net>
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On Fri, Feb 27, 2015 at 04:08:51PM -0800, Alexei Starovoitov wrote:
> bpf_ktime_get_ns() is used by programs to compue time delta between events
> or as a timestamp
> 
> Signed-off-by: Alexei Starovoitov <ast@plumgrid.com>
> ---
>  include/uapi/linux/bpf.h |    1 +
>  kernel/trace/bpf_trace.c |   10 ++++++++++
>  2 files changed, 11 insertions(+)
> 
> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> index 4486d36d2e9e..101e509d1001 100644
> --- a/include/uapi/linux/bpf.h
> +++ b/include/uapi/linux/bpf.h
> @@ -165,6 +165,7 @@ enum bpf_func_id {
>  	BPF_FUNC_map_update_elem, /* int map_update_elem(&map, &key, &value, flags) */
>  	BPF_FUNC_map_delete_elem, /* int map_delete_elem(&map, &key) */
>  	BPF_FUNC_probe_read,      /* int bpf_probe_read(void *dst, int size, void *src) */
> +	BPF_FUNC_ktime_get_ns,    /* u64 bpf_ktime_get_ns(void) */
>  	__BPF_FUNC_MAX_ID,
>  };
>  
> diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
> index bcce9b238dad..64b918488607 100644
> --- a/kernel/trace/bpf_trace.c
> +++ b/kernel/trace/bpf_trace.c
> @@ -36,6 +36,11 @@ static u64 bpf_probe_read(u64 r1, u64 r2, u64 r3, u64 r4, u64 r5)
>  	return probe_kernel_read(dst, unsafe_ptr, size);
>  }
>  
> +static u64 bpf_ktime_get_ns(u64 r1, u64 r2, u64 r3, u64 r4, u64 r5)
> +{
> +	return ktime_get_ns();
> +}

Please use ktime_get_mono_fast_ns() instead. If you ever want to allow
running BPF stuff from general tracepoints and the like you need to
be NMI safe.