linux-kernel - Re: [PATCH v3 linux-trace 1/8] tracing: attach eBPF programs to tracepoints and syscalls

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Date:	Mon, 9 Feb 2015 23:46:05 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Alexei Starovoitov <ast@...mgrid.com>
Cc:	Ingo Molnar <mingo@...nel.org>, Namhyung Kim <namhyung@...nel.org>,
	Arnaldo Carvalho de Melo <acme@...radead.org>,
	Jiri Olsa <jolsa@...hat.com>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	linux-api@...r.kernel.org, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 linux-trace 1/8] tracing: attach eBPF programs to
 tracepoints and syscalls

On Mon,  9 Feb 2015 19:45:54 -0800
Alexei Starovoitov <ast@...mgrid.com> wrote:

> +#endif /* _LINUX_KERNEL_BPF_TRACE_H */
> diff --git a/include/trace/ftrace.h b/include/trace/ftrace.h
> index 139b5067345b..4c275ce2dcf0 100644
> --- a/include/trace/ftrace.h
> +++ b/include/trace/ftrace.h
> @@ -17,6 +17,7 @@
>   */
>  
>  #include <linux/ftrace_event.h>
> +#include <trace/bpf_trace.h>
>  
>  /*
>   * DECLARE_EVENT_CLASS can be used to add a generic function
> @@ -755,12 +756,32 @@ __attribute__((section("_ftrace_events"))) *__event_##call = &event_##call
>  #undef __perf_task
>  #define __perf_task(t)	(__task = (t))
>  
> +/* zero extend integer, pointer or aggregate type to u64 without warnings */
> +#define __CAST_TO_U64(EXPR) ({ \
> +	u64 ret = 0; \
> +	typeof(EXPR) expr = EXPR; \
> +	switch (sizeof(expr)) { \
> +	case 8: ret = *(u64 *) &expr; break; \
> +	case 4: ret = *(u32 *) &expr; break; \
> +	case 2: ret = *(u16 *) &expr; break; \
> +	case 1: ret = *(u8 *) &expr; break; \
> +	} \
> +	ret; })
> +
> +#define __BPF_CAST1(a,...) __CAST_TO_U64(a)
> +#define __BPF_CAST2(a,...) __CAST_TO_U64(a), __BPF_CAST1(__VA_ARGS__)
> +#define __BPF_CAST3(a,...) __CAST_TO_U64(a), __BPF_CAST2(__VA_ARGS__)
> +#define __BPF_CAST4(a,...) __CAST_TO_U64(a), __BPF_CAST3(__VA_ARGS__)
> +#define __BPF_CAST5(a,...) __CAST_TO_U64(a), __BPF_CAST4(__VA_ARGS__)
> +#define __BPF_CAST6(a,...) __CAST_TO_U64(a), __BPF_CAST5(__VA_ARGS__)
> +
>  #undef DECLARE_EVENT_CLASS
>  #define DECLARE_EVENT_CLASS(call, proto, args, tstruct, assign, print)	\
>  static notrace void							\
>  perf_trace_##call(void *__data, proto)					\
>  {									\
>  	struct ftrace_event_call *event_call = __data;			\
> +	struct bpf_prog *prog = event_call->prog;			\


Looks like this is entirely perf based and does not interact with
ftrace at all. In other words, it's perf not tracing.

It makes more sense to go through tip than the tracing tree.

But I still do not want any hard coded event structures. All access to
data from the binary code must be parsed by looking at the event/format
files. Otherwise you will lock internals of the kernel as userspace
ABI, because eBPF programs will break if those internals change, and
that could severely limit progress in the future.

-- Steve

>  	struct ftrace_data_offsets_##call __maybe_unused __data_offsets;\
>  	struct ftrace_raw_##call *entry;				\
>  	struct pt_regs __regs;						\
> @@ -771,6 +792,16 @@ perf_trace_##call(void *__data, proto)					\
>  	int __data_size;						\
>  	int rctx;							\
>  									\
> +	if (prog) {							\
> +		__maybe_unused const u64 z = 0;				\
> +		struct bpf_context __ctx = ((struct bpf_context) {	\
> +				__BPF_CAST6(args, z, z, z, z, z)	\
> +			});						\
> +									\
> +		if (!trace_call_bpf(prog, &__ctx))			\
> +			return;						\
> +	}								\
> +									\
>  	__data_size = ftrace_get_offsets_##call(&__data_offsets, args); \
>  									\
>  	head = this_cpu_ptr(event_call->perf_events);			\
--
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