linux-kernel - Re: [PATCH 11/13] perf bpf: Add helper header files

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Date:   Mon, 12 Mar 2018 11:44:57 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Jiri Olsa <jolsa@...nel.org>
Cc:     Arnaldo Carvalho de Melo <acme@...nel.org>,
        lkml <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Namhyung Kim <namhyung@...nel.org>,
        David Ahern <dsahern@...il.com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Peter Zijlstra <a.p.zijlstra@...llo.nl>,
        Daniel Borkmann <daniel@...earbox.net>,
        Wangnan <wangnan0@...wei.com>
Subject: Re: [PATCH 11/13] perf bpf: Add helper header files

On Mon, Mar 12, 2018 at 10:43:11AM +0100, Jiri Olsa wrote:
> diff --git a/tools/perf/util/bpf-userapi.h b/tools/perf/util/bpf-userapi.h
> new file mode 100644
> index 000000000000..63f2b4c13a5c
> --- /dev/null
> +++ b/tools/perf/util/bpf-userapi.h
> @@ -0,0 +1,11 @@
> +#ifndef __BPF_USERAPI_H
> +#define __BPF_USERAPI_H
> +
> +enum {
> +	BPF_FUNC_USER_START			= 10000,
> +	BPF_FUNC_USER_print			= BPF_FUNC_USER_START,
> +	BPF_FUNC_USER_bpf_map_get_next_key,
> +	BPF_FUNC_USER_bpf_map_lookup_elem,
> +};
> +
> +#endif /* __BPF_USERFUNCS_H */
> diff --git a/tools/perf/util/bpf-userfuncs.h b/tools/perf/util/bpf-userfuncs.h
> new file mode 100644
> index 000000000000..e920420237ee
> --- /dev/null
> +++ b/tools/perf/util/bpf-userfuncs.h
> @@ -0,0 +1,19 @@
> +#ifndef __BPF_USERFUNCS_H
> +#define __BPF_USERFUNCS_H
> +
> +#include <bpf-userapi.h>
> +
> +static int (*bpfu_print)(const char *fmt, ...) =
> +	(void *) BPF_FUNC_USER_print;
> +static int (*bpfu_map_get_next_key)(void *map, void *key, void *value) =
> +	(void *) BPF_FUNC_USER_bpf_map_get_next_key;
> +static int (*bpfu_map_lookup_elem)(void *map, void *key, void *value) =
> +	(void *) BPF_FUNC_USER_bpf_map_lookup_elem;
> +
> +#define print(fmt, ...)                                  \
> +({                                                       \
> +	char ____fmt[] = fmt;                            \
> +	bpfu_print(____fmt, ##__VA_ARGS__);              \
> +})

since there is no verifier for this user space bpf interpreter
there is no need to restrict what BEGIN/END progs can do and can call.
llvm will happily compile any C code into bpf instructions.
With little bit of elf magic it's possible to let it call any libc
function instead of only 3 above.
perf loader will see bpf_call into normal printf, memcpy, memcmp, etc
then during the loading need to associate symbol with actual address
of these functions inside perf binary and let call_cb do the call.
I think this way begin/end will be much more powerful and useful.
Potentially allowing some of the bcc scripts to be written this way.