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Date: Thu, 15 Oct 2015 14:26:23 +0900
From: Namhyung Kim <namhyung@...nel.org>
To: Wang Nan <wangnan0@...wei.com>
Cc: acme@...nel.org, ast@...mgrid.com, brendan.d.gregg@...il.com,
a.p.zijlstra@...llo.nl, daniel@...earbox.net, dsahern@...il.com,
hekuang@...wei.com, jolsa@...nel.org, lizefan@...wei.com,
masami.hiramatsu.pt@...achi.com, paulus@...ba.org,
linux-kernel@...r.kernel.org, pi3orama@....com,
xiakaixu@...wei.com, Arnaldo Carvalho de Melo <acme@...hat.com>
Subject: Re: [PATCH 16/31] perf tools: Add prologue for BPF programs for
fetching arguments
On Wed, Oct 14, 2015 at 12:41:27PM +0000, Wang Nan wrote:
> From: He Kuang <hekuang@...wei.com>
>
> This patch generates prologue for a BPF program which fetch arguments
> for it. With this patch, the program can have arguments as follow:
>
> SEC("lock_page=__lock_page page->flags")
> int lock_page(struct pt_regs *ctx, int err, unsigned long flags)
> {
> return 1;
> }
>
> This patch passes at most 3 arguments from r3, r4 and r5. r1 is still
> the ctx pointer. r2 is used to indicate the successfulness of
> dereferencing.
>
> This patch uses r6 to hold ctx (struct pt_regs) and r7 to hold stack
> pointer for result. Result of each arguments first store on stack:
>
> low address
> BPF_REG_FP - 24 ARG3
> BPF_REG_FP - 16 ARG2
> BPF_REG_FP - 8 ARG1
> BPF_REG_FP
> high address
>
> Then loaded into r3, r4 and r5.
>
> The output prologue for offn(...off2(off1(reg)))) should be:
>
> r6 <- r1 // save ctx into a callee saved register
> r7 <- fp
> r7 <- r7 - stack_offset // pointer to result slot
> /* load r3 with the offset in pt_regs of 'reg' */
> (r7) <- r3 // make slot valid
> r3 <- r3 + off1 // prepare to read unsafe pointer
> r2 <- 8
> r1 <- r7 // result put onto stack
> call probe_read // read unsafe pointer
> jnei r0, 0, err // error checking
> r3 <- (r7) // read result
> r3 <- r3 + off2 // prepare to read unsafe pointer
> r2 <- 8
> r1 <- r7
> call probe_read
> jnei r0, 0, err
> ...
> /* load r2, r3, r4 from stack */
> goto success
> err:
> r2 <- 1
> /* load r3, r4, r5 with 0 */
> goto usercode
> success:
> r2 <- 0
> usercode:
> r1 <- r6 // restore ctx
> // original user code
>
> If all of arguments reside in register (dereferencing is not
> required), gen_prologue_fastpath() will be used to create
> fast prologue:
>
> r3 <- (r1 + offset of reg1)
> r4 <- (r1 + offset of reg2)
> r5 <- (r1 + offset of reg3)
> r2 <- 0
>
> P.S.
>
> eBPF calling convention is defined as:
>
> * r0 - return value from in-kernel function, and exit value
> for eBPF program
> * r1 - r5 - arguments from eBPF program to in-kernel function
> * r6 - r9 - callee saved registers that in-kernel function will
> preserve
> * r10 - read-only frame pointer to access stack
>
> Signed-off-by: He Kuang <hekuang@...wei.com>
> Signed-off-by: Wang Nan <wangnan0@...wei.com>
> Cc: Alexei Starovoitov <ast@...mgrid.com>
> Cc: Brendan Gregg <brendan.d.gregg@...il.com>
> Cc: Daniel Borkmann <daniel@...earbox.net>
> Cc: David Ahern <dsahern@...il.com>
> Cc: He Kuang <hekuang@...wei.com>
> Cc: Jiri Olsa <jolsa@...nel.org>
> Cc: Kaixu Xia <xiakaixu@...wei.com>
> Cc: Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>
> Cc: Namhyung Kim <namhyung@...nel.org>
> Cc: Paul Mackerras <paulus@...ba.org>
> Cc: Peter Zijlstra <a.p.zijlstra@...llo.nl>
> Cc: Zefan Li <lizefan@...wei.com>
> Cc: pi3orama@....com
> Cc: Arnaldo Carvalho de Melo <acme@...hat.com>
> Link: http://lkml.kernel.org/n/ebpf-6yw9eg0ej3l4jnqhinngkw86@git.kernel.org
> ---
[SNIP]
> +int bpf__gen_prologue(struct probe_trace_arg *args, int nargs,
> + struct bpf_insn *new_prog, size_t *new_cnt,
> + size_t cnt_space)
> +{
> + struct bpf_insn *success_code = NULL;
> + struct bpf_insn *error_code = NULL;
> + struct bpf_insn *user_code = NULL;
> + struct bpf_insn_pos pos;
> + bool fastpath = true;
> + int i;
> +
> + if (!new_prog || !new_cnt)
> + return -EINVAL;
> +
> + pos.begin = new_prog;
> + pos.end = new_prog + cnt_space;
> + pos.pos = new_prog;
> +
> + if (!nargs) {
> + ins(BPF_ALU64_IMM(BPF_MOV, BPF_PROLOGUE_FETCH_RESULT_REG, 0),
> + &pos);
> +
> + if (check_pos(&pos))
> + goto errout;
> +
> + *new_cnt = pos_get_cnt(&pos);
> + return 0;
> + }
> +
> + if (nargs > BPF_PROLOGUE_MAX_ARGS)
> + nargs = BPF_PROLOGUE_MAX_ARGS;
Wouldn't it be better to inform user if it ignored some arguments?
Thanks,
Namhyung
> + if (cnt_space > BPF_MAXINSNS)
> + cnt_space = BPF_MAXINSNS;
> +
> + /* First pass: validation */
> + for (i = 0; i < nargs; i++) {
> + struct probe_trace_arg_ref *ref = args[i].ref;
> +
> + if (args[i].value[0] == '@') {
> + /* TODO: fetch global variable */
> + pr_err("bpf: prologue: global %s%+ld not support\n",
> + args[i].value, ref ? ref->offset : 0);
> + return -ENOTSUP;
> + }
> +
> + while (ref) {
> + /* fastpath is true if all args has ref == NULL */
> + fastpath = false;
> +
> + /*
> + * Instruction encodes immediate value using
> + * s32, ref->offset is long. On systems which
> + * can't fill long in s32, refuse to process if
> + * ref->offset too large (or small).
> + */
> +#ifdef __LP64__
> +#define OFFSET_MAX ((1LL << 31) - 1)
> +#define OFFSET_MIN ((1LL << 31) * -1)
> + if (ref->offset > OFFSET_MAX ||
> + ref->offset < OFFSET_MIN) {
> + pr_err("bpf: prologue: offset out of bound: %ld\n",
> + ref->offset);
> + return -E2BIG;
> + }
> +#endif
> + ref = ref->next;
> + }
> + }
> + pr_debug("prologue: pass validation\n");
> +
> + if (fastpath) {
> + /* If all variables are registers... */
> + pr_debug("prologue: fast path\n");
> + if (gen_prologue_fastpath(&pos, args, nargs))
> + goto errout;
> + } else {
> + pr_debug("prologue: slow path\n");
> +
> + /* Initialization: move ctx to a callee saved register. */
> + ins(BPF_MOV64_REG(BPF_REG_CTX, BPF_REG_ARG1), &pos);
> +
> + if (gen_prologue_slowpath(&pos, args, nargs))
> + goto errout;
> + /*
> + * start of ERROR_CODE (only slow pass needs error code)
> + * mov r2 <- 1
> + * goto usercode
> + */
> + error_code = pos.pos;
> + ins(BPF_ALU64_IMM(BPF_MOV, BPF_PROLOGUE_FETCH_RESULT_REG, 1),
> + &pos);
> +
> + for (i = 0; i < nargs; i++)
> + ins(BPF_ALU64_IMM(BPF_MOV,
> + BPF_PROLOGUE_START_ARG_REG + i,
> + 0),
> + &pos);
> + ins(BPF_JMP_IMM(BPF_JA, BPF_REG_0, 0, JMP_TO_USER_CODE),
> + &pos);
> + }
> +
> + /*
> + * start of SUCCESS_CODE:
> + * mov r2 <- 0
> + * goto usercode // skip
> + */
> + success_code = pos.pos;
> + ins(BPF_ALU64_IMM(BPF_MOV, BPF_PROLOGUE_FETCH_RESULT_REG, 0), &pos);
> +
> + /*
> + * start of USER_CODE:
> + * Restore ctx to r1
> + */
> + user_code = pos.pos;
> + if (!fastpath) {
> + /*
> + * Only slow path needs restoring of ctx. In fast path,
> + * register are loaded directly from r1.
> + */
> + ins(BPF_MOV64_REG(BPF_REG_ARG1, BPF_REG_CTX), &pos);
> + if (prologue_relocate(&pos, error_code, success_code,
> + user_code))
> + goto errout;
> + }
> +
> + if (check_pos(&pos))
> + goto errout;
> +
> + *new_cnt = pos_get_cnt(&pos);
> + return 0;
> +errout:
> + return -ERANGE;
> +}
> diff --git a/tools/perf/util/bpf-prologue.h b/tools/perf/util/bpf-prologue.h
> new file mode 100644
> index 0000000..f1e4c5d
> --- /dev/null
> +++ b/tools/perf/util/bpf-prologue.h
> @@ -0,0 +1,34 @@
> +/*
> + * Copyright (C) 2015, He Kuang <hekuang@...wei.com>
> + * Copyright (C) 2015, Huawei Inc.
> + */
> +#ifndef __BPF_PROLOGUE_H
> +#define __BPF_PROLOGUE_H
> +
> +#include <linux/compiler.h>
> +#include <linux/filter.h>
> +#include "probe-event.h"
> +
> +#define BPF_PROLOGUE_MAX_ARGS 3
> +#define BPF_PROLOGUE_START_ARG_REG BPF_REG_3
> +#define BPF_PROLOGUE_FETCH_RESULT_REG BPF_REG_2
> +
> +#ifdef HAVE_BPF_PROLOGUE
> +int bpf__gen_prologue(struct probe_trace_arg *args, int nargs,
> + struct bpf_insn *new_prog, size_t *new_cnt,
> + size_t cnt_space);
> +#else
> +static inline int
> +bpf__gen_prologue(struct probe_trace_arg *args __maybe_unused,
> + int nargs __maybe_unused,
> + struct bpf_insn *new_prog __maybe_unused,
> + size_t *new_cnt,
> + size_t cnt_space __maybe_unused)
> +{
> + if (!new_cnt)
> + return -EINVAL;
> + *new_cnt = 0;
> + return 0;
> +}
> +#endif
> +#endif /* __BPF_PROLOGUE_H */
> --
> 1.8.3.4
>
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