[RFC PATCH 3/5] bpf: Add helper function for fetching variables at probe point

[He Kuang <hekuang@huawei.com>]

This helper function uses kernel structure trace_probe and related fetch
functions for fetching variables described in 'SEC' to bpf stack.

Signed-off-by: He Kuang <hekuang@huawei.com>
---
 include/uapi/linux/bpf.h  |  1 +
 kernel/trace/bpf_trace.c  | 38 ++++++++++++++++++++++++++++++++++++++
 samples/bpf/bpf_helpers.h |  2 ++
 3 files changed, 41 insertions(+)

diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
index a9ebdf5..b1a7685 100644
--- a/include/uapi/linux/bpf.h
+++ b/include/uapi/linux/bpf.h
@@ -210,6 +210,7 @@ enum bpf_func_id {
 	 * Return: 0 on success
 	 */
 	BPF_FUNC_l4_csum_replace,
+	BPF_FUNC_fetch_args,
 	__BPF_FUNC_MAX_ID,
 };
 
diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
index 2d56ce5..ba601da 100644
--- a/kernel/trace/bpf_trace.c
+++ b/kernel/trace/bpf_trace.c
@@ -12,6 +12,7 @@
 #include <linux/uaccess.h>
 #include <linux/ctype.h>
 #include "trace.h"
+#include "trace_probe.h"
 
 static DEFINE_PER_CPU(int, bpf_prog_active);
 
@@ -159,6 +160,39 @@ static const struct bpf_func_proto bpf_trace_printk_proto = {
 	.arg2_type	= ARG_CONST_STACK_SIZE,
 };
 
+/* Store the value of each argument */
+static void
+bpf_store_trace_args(struct pt_regs *regs, struct trace_probe *tp,
+		u8 *data)
+{
+	int i;
+
+	for (i = 0; i < tp->nr_args; i++) {
+		/* Just fetching data normally */
+		call_fetch(&tp->args[i].fetch, regs,
+			data + tp->args[i].offset);
+	}
+}
+
+static u64 bpf_fetch_args(u64 r1, u64 r2, u64 r3, u64 r4, u64 r5)
+{
+	struct pt_regs *regs = (struct pt_regs *)(long)r1;
+	struct trace_probe *tp = ((struct bpf_pt_regs *)regs)->tp;
+	void *data = (void *)(long)r2;
+
+	bpf_store_trace_args(regs, tp, data);
+
+	return 0;
+}
+
+static const struct bpf_func_proto bpf_fetch_args_proto = {
+	.func		= bpf_fetch_args,
+	.gpl_only	= true,
+	.ret_type	= RET_INTEGER,
+	.arg1_type	= ARG_PTR_TO_CTX,
+	.arg2_type	= ARG_PTR_TO_STACK,
+};
+
 static const struct bpf_func_proto *kprobe_prog_func_proto(enum bpf_func_id func_id)
 {
 	switch (func_id) {
@@ -181,6 +215,10 @@ static const struct bpf_func_proto *kprobe_prog_func_proto(enum bpf_func_id func
 		trace_printk_init_buffers();
 
 		return &bpf_trace_printk_proto;
+
+	case BPF_FUNC_fetch_args:
+		return &bpf_fetch_args_proto;
+
 	default:
 		return NULL;
 	}
diff --git a/samples/bpf/bpf_helpers.h b/samples/bpf/bpf_helpers.h
index f960b5f..578a8e3 100644
--- a/samples/bpf/bpf_helpers.h
+++ b/samples/bpf/bpf_helpers.h
@@ -21,6 +21,8 @@ static unsigned long long (*bpf_ktime_get_ns)(void) =
 	(void *) BPF_FUNC_ktime_get_ns;
 static int (*bpf_trace_printk)(const char *fmt, int fmt_size, ...) =
 	(void *) BPF_FUNC_trace_printk;
+static int (*bpf_fetch_args)(void *ctx, void *data) =
+	(void *) BPF_FUNC_fetch_args;
 
 /* llvm builtin functions that eBPF C program may use to
  * emit BPF_LD_ABS and BPF_LD_IND instructions
-- 
1.8.5.2