[RFC PATCH v2 14/15] samples/bpf: Add sample for testing bpf fetch args

[He Kuang <hekuang@huawei.com>]

Sample code for testing bpf fetch args.

Works as following steps:

  $ perf bpf record --object sample_bpf_fetch_args.o -- dd if=/dev/zero of=/mnt/data/test bs=4k count=3

show result in ringbuffer:

  $ perf script
  dd  1176 [000] 87706.975390: perf_bpf_probe:generic_perform_write: (ffffffff811307ea) a_ops=0xffffffff81a200e0 bytes=0x1000 page=0xffff88007c61fd80 pos=0
  dd  1176 [000] 87706.975390: perf_bpf_probe:generic_perform_write: (ffffffff811307ea) a_ops=0xffffffff81a200e0 bytes=0x1000 page=0xffffea0001c44800 pos=4096
  dd  1176 [000] 87706.975390: perf_bpf_probe:generic_perform_write: (ffffffff811307ea) a_ops=0xffffffff81a200e0 bytes=0x1000 page=0xffffea0001ed24c0 pos=8192
  dd  1176 [000] 87706.975390: perf_bpf_probe:submit_bio: (ffffffff81310cdf) count=0x3000 rw=1 bio=0xffff88007c237a80

show result in bpf prog:

  $ cat /sys/kernel/debug/tracing/trace |grep dd
  dd-1176  [000] d... 87778.531511: : generic_perform_write(1): a_ops=ffffffff81a200e0, bytes=4096
  dd-1176  [000] d... 87778.531550: : generic_perform_write(2): page =ffff88007c61fd80, pos  =0
  dd-1176  [000] d... 87778.532142: : generic_perform_write(1): a_ops=ffffffff81a200e0, bytes=4096
  dd-1176  [000] d... 87778.532154: : generic_perform_write(2): page =ffffea0001c44800, pos  =4096
  dd-1176  [000] d... 87778.532342: : generic_perform_write(1): a_ops=ffffffff81a200e0, bytes=4096
  dd-1176  [000] d... 87778.532354: : generic_perform_write(2): page =ffffea0001ed24c0, pos  =8192
  dd-1176  [000] d.h. 87778.533049: : submit_bio count=12288, rw=1, bio=ffff88007c237a80

Signed-off-by: He Kuang <hekuang@huawei.com>
---
 samples/bpf/Makefile                |  1 +
 samples/bpf/sample_bpf_fetch_args.c | 62 +++++++++++++++++++++++++++++++++++++
 2 files changed, 63 insertions(+)
 create mode 100644 samples/bpf/sample_bpf_fetch_args.c

diff --git a/samples/bpf/Makefile b/samples/bpf/Makefile
index 8fdbd73..dc0b0e8 100644
--- a/samples/bpf/Makefile
+++ b/samples/bpf/Makefile
@@ -30,6 +30,7 @@ always += tracex2_kern.o
 always += tracex3_kern.o
 always += tracex4_kern.o
 always += tcbpf1_kern.o
+always += sample_bpf_fetch_args.o
 
 HOSTCFLAGS += -I$(objtree)/usr/include
 
diff --git a/samples/bpf/sample_bpf_fetch_args.c b/samples/bpf/sample_bpf_fetch_args.c
new file mode 100644
index 0000000..5d2c80c
--- /dev/null
+++ b/samples/bpf/sample_bpf_fetch_args.c
@@ -0,0 +1,62 @@
+/*
+  Sample code for bpf_fetch_args().
+*/
+
+#include <linux/writeback.h>
+#include <linux/blkdev.h>
+
+#include <uapi/linux/bpf.h>
+#include <linux/version.h>
+#include "bpf_helpers.h"
+
+/*
+ * Prototype:
+ * ssize_t generic_perform_write(struct file *file,
+ *				struct iov_iter *i, loff_t pos)
+ * Local variables:
+ *	const struct address_space_operations *a_ops;
+ *	unsigned long bytes;
+ *	struct page *page;
+ *	loff_t pos;
+ */
+SEC("generic_perform_write=generic_perform_write+122 file->f_mapping->a_ops bytes page pos")
+int NODE_generic_perform_write(struct pt_regs *ctx,
+			void *a_ops,
+			void *bytes,
+			void *page,
+			void *pos)
+{
+	/* Too many args for bpf_trace_printk, show in 2 lines */
+	char fmt1[] = "generic_perform_write(1): a_ops=%p, bytes=%lu\n";
+	char fmt2[] = "generic_perform_write(2): page =%p, pos  =%lu\n";
+
+	bpf_trace_printk(fmt1, sizeof(fmt1),
+			a_ops, (unsigned long)bytes);
+	bpf_trace_printk(fmt2, sizeof(fmt2),
+			page, (unsigned long)pos);
+
+	return 1;
+}
+
+/*
+ * Prototype:
+ * void submit_bio(int rw, struct bio *bio)
+ * Local variables:
+ *	unsigned int count;
+ */
+SEC("submit_bio=submit_bio+63 count rw bio")
+int NODE_submit_bio(struct pt_regs *ctx,
+			void *count,
+			void *rw,
+			void *bio)
+{
+	char fmt[] = "submit_bio count=%u, rw=%d, bio=%p\n";
+
+	bpf_trace_printk(fmt, sizeof(fmt),
+			(unsigned int)count, (int)rw, bio);
+
+	return 1;
+}
+
+char _license[] SEC("license") = "GPL";
+u32 _version SEC("version") = LINUX_VERSION_CODE;
-- 
1.8.5.2