On Jun 23, 2020, at 3:27 PM, Yonghong Song [off-list ref] wrote:



On 6/23/20 3:07 PM, Song Liu wrote:

quoted

quoted

On Jun 23, 2020, at 11:57 AM, Yonghong Song [off-list ref] wrote:



On 6/23/20 12:08 AM, Song Liu wrote:

quoted

The new test is similar to other bpf_iter tests.
Signed-off-by: Song Liu <redacted>
---
 .../selftests/bpf/prog_tests/bpf_iter.c       | 17 +++++++
 .../selftests/bpf/progs/bpf_iter_task_stack.c | 50 +++++++++++++++++++
 2 files changed, 67 insertions(+)
 create mode 100644 tools/testing/selftests/bpf/progs/bpf_iter_task_stack.c

diff --git a/tools/testing/selftests/bpf/prog_tests/bpf_iter.c b/tools/testing/selftests/bpf/prog_tests/bpf_iter.c
index 87c29dde1cf96..baa83328f810d 100644
--- a/tools/testing/selftests/bpf/prog_tests/bpf_iter.c
+++ b/tools/testing/selftests/bpf/prog_tests/bpf_iter.c

@@ -5,6 +5,7 @@
 #include "bpf_iter_netlink.skel.h"
 #include "bpf_iter_bpf_map.skel.h"
 #include "bpf_iter_task.skel.h"
+#include "bpf_iter_task_stack.skel.h"
 #include "bpf_iter_task_file.skel.h"
 #include "bpf_iter_test_kern1.skel.h"
 #include "bpf_iter_test_kern2.skel.h"

@@ -106,6 +107,20 @@ static void test_task(void)
 	bpf_iter_task__destroy(skel);
 }
 +static void test_task_stack(void)
+{
+	struct bpf_iter_task_stack *skel;
+
+	skel = bpf_iter_task_stack__open_and_load();
+	if (CHECK(!skel, "bpf_iter_task_stack__open_and_load",
+		  "skeleton open_and_load failed\n"))
+		return;
+
+	do_dummy_read(skel->progs.dump_task_stack);
+
+	bpf_iter_task_stack__destroy(skel);
+}
+
 static void test_task_file(void)
 {
 	struct bpf_iter_task_file *skel;

@@ -392,6 +407,8 @@ void test_bpf_iter(void)
 		test_bpf_map();
 	if (test__start_subtest("task"))
 		test_task();
+	if (test__start_subtest("task_stack"))
+		test_task_stack();
 	if (test__start_subtest("task_file"))
 		test_task_file();
 	if (test__start_subtest("anon"))

diff --git a/tools/testing/selftests/bpf/progs/bpf_iter_task_stack.c b/tools/testing/selftests/bpf/progs/bpf_iter_task_stack.c
new file mode 100644
index 0000000000000..4fc939e0fca77
--- /dev/null
+++ b/tools/testing/selftests/bpf/progs/bpf_iter_task_stack.c

@@ -0,0 +1,50 @@
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2020 Facebook */
+/* "undefine" structs in vmlinux.h, because we "override" them below */
+#define bpf_iter_meta bpf_iter_meta___not_used
+#define bpf_iter__task bpf_iter__task___not_used
+#include "vmlinux.h"
+#undef bpf_iter_meta
+#undef bpf_iter__task
+#include <bpf/bpf_helpers.h>
+#include <bpf/bpf_tracing.h>
+
+char _license[] SEC("license") = "GPL";
+
+struct bpf_iter_meta {
+	struct seq_file *seq;
+	__u64 session_id;
+	__u64 seq_num;
+} __attribute__((preserve_access_index));
+
+struct bpf_iter__task {
+	struct bpf_iter_meta *meta;
+	struct task_struct *task;
+} __attribute__((preserve_access_index));
+
+#define MAX_STACK_TRACE_DEPTH   64
+unsigned long entries[MAX_STACK_TRACE_DEPTH];
+
+SEC("iter/task")
+int dump_task_stack(struct bpf_iter__task *ctx)
+{
+	struct seq_file *seq = ctx->meta->seq;
+	struct task_struct *task = ctx->task;
+	unsigned int i, num_entries;
+
+	if (task == (void *)0)
+		return 0;
+
+	num_entries = bpf_get_task_stack_trace(task, entries, MAX_STACK_TRACE_DEPTH);
+
+	BPF_SEQ_PRINTF(seq, "pid: %8u\n", task->pid);
+
+	for (i = 0; i < MAX_STACK_TRACE_DEPTH; i++) {
+		if (num_entries > i)
+			BPF_SEQ_PRINTF(seq, "[<0>] %pB\n", (void *)entries[i]);

We may have an issue on 32bit issue.
On 32bit system, the following is called in the kernel
+	return stack_trace_save_tsk(task, (unsigned long *)entries, size, 0);
it will pack addresses at 4 byte increment.
But in BPF program, the reading is in 8 byte increment.

Can we avoid potential issues by requiring size % 8 == 0? Or maybe round down
size to closest multiple of 8?

This is what I mean:
 for bpf program: "long" means u64, so we allocate 64 * 8 buffer size
                  and pass it to the helper
 in the helper, the address will be increased along sizeof(long), which
                is 4 for 32bit system.
         So address is recorded at buf, buf + 4, buf + 8, buf + 12, ...
 After the helper returns, the bpf program tries to retrieve
         the address at buf, buf + 8, buf + 16.

The helper itself is okay. But BPF_SEQ_PRINTF above is wrong.
Is this interpretation correct?

Thanks for the clarification. I guess the best solution is to fix this 
once in the kernel, so BPF programs don't have to worry about it. 

Song