[PATCH v3 1/2] bpf: Adjust BPF stack helper functions to accommodate skip > 0

[PATCH v3 1/2] bpf: Adjust BPF stack helper functions to accommodate skip > 0

[Namhyung Kim]

Let's say that the caller has storage for num_elem stack frames.  Then,
the BPF stack helper functions walk the stack for only num_elem frames.
This means that if skip > 0, one keeps only 'num_elem - skip' frames.

This is because it sets init_nr in the perf_callchain_entry to the end
of the buffer to save num_elem entries only.  I believe it was because
the perf callchain code unwound the stack frames until it reached the
global max size (sysctl_perf_event_max_stack).

However it now has perf_callchain_entry_ctx.max_stack to limit the
iteration locally.  This simplifies the code to handle init_nr in the
BPF callstack entries and removes the confusion with the perf_event's
__PERF_SAMPLE_CALLCHAIN_EARLY which sets init_nr to 0.

Also change the comment on bpf_get_stack() in the header file to be
more explicit what the return value means.

Link: https://lore.kernel.org/bpf/30a7b5d5-6726-1cc2-eaee-8da2828a9a9c@oracle.com

Fixes: c195651e565a ("bpf: add bpf_get_stack helper")
Based-on-patch-by: Eugene Loh <eugene.loh@oracle.com>
Acked-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Namhyung Kim <namhyung@kernel.org>
---
 include/uapi/linux/bpf.h |  8 +++---
 kernel/bpf/stackmap.c    | 56 +++++++++++++++++-----------------------
 2 files changed, 28 insertions(+), 36 deletions(-)

diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
index b0383d371b9a..f09f20845904 100644
--- a/include/uapi/linux/bpf.h
+++ b/include/uapi/linux/bpf.h
@@ -2975,8 +2975,8 @@ union bpf_attr {
  *
  * 			# sysctl kernel.perf_event_max_stack=<new value>
  * 	Return
- * 		A non-negative value equal to or less than *size* on success,
- * 		or a negative error in case of failure.
+ * 		The non-negative copied *buf* length equal to or less than
+ * 		*size* on success, or a negative error in case of failure.
  *
  * long bpf_skb_load_bytes_relative(const void *skb, u32 offset, void *to, u32 len, u32 start_header)
  * 	Description
@@ -4279,8 +4279,8 @@ union bpf_attr {
  *
  *			# sysctl kernel.perf_event_max_stack=<new value>
  *	Return
- *		A non-negative value equal to or less than *size* on success,
- *		or a negative error in case of failure.
+ * 		The non-negative copied *buf* length equal to or less than
+ * 		*size* on success, or a negative error in case of failure.
  *
  * long bpf_load_hdr_opt(struct bpf_sock_ops *skops, void *searchby_res, u32 len, u64 flags)
  *	Description
diff --git a/kernel/bpf/stackmap.c b/kernel/bpf/stackmap.c
index 22c8ae94e4c1..2823dcefae10 100644
--- a/kernel/bpf/stackmap.c
+++ b/kernel/bpf/stackmap.c
@@ -166,7 +166,7 @@ static void stack_map_get_build_id_offset(struct bpf_stack_build_id *id_offs,
 }
 
 static struct perf_callchain_entry *
-get_callchain_entry_for_task(struct task_struct *task, u32 init_nr)
+get_callchain_entry_for_task(struct task_struct *task, u32 max_depth)
 {
 #ifdef CONFIG_STACKTRACE
 	struct perf_callchain_entry *entry;
@@ -177,9 +177,8 @@ get_callchain_entry_for_task(struct task_struct *task, u32 init_nr)
 	if (!entry)
 		return NULL;
 
-	entry->nr = init_nr +
-		stack_trace_save_tsk(task, (unsigned long *)(entry->ip + init_nr),
-				     sysctl_perf_event_max_stack - init_nr, 0);
+	entry->nr = stack_trace_save_tsk(task, (unsigned long *)entry->ip,
+					 max_depth, 0);
 
 	/* stack_trace_save_tsk() works on unsigned long array, while
 	 * perf_callchain_entry uses u64 array. For 32-bit systems, it is
@@ -191,7 +190,7 @@ get_callchain_entry_for_task(struct task_struct *task, u32 init_nr)
 		int i;
 
 		/* copy data from the end to avoid using extra buffer */
-		for (i = entry->nr - 1; i >= (int)init_nr; i--)
+		for (i = entry->nr - 1; i >= 0; i--)
 			to[i] = (u64)(from[i]);
 	}
 
@@ -208,27 +207,19 @@ static long __bpf_get_stackid(struct bpf_map *map,
 {
 	struct bpf_stack_map *smap = container_of(map, struct bpf_stack_map, map);
 	struct stack_map_bucket *bucket, *new_bucket, *old_bucket;
-	u32 max_depth = map->value_size / stack_map_data_size(map);
-	/* stack_map_alloc() checks that max_depth <= sysctl_perf_event_max_stack */
-	u32 init_nr = sysctl_perf_event_max_stack - max_depth;
 	u32 skip = flags & BPF_F_SKIP_FIELD_MASK;
 	u32 hash, id, trace_nr, trace_len;
 	bool user = flags & BPF_F_USER_STACK;
 	u64 *ips;
 	bool hash_matches;
 
-	/* get_perf_callchain() guarantees that trace->nr >= init_nr
-	 * and trace-nr <= sysctl_perf_event_max_stack, so trace_nr <= max_depth
-	 */
-	trace_nr = trace->nr - init_nr;
-
-	if (trace_nr <= skip)
+	if (trace->nr <= skip)
 		/* skipping more than usable stack trace */
 		return -EFAULT;
 
-	trace_nr -= skip;
+	trace_nr = trace->nr - skip;
 	trace_len = trace_nr * sizeof(u64);
-	ips = trace->ip + skip + init_nr;
+	ips = trace->ip + skip;
 	hash = jhash2((u32 *)ips, trace_len / sizeof(u32), 0);
 	id = hash & (smap->n_buckets - 1);
 	bucket = READ_ONCE(smap->buckets[id]);
@@ -285,8 +276,7 @@ BPF_CALL_3(bpf_get_stackid, struct pt_regs *, regs, struct bpf_map *, map,
 	   u64, flags)
 {
 	u32 max_depth = map->value_size / stack_map_data_size(map);
-	/* stack_map_alloc() checks that max_depth <= sysctl_perf_event_max_stack */
-	u32 init_nr = sysctl_perf_event_max_stack - max_depth;
+	u32 skip = flags & BPF_F_SKIP_FIELD_MASK;
 	bool user = flags & BPF_F_USER_STACK;
 	struct perf_callchain_entry *trace;
 	bool kernel = !user;
@@ -295,8 +285,12 @@ BPF_CALL_3(bpf_get_stackid, struct pt_regs *, regs, struct bpf_map *, map,
 			       BPF_F_FAST_STACK_CMP | BPF_F_REUSE_STACKID)))
 		return -EINVAL;
 
-	trace = get_perf_callchain(regs, init_nr, kernel, user,
-				   sysctl_perf_event_max_stack, false, false);
+	max_depth += skip;
+	if (max_depth > sysctl_perf_event_max_stack)
+		max_depth = sysctl_perf_event_max_stack;
+
+	trace = get_perf_callchain(regs, 0, kernel, user, max_depth,
+				   false, false);
 
 	if (unlikely(!trace))
 		/* couldn't fetch the stack trace */
@@ -387,7 +381,7 @@ static long __bpf_get_stack(struct pt_regs *regs, struct task_struct *task,
 			    struct perf_callchain_entry *trace_in,
 			    void *buf, u32 size, u64 flags)
 {
-	u32 init_nr, trace_nr, copy_len, elem_size, num_elem;
+	u32 trace_nr, copy_len, elem_size, num_elem, max_depth;
 	bool user_build_id = flags & BPF_F_USER_BUILD_ID;
 	u32 skip = flags & BPF_F_SKIP_FIELD_MASK;
 	bool user = flags & BPF_F_USER_STACK;
@@ -412,30 +406,28 @@ static long __bpf_get_stack(struct pt_regs *regs, struct task_struct *task,
 		goto err_fault;
 
 	num_elem = size / elem_size;
-	if (sysctl_perf_event_max_stack < num_elem)
-		init_nr = 0;
-	else
-		init_nr = sysctl_perf_event_max_stack - num_elem;
+	max_depth = num_elem + skip;
+	if (sysctl_perf_event_max_stack < max_depth)
+		max_depth = sysctl_perf_event_max_stack;
 
 	if (trace_in)
 		trace = trace_in;
 	else if (kernel && task)
-		trace = get_callchain_entry_for_task(task, init_nr);
+		trace = get_callchain_entry_for_task(task, max_depth);
 	else
-		trace = get_perf_callchain(regs, init_nr, kernel, user,
-					   sysctl_perf_event_max_stack,
+		trace = get_perf_callchain(regs, 0, kernel, user, max_depth,
 					   false, false);
 	if (unlikely(!trace))
 		goto err_fault;
 
-	trace_nr = trace->nr - init_nr;
-	if (trace_nr < skip)
+	if (trace->nr < skip)
 		goto err_fault;
 
-	trace_nr -= skip;
+	trace_nr = trace->nr - skip;
 	trace_nr = (trace_nr <= num_elem) ? trace_nr : num_elem;
 	copy_len = trace_nr * elem_size;
-	ips = trace->ip + skip + init_nr;
+
+	ips = trace->ip + skip;
 	if (user && user_build_id)
 		stack_map_get_build_id_offset(buf, ips, trace_nr, user);
 	else
-- 
2.35.1.723.g4982287a31-goog

[PATCH v3 2/2] bpf/selftests: Test skipping stacktrace

[Namhyung Kim]

Add a test case for stacktrace with skip > 0 using a small sized
buffer.  It didn't support skipping entries greater than or equal to
the size of buffer and filled the skipped part with 0.

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
---
v3)
 * add pid filter
 * change assert condition

 .../bpf/prog_tests/stacktrace_map_skip.c      | 63 +++++++++++++++++
 .../selftests/bpf/progs/stacktrace_map_skip.c | 68 +++++++++++++++++++
 2 files changed, 131 insertions(+)
 create mode 100644 tools/testing/selftests/bpf/prog_tests/stacktrace_map_skip.c
 create mode 100644 tools/testing/selftests/bpf/progs/stacktrace_map_skip.c

diff --git a/tools/testing/selftests/bpf/prog_tests/stacktrace_map_skip.c b/tools/testing/selftests/bpf/prog_tests/stacktrace_map_skip.c
new file mode 100644
index 000000000000..1932b1e0685c
--- /dev/null
+++ b/tools/testing/selftests/bpf/prog_tests/stacktrace_map_skip.c
@@ -0,0 +1,63 @@
+// SPDX-License-Identifier: GPL-2.0
+#include <test_progs.h>
+#include "stacktrace_map_skip.skel.h"
+
+#define TEST_STACK_DEPTH  2
+
+void test_stacktrace_map_skip(void)
+{
+	struct stacktrace_map_skip *skel;
+	int stackid_hmap_fd, stackmap_fd, stack_amap_fd;
+	int err, stack_trace_len;
+
+	skel = stacktrace_map_skip__open_and_load();
+	if (!ASSERT_OK_PTR(skel, "skel_open_and_load"))
+		return;
+
+	/* find map fds */
+	stackid_hmap_fd = bpf_map__fd(skel->maps.stackid_hmap);
+	if (!ASSERT_GE(stackid_hmap_fd, 0, "stackid_hmap fd"))
+		goto out;
+
+	stackmap_fd = bpf_map__fd(skel->maps.stackmap);
+	if (!ASSERT_GE(stackmap_fd, 0, "stackmap fd"))
+		goto out;
+
+	stack_amap_fd = bpf_map__fd(skel->maps.stack_amap);
+	if (!ASSERT_GE(stack_amap_fd, 0, "stack_amap fd"))
+		goto out;
+
+	skel->bss->pid = getpid();
+
+	err = stacktrace_map_skip__attach(skel);
+	if (!ASSERT_OK(err, "skel_attach"))
+		goto out;
+
+	/* give some time for bpf program run */
+	sleep(1);
+
+	/* disable stack trace collection */
+	skel->bss->control = 1;
+
+	/* for every element in stackid_hmap, we can find a corresponding one
+	 * in stackmap, and vise versa.
+	 */
+	err = compare_map_keys(stackid_hmap_fd, stackmap_fd);
+	if (!ASSERT_OK(err, "compare_map_keys stackid_hmap vs. stackmap"))
+		goto out;
+
+	err = compare_map_keys(stackmap_fd, stackid_hmap_fd);
+	if (!ASSERT_OK(err, "compare_map_keys stackmap vs. stackid_hmap"))
+		goto out;
+
+	stack_trace_len = TEST_STACK_DEPTH * sizeof(__u64);
+	err = compare_stack_ips(stackmap_fd, stack_amap_fd, stack_trace_len);
+	if (!ASSERT_OK(err, "compare_stack_ips stackmap vs. stack_amap"))
+		goto out;
+
+	if (!ASSERT_EQ(skel->bss->failed, 0, "skip_failed"))
+		goto out;
+
+out:
+	stacktrace_map_skip__destroy(skel);
+}
diff --git a/tools/testing/selftests/bpf/progs/stacktrace_map_skip.c b/tools/testing/selftests/bpf/progs/stacktrace_map_skip.c
new file mode 100644
index 000000000000..2eb297df3dd6
--- /dev/null
+++ b/tools/testing/selftests/bpf/progs/stacktrace_map_skip.c
@@ -0,0 +1,68 @@
+// SPDX-License-Identifier: GPL-2.0
+#include <vmlinux.h>
+#include <bpf/bpf_helpers.h>
+
+#define TEST_STACK_DEPTH	2
+#define TEST_MAX_ENTRIES	16384
+
+typedef __u64 stack_trace_t[TEST_STACK_DEPTH];
+
+struct {
+	__uint(type, BPF_MAP_TYPE_STACK_TRACE);
+	__uint(max_entries, TEST_MAX_ENTRIES);
+	__type(key, __u32);
+	__type(value, stack_trace_t);
+} stackmap SEC(".maps");
+
+struct {
+	__uint(type, BPF_MAP_TYPE_HASH);
+	__uint(max_entries, TEST_MAX_ENTRIES);
+	__type(key, __u32);
+	__type(value, __u32);
+} stackid_hmap SEC(".maps");
+
+struct {
+	__uint(type, BPF_MAP_TYPE_ARRAY);
+	__uint(max_entries, TEST_MAX_ENTRIES);
+	__type(key, __u32);
+	__type(value, stack_trace_t);
+} stack_amap SEC(".maps");
+
+int pid = 0;
+int control = 0;
+int failed = 0;
+
+SEC("tracepoint/sched/sched_switch")
+int oncpu(struct trace_event_raw_sched_switch *ctx)
+{
+	__u32 max_len = TEST_STACK_DEPTH * sizeof(__u64);
+	__u32 key = 0, val = 0;
+	__u64 *stack_p;
+
+	if (pid != (bpf_get_current_pid_tgid() >> 32))
+		return 0;
+
+	if (control)
+		return 0;
+
+	/* it should allow skipping whole buffer size entries */
+	key = bpf_get_stackid(ctx, &stackmap, TEST_STACK_DEPTH);
+	if ((int)key >= 0) {
+		/* The size of stackmap and stack_amap should be the same */
+		bpf_map_update_elem(&stackid_hmap, &key, &val, 0);
+		stack_p = bpf_map_lookup_elem(&stack_amap, &key);
+		if (stack_p) {
+			bpf_get_stack(ctx, stack_p, max_len, TEST_STACK_DEPTH);
+			/* it wrongly skipped all the entries and filled zero */
+			if (stack_p[0] == 0)
+				failed = 1;
+		}
+	} else {
+		/* old kernel doesn't support skipping that many entries */
+		failed = 2;
+	}
+
+	return 0;
+}
+
+char _license[] SEC("license") = "GPL";
-- 
2.35.1.723.g4982287a31-goog

Re: [PATCH v3 2/2] bpf/selftests: Test skipping stacktrace

[Yonghong Song]

On 3/14/22 11:20 AM, Namhyung Kim wrote:
> Add a test case for stacktrace with skip > 0 using a small sized
> buffer.  It didn't support skipping entries greater than or equal to
> the size of buffer and filled the skipped part with 0.
> 
> Signed-off-by: Namhyung Kim <namhyung@kernel.org>

Acked-by: Yonghong Song <yhs@fb.com>

Re: [PATCH v3 1/2] bpf: Adjust BPF stack helper functions to accommodate skip > 0

[patchwork-bot+netdevbpf]

Hello:

This series was applied to bpf/bpf-next.git (master)
by Alexei Starovoitov <ast@kernel.org>:

On Mon, 14 Mar 2022 11:20:41 -0700 you wrote:
> Let's say that the caller has storage for num_elem stack frames.  Then,
> the BPF stack helper functions walk the stack for only num_elem frames.
> This means that if skip > 0, one keeps only 'num_elem - skip' frames.
> 
> This is because it sets init_nr in the perf_callchain_entry to the end
> of the buffer to save num_elem entries only.  I believe it was because
> the perf callchain code unwound the stack frames until it reached the
> global max size (sysctl_perf_event_max_stack).
> 
> [...]

Here is the summary with links:
  - [v3,1/2] bpf: Adjust BPF stack helper functions to accommodate skip > 0
    https://git.kernel.org/bpf/bpf-next/c/ee2a098851bf
  - [v3,2/2] bpf/selftests: Test skipping stacktrace
    https://git.kernel.org/bpf/bpf-next/c/e1cc1f39981b

You are awesome, thank you!
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