[PATCH bpf-next 9/9] selftests/bpf: Tracing prog can still do lookup under busy lock

Martin KaFai Lau
2022-10-25T18:46:30Z

This patch modifies the task_ls_recursion test to check that
the first bpf_task_storage_get(&map_a, ...) in BPF_PROG(on_update)
can still do the lockless lookup even it cannot acquire the percpu
busy lock. If the lookup succeeds, it will increment the value
by 1 and the value in the task storage map_a will become 200+1=201.
After that, BPF_PROG(on_update) tries to delete from map_a and
should get -EBUSY because it cannot acquire the percpu busy lock
after finding the data.

Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
---
.../bpf/prog_tests/task_local_storage.c | 48 ++++++++++++++++++-
.../selftests/bpf/progs/task_ls_recursion.c | 43 +++++++++++++++--
2 files changed, 86 insertions(+), 5 deletions(-)

diff --git a/tools/testing/selftests/bpf/prog_tests/task_local_storage.c b/tools/testing/selftests/bpf/prog_tests/task_local_storage.c
index ae535f5de6a2..a176bd75a748 100644
--- a/tools/testing/selftests/bpf/prog_tests/task_local_storage.c
+++ b/tools/testing/selftests/bpf/prog_tests/task_local_storage.c
@@ -8,6 +8,7 @@
#include <sys/syscall.h> /* For SYS_xxx definitions */
#include <sys/types.h>
#include <test_progs.h>
+#include "task_local_storage_helpers.h"
#include "task_local_storage.skel.h"
#include "task_local_storage_exit_creds.skel.h"
#include "task_ls_recursion.skel.h"
@@ -78,21 +79,64 @@ static void test_exit_creds(void)

static void test_recursion(void)
{
+ int err, map_fd, prog_fd, task_fd;
struct task_ls_recursion *skel;
- int err;
+ struct bpf_prog_info info;
+ __u32 info_len = sizeof(info);
+ long value;
+
+ task_fd = sys_pidfd_open(getpid(), 0);
+ if (!ASSERT_NEQ(task_fd, -1, "sys_pidfd_open"))
+ return;

skel = task_ls_recursion__open_and_load();
if (!ASSERT_OK_PTR(skel, "skel_open_and_load"))
- return;
+ goto out;

err = task_ls_recursion__attach(skel);
if (!ASSERT_OK(err, "skel_attach"))
goto out;

/* trigger sys_enter, make sure it does not cause deadlock */
+ skel->bss->test_pid = getpid();
syscall(SYS_gettid);
+ skel->bss->test_pid = 0;
+ task_ls_recursion__detach(skel);
+
+ /* Refer to the comment in BPF_PROG(on_update) for
+ * the explanation on the value 201 and 100.
+ */
+ map_fd = bpf_map__fd(skel->maps.map_a);
+ err = bpf_map_lookup_elem(map_fd, &task_fd, &value);
+ ASSERT_OK(err, "lookup map_a");
+ ASSERT_EQ(value, 201, "map_a value");
+ ASSERT_EQ(skel->bss->nr_del_errs, 1, "bpf_task_storage_delete busy");
+
+ map_fd = bpf_map__fd(skel->maps.map_b);
+ err = bpf_map_lookup_elem(map_fd, &task_fd, &value);
+ ASSERT_OK(err, "lookup map_b");
+ ASSERT_EQ(value, 100, "map_b value");
+
+ prog_fd = bpf_program__fd(skel->progs.on_lookup);
+ memset(&info, 0, sizeof(info));
+ err = bpf_obj_get_info_by_fd(prog_fd, &info, &info_len);
+ ASSERT_OK(err, "get prog info");
+ ASSERT_GT(info.recursion_misses, 0, "on_lookup prog recursion");
+
+ prog_fd = bpf_program__fd(skel->progs.on_update);
+ memset(&info, 0, sizeof(info));
+ err = bpf_obj_get_info_by_fd(prog_fd, &info, &info_len);
+ ASSERT_OK(err, "get prog info");
+ ASSERT_EQ(info.recursion_misses, 0, "on_update prog recursion");
+
+ prog_fd = bpf_program__fd(skel->progs.on_enter);
+ memset(&info, 0, sizeof(info));
+ err = bpf_obj_get_info_by_fd(prog_fd, &info, &info_len);
+ ASSERT_OK(err, "get prog info");
+ ASSERT_EQ(info.recursion_misses, 0, "on_enter prog recursion");

out:
+ close(task_fd);
task_ls_recursion__destroy(skel);
}

diff --git a/tools/testing/selftests/bpf/progs/task_ls_recursion.c b/tools/testing/selftests/bpf/progs/task_ls_recursion.c
index 564583dca7c8..4542dc683b44 100644
--- a/tools/testing/selftests/bpf/progs/task_ls_recursion.c
+++ b/tools/testing/selftests/bpf/progs/task_ls_recursion.c
@@ -5,7 +5,13 @@
#include <bpf/bpf_helpers.h>
#include <bpf/bpf_tracing.h>

+#ifndef EBUSY
+#define EBUSY 16
+#endif
+
char _license[] SEC("license") = "GPL";
+int nr_del_errs = 0;
+int test_pid = 0;

struct {
__uint(type, BPF_MAP_TYPE_TASK_STORAGE);
@@ -26,6 +32,13 @@ int BPF_PROG(on_lookup)
{
struct task_struct *task = bpf_get_current_task_btf();

+ if (!test_pid || task->pid != test_pid)
+ return 0;
+
+ /* The bpf_task_storage_delete will call
+ * bpf_local_storage_lookup. The prog->active will
+ * stop the recursion.
+ */
bpf_task_storage_delete(&map_a, task);
bpf_task_storage_delete(&map_b, task);
return 0;
@@ -37,11 +50,32 @@ int BPF_PROG(on_update)
struct task_struct *task = bpf_get_current_task_btf();
long *ptr;

+ if (!test_pid || task->pid != test_pid)
+ return 0;
+
ptr = bpf_task_storage_get(&map_a, task, 0,
BPF_LOCAL_STORAGE_GET_F_CREATE);
- if (ptr)
+ /* ptr will not be NULL when it is called from
+ * the bpf_task_storage_get(&map_b,...F_CREATE) in
+ * the BPF_PROG(on_enter) below. It is because
+ * the value can be found in map_a and the kernel
+ * does not need to acquire any spin_lock.
+ */
+ if (ptr) {
+ int err;
+
*ptr += 1;
+ err = bpf_task_storage_delete(&map_a, task);
+ if (err == -EBUSY)
+ nr_del_errs++;
+ }

+ /* This will still fail because map_b is empty and
+ * this BPF_PROG(on_update) has failed to acquire
+ * the percpu busy lock => meaning potential
+ * deadlock is detected and it will fail to create
+ * new storage.
+ */
ptr = bpf_task_storage_get(&map_b, task, 0,
BPF_LOCAL_STORAGE_GET_F_CREATE);
if (ptr)
@@ -57,14 +91,17 @@ int BPF_PROG(on_enter, struct pt_regs *regs, long id)
long *ptr;

task = bpf_get_current_task_btf();
+ if (!test_pid || task->pid != test_pid)
+ return 0;
+
ptr = bpf_task_storage_get(&map_a, task, 0,
BPF_LOCAL_STORAGE_GET_F_CREATE);
- if (ptr)
+ if (ptr && !*ptr)
*ptr = 200;

ptr = bpf_task_storage_get(&map_b, task, 0,
BPF_LOCAL_STORAGE_GET_F_CREATE);
- if (ptr)
+ if (ptr && !*ptr)
*ptr = 100;
return 0;
}
--
2.30.2