Re: [PATCH v2 2/2] bpf/selftests: Add selftests for new task kfuncs

[Kumar Kartikeya Dwivedi]

On Sat, 1 Oct 2022 at 16:47, David Vernet <void@xxxxxxxxxxxxx> wrote:
>
> A previous change added a series of kfuncs for storing struct
> task_struct objects as referenced kptrs. This patch adds a new
> task_kfunc test suite for validating their expected behavior.
>
> Signed-off-by: David Vernet <void@xxxxxxxxxxxxx>
> ---
>  tools/testing/selftests/bpf/DENYLIST.s390x    |   1 +
>  .../selftests/bpf/prog_tests/task_kfunc.c     | 155 ++++++++++++
>  .../selftests/bpf/progs/task_kfunc_common.h   |  83 +++++++
>  .../selftests/bpf/progs/task_kfunc_failure.c  | 225 ++++++++++++++++++
>  .../selftests/bpf/progs/task_kfunc_success.c  | 113 +++++++++
>  5 files changed, 577 insertions(+)
>  create mode 100644 tools/testing/selftests/bpf/prog_tests/task_kfunc.c
>  create mode 100644 tools/testing/selftests/bpf/progs/task_kfunc_common.h
>  create mode 100644 tools/testing/selftests/bpf/progs/task_kfunc_failure.c
>  create mode 100644 tools/testing/selftests/bpf/progs/task_kfunc_success.c
>
> diff --git a/tools/testing/selftests/bpf/DENYLIST.s390x b/tools/testing/selftests/bpf/DENYLIST.s390x
> index 17e074eb42b8..4c34818ec1ee 100644
> --- a/tools/testing/selftests/bpf/DENYLIST.s390x
> +++ b/tools/testing/selftests/bpf/DENYLIST.s390x
> @@ -75,3 +75,4 @@ user_ringbuf                             # failed to find kernel BTF type ID of
>  lookup_key                               # JIT does not support calling kernel function                                (kfunc)
>  verify_pkcs7_sig                         # JIT does not support calling kernel function                                (kfunc)
>  kfunc_dynptr_param                       # JIT does not support calling kernel function                                (kfunc)
> +task_kfunc                               # JIT does not support calling kernel function
> diff --git a/tools/testing/selftests/bpf/prog_tests/task_kfunc.c b/tools/testing/selftests/bpf/prog_tests/task_kfunc.c
> new file mode 100644
> index 000000000000..6c577fbca8f7
> --- /dev/null
> +++ b/tools/testing/selftests/bpf/prog_tests/task_kfunc.c
> @@ -0,0 +1,155 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/* Copyright (c) 2022 Meta Platforms, Inc. and affiliates. */
> +
> +#define _GNU_SOURCE
> +#include <sys/wait.h>
> +#include <test_progs.h>
> +#include <unistd.h>
> +
> +#include "task_kfunc_failure.skel.h"
> +#include "task_kfunc_success.skel.h"
> +
> +static size_t log_buf_sz = 1 << 20; /* 1 MB */
> +static char obj_log_buf[1048576];
> +
> +static struct task_kfunc_success *open_load_task_kfunc_skel(void)
> +{
> +       struct task_kfunc_success *skel;
> +       int err;
> +
> +       skel = task_kfunc_success__open();
> +       if (!ASSERT_OK_PTR(skel, "skel_open"))
> +               return NULL;
> +
> +       skel->bss->pid = getpid();
> +
> +       err = task_kfunc_success__load(skel);
> +       if (!ASSERT_OK(err, "skel_load"))
> +               goto cleanup;
> +
> +       return skel;
> +
> +cleanup:
> +       task_kfunc_success__destroy(skel);
> +       return NULL;
> +}
> +
> +static void run_success_test(const char *prog_name)
> +{
> +       struct task_kfunc_success *skel;
> +       int status;
> +       pid_t child_pid;
> +       struct bpf_program *prog;
> +       struct bpf_link *link = NULL;
> +
> +       skel = open_load_task_kfunc_skel();
> +       if (!ASSERT_OK_PTR(skel, "open_load_skel"))
> +               return;
> +
> +       if (!ASSERT_OK(skel->bss->err, "pre_spawn_err"))
> +               goto cleanup;
> +
> +       prog = bpf_object__find_program_by_name(skel->obj, prog_name);
> +       if (!ASSERT_OK_PTR(prog, "bpf_object__find_program_by_name"))
> +               goto cleanup;
> +
> +       link = bpf_program__attach(prog);
> +       if (!ASSERT_OK_PTR(link, "attached_link"))
> +               goto cleanup;
> +
> +       child_pid = fork();
> +       if (!ASSERT_GT(child_pid, -1, "child_pid"))
> +               goto cleanup;
> +       if (child_pid == 0)
> +               _exit(0);
> +       waitpid(child_pid, &status, 0);
> +
> +       ASSERT_OK(skel->bss->err, "post_wait_err");
> +
> +cleanup:
> +       bpf_link__destroy(link);
> +       task_kfunc_success__destroy(skel);
> +}
> +
> +static const char * const success_tests[] = {
> +       "test_task_acquire_release",
> +       "test_task_acquire_leave_in_map",
> +       "test_task_xchg_release",
> +       "test_task_get_release",
> +};
> +
> +static struct {
> +       const char *prog_name;
> +       const char *expected_err_msg;
> +} failure_tests[] = {
> +       {"task_kfunc_acquire_untrusted", "arg#0 pointer type STRUCT task_struct must point"},
> +       {"task_kfunc_acquire_null", "arg#0 pointer type STRUCT task_struct must point"},
> +       {"task_kfunc_acquire_unreleased", "Unreleased reference"},
> +       {"task_kfunc_get_non_kptr_param", "arg#0 expected pointer to map value"},
> +       {"task_kfunc_get_non_kptr_acquired", "arg#0 expected pointer to map value"},
> +       {"task_kfunc_get_null", "arg#0 expected pointer to map value"},
> +       {"task_kfunc_xchg_unreleased", "Unreleased reference"},
> +       {"task_kfunc_get_unreleased", "Unreleased reference"},
> +       {"task_kfunc_release_untrusted", "arg#0 pointer type STRUCT task_struct must point"},
> +       {"task_kfunc_release_null", "arg#0 pointer type STRUCT task_struct must point"},
> +       {"task_kfunc_release_unacquired", "R1 must be referenced"},
> +};
> +
> +static void verify_fail(const char *prog_name, const char *expected_err_msg)
> +{
> +       LIBBPF_OPTS(bpf_object_open_opts, opts);
> +       struct task_kfunc_failure *skel;
> +       int err, i;
> +
> +       opts.kernel_log_buf = obj_log_buf;
> +       opts.kernel_log_size = log_buf_sz;
> +       opts.kernel_log_level = 1;
> +
> +       skel = task_kfunc_failure__open_opts(&opts);
> +       if (!ASSERT_OK_PTR(skel, "task_kfunc_failure__open_opts"))
> +               goto cleanup;
> +
> +       skel->bss->pid = getpid();
> +
> +       for (i = 0; i < ARRAY_SIZE(failure_tests); i++) {
> +               struct bpf_program *prog;
> +               const char *curr_name = failure_tests[i].prog_name;
> +
> +               prog = bpf_object__find_program_by_name(skel->obj, curr_name);
> +               if (!ASSERT_OK_PTR(prog, "bpf_object__find_program_by_name"))
> +                       goto cleanup;
> +
> +               bpf_program__set_autoload(prog, !strcmp(curr_name, prog_name));
> +       }
> +
> +       err = task_kfunc_failure__load(skel);
> +       if (!ASSERT_ERR(err, "unexpected load success"))
> +               goto cleanup;
> +
> +       if (!ASSERT_OK_PTR(strstr(obj_log_buf, expected_err_msg), "expected_err_msg")) {
> +               fprintf(stderr, "Expected err_msg: %s\n", expected_err_msg);
> +               fprintf(stderr, "Verifier output: %s\n", obj_log_buf);
> +       }
> +
> +cleanup:
> +       task_kfunc_failure__destroy(skel);
> +}
> +
> +void test_task_kfunc(void)
> +{
> +       int i;
> +
> +       for (i = 0; i < ARRAY_SIZE(success_tests); i++) {
> +               if (!test__start_subtest(success_tests[i]))
> +                       continue;
> +
> +               run_success_test(success_tests[i]);
> +       }
> +
> +       for (i = 0; i < ARRAY_SIZE(failure_tests); i++) {
> +               if (!test__start_subtest(failure_tests[i].prog_name))
> +                       continue;
> +
> +               verify_fail(failure_tests[i].prog_name, failure_tests[i].expected_err_msg);
> +       }
> +}
> diff --git a/tools/testing/selftests/bpf/progs/task_kfunc_common.h b/tools/testing/selftests/bpf/progs/task_kfunc_common.h
> new file mode 100644
> index 000000000000..bbb0a40572fd
> --- /dev/null
> +++ b/tools/testing/selftests/bpf/progs/task_kfunc_common.h
> @@ -0,0 +1,83 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +/* Copyright (c) 2022 Meta Platforms, Inc. and affiliates. */
> +
> +#ifndef _TASK_KFUNC_COMMON_H
> +#define _TASK_KFUNC_COMMON_H
> +
> +#include <errno.h>
> +#include <vmlinux.h>
> +#include <bpf/bpf_tracing.h>
> +#include <bpf/bpf_helpers.h>
> +
> +struct __tasks_kfunc_map_value {
> +       struct task_struct __kptr_ref * task;
> +};
> +
> +struct hash_map {
> +       __uint(type, BPF_MAP_TYPE_HASH);
> +       __type(key, int);
> +       __type(value, struct __tasks_kfunc_map_value);
> +       __uint(max_entries, 1);
> +} __tasks_kfunc_map SEC(".maps");
> +
> +struct task_struct *bpf_task_acquire(struct task_struct *p) __ksym;
> +struct task_struct *bpf_task_kptr_get(struct task_struct **pp) __ksym;
> +void bpf_task_release(struct task_struct *p) __ksym;
> +
> +#define TEST_NAME_SZ 128
> +
> +/* The pid of the test process used to determine if a newly created task is the test task. */
> +int pid;
> +
> +static inline struct __tasks_kfunc_map_value *tasks_kfunc_map_value_lookup(struct task_struct *p)
> +{
> +       s32 pid;
> +       long status;
> +
> +       status = bpf_probe_read_kernel(&pid, sizeof(pid), &p->pid);
> +       if (status)
> +               return NULL;
> +
> +       return bpf_map_lookup_elem(&__tasks_kfunc_map, &pid);
> +}
> +
> +static inline int tasks_kfunc_map_insert(struct task_struct *p)
> +{
> +       struct __tasks_kfunc_map_value local, *v;
> +       long status;
> +       struct task_struct *acquired, *old;
> +       s32 pid;
> +
> +       status = bpf_probe_read_kernel(&pid, sizeof(pid), &p->pid);
> +       if (status)
> +               return status;
> +
> +       local.task = NULL;
> +       status = bpf_map_update_elem(&__tasks_kfunc_map, &pid, &local, BPF_NOEXIST);
> +       if (status)
> +               return status;
> +
> +       v = bpf_map_lookup_elem(&__tasks_kfunc_map, &pid);
> +       if (!v) {
> +               bpf_map_delete_elem(&__tasks_kfunc_map, &pid);
> +               return status;
> +       }
> +
> +       acquired = bpf_task_acquire(p);
> +       old = bpf_kptr_xchg(&v->task, acquired);
> +       if (old) {
> +               bpf_task_release(old);
> +               return -EEXIST;
> +       }
> +
> +       return 0;
> +}
> +
> +static inline bool is_test_kfunc_task(struct task_struct *task)
> +{
> +       int cur_pid = bpf_get_current_pid_tgid() >> 32;
> +
> +       return pid == cur_pid;
> +}
> +
> +#endif /* _TASK_KFUNC_COMMON_H */
> diff --git a/tools/testing/selftests/bpf/progs/task_kfunc_failure.c b/tools/testing/selftests/bpf/progs/task_kfunc_failure.c
> new file mode 100644
> index 000000000000..4cf01bbc8a16
> --- /dev/null
> +++ b/tools/testing/selftests/bpf/progs/task_kfunc_failure.c
> @@ -0,0 +1,225 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/* Copyright (c) 2022 Meta Platforms, Inc. and affiliates. */
> +
> +#include <vmlinux.h>
> +#include <bpf/bpf_tracing.h>
> +#include <bpf/bpf_helpers.h>
> +
> +#include "task_kfunc_common.h"
> +
> +char _license[] SEC("license") = "GPL";
> +
> +/* Prototype for all of the program trace events below:
> + *
> + * TRACE_EVENT(task_newtask,
> + *         TP_PROTO(struct task_struct *p, u64 clone_flags)
> + */
> +
> +SEC("tp_btf/task_newtask")
> +int BPF_PROG(task_kfunc_acquire_untrusted, struct task_struct *task, u64 clone_flags)
> +{
> +       struct task_struct *acquired, *stack_ptr;
> +
> +       if (!is_test_kfunc_task(task))
> +               return 0;
> +
> +       /* Can't invoke bpf_task_acquire() on an untrusted, random pointer. */
> +       stack_ptr = (struct task_struct *)0xcafef00d;

This seems like a misleading comment. 'stack_ptr' would just be a
scalar, not a pointer. Maybe you should be testing this for an actual
PTR_UNTRUSTED pointer instead. Load of a __kptr tagged pointer would
be a good way.

Very soon a lot of other pointers obtained from pointer walking are
going to be marked PTR_UNTRUSTED, so then we would cover those as well
similar to this test.

Also, could you include a test to make sure sleepable programs cannot
call bpf_task_acquire? It seems to assume RCU read lock is held while
that may not be true. If already not possible, maybe a WARN_ON_ONCE
inside the helper to ensure future cases don't creep in.

> [...]