Re: [PATCH bpf-next v3 2/2] selftests/bpf: Add testcase for bpf_task_under_cgroup

[Yonghong Song <yhs@meta.com>]

On 4/26/23 7:30 PM, Feng zhou wrote:
> From: Feng Zhou <zhoufeng.zf@bytedance.com>
> 
> test_progs:
> Tests new kfunc bpf_task_under_cgroup().
> 
> The bpf program saves the pid which call the getpgid syscall within a
> given cgroup to the remote_pid, which is convenient for the user-mode
> program to verify the test correctness.
> 
> The user-mode program creates its own mount namespace, and mounts the
> cgroupsv2 hierarchy in there, call the getpgid syscall, then check if
> remote_pid and local_pid are equal.
> 
> Signed-off-by: Feng Zhou <zhoufeng.zf@bytedance.com>
> ---
>   tools/testing/selftests/bpf/DENYLIST.s390x    |  1 +
>   .../bpf/prog_tests/task_under_cgroup.c        | 47 +++++++++++++++++++
>   .../selftests/bpf/progs/cgrp_kfunc_common.h   |  1 +
>   .../bpf/progs/test_task_under_cgroup.c        | 37 +++++++++++++++
>   4 files changed, 86 insertions(+)
>   create mode 100644 tools/testing/selftests/bpf/prog_tests/task_under_cgroup.c
>   create mode 100644 tools/testing/selftests/bpf/progs/test_task_under_cgroup.c
> 
> diff --git a/tools/testing/selftests/bpf/DENYLIST.s390x b/tools/testing/selftests/bpf/DENYLIST.s390x
> index c7463f3ec3c0..5061d9e24c16 100644
> --- a/tools/testing/selftests/bpf/DENYLIST.s390x
> +++ b/tools/testing/selftests/bpf/DENYLIST.s390x
> @@ -26,3 +26,4 @@ user_ringbuf                             # failed to find kernel BTF type ID of
>   verif_stats                              # trace_vprintk__open_and_load unexpected error: -9                           (?)
>   xdp_bonding                              # failed to auto-attach program 'trace_on_entry': -524                        (trampoline)
>   xdp_metadata                             # JIT does not support calling kernel function                                (kfunc)
> +test_task_under_cgroup                   # JIT does not support calling kernel function                                (kfunc)
> diff --git a/tools/testing/selftests/bpf/prog_tests/task_under_cgroup.c b/tools/testing/selftests/bpf/prog_tests/task_under_cgroup.c
> new file mode 100644
> index 000000000000..6d5709a8203d
> --- /dev/null
> +++ b/tools/testing/selftests/bpf/prog_tests/task_under_cgroup.c
> @@ -0,0 +1,47 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/* Copyright (c) 2023 Bytedance */
> +
> +#include <sys/syscall.h>
> +#include <test_progs.h>
> +#include <cgroup_helpers.h>
> +#include "test_task_under_cgroup.skel.h"
> +
> +#define FOO	"/foo"
> +
> +void test_task_under_cgroup(void)
> +{
> +	struct test_task_under_cgroup *skel;
> +	int ret, foo = -1;
> +
> +	foo = test__join_cgroup(FOO);
> +	if (!ASSERT_OK(foo < 0, "cgroup_join_foo"))
> +		return;
> +
> +	skel = test_task_under_cgroup__open();
> +	if (!ASSERT_OK_PTR(skel, "test_task_under_cgroup__open"))
> +		goto cleanup;
> +
> +	skel->rodata->local_pid = getpid();
> +	skel->rodata->cgid = get_cgroup_id(FOO);
> +
> +	ret = test_task_under_cgroup__load(skel);
> +	if (!ASSERT_OK(ret, "test_task_under_cgroup__load"))
> +		goto cleanup;
> +
> +	ret = test_task_under_cgroup__attach(skel);
> +	if (!ASSERT_OK(ret, "test_task_under_cgroup__attach"))
> +		goto cleanup;
> +
> +	syscall(SYS_getpgid);
> +
> +	test_task_under_cgroup__detach(skel);
> +
> +	ASSERT_EQ(skel->bss->remote_pid, skel->rodata->local_pid,
> +		  "test task_under_cgroup");
> +
> +cleanup:
> +	if (foo >= 0)
> +		close(foo);
> +
> +	test_task_under_cgroup__destroy(skel);
> +}
> diff --git a/tools/testing/selftests/bpf/progs/cgrp_kfunc_common.h b/tools/testing/selftests/bpf/progs/cgrp_kfunc_common.h
> index 22914a70db54..001c416b42bc 100644
> --- a/tools/testing/selftests/bpf/progs/cgrp_kfunc_common.h
> +++ b/tools/testing/selftests/bpf/progs/cgrp_kfunc_common.h
> @@ -26,6 +26,7 @@ struct cgroup *bpf_cgroup_ancestor(struct cgroup *cgrp, int level) __ksym;
>   struct cgroup *bpf_cgroup_from_id(u64 cgid) __ksym;
>   void bpf_rcu_read_lock(void) __ksym;
>   void bpf_rcu_read_unlock(void) __ksym;
> +int bpf_task_under_cgroup(struct task_struct *task, struct cgroup *ancestor) __ksym;

return type 'long'?

>   
>   static inline struct __cgrps_kfunc_map_value *cgrps_kfunc_map_value_lookup(struct cgroup *cgrp)
>   {
> diff --git a/tools/testing/selftests/bpf/progs/test_task_under_cgroup.c b/tools/testing/selftests/bpf/progs/test_task_under_cgroup.c
> new file mode 100644
> index 000000000000..8f23a2933fde
> --- /dev/null
> +++ b/tools/testing/selftests/bpf/progs/test_task_under_cgroup.c
> @@ -0,0 +1,37 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/* Copyright (c) 2023 Bytedance */
> +
> +#include <vmlinux.h>
> +#include <bpf/bpf_tracing.h>
> +#include <bpf/bpf_helpers.h>
> +
> +#include "bpf_misc.h"
> +#include "cgrp_kfunc_common.h"
> +
> +const volatile int local_pid;
> +const volatile long cgid;
> +int remote_pid;
> +
> +SEC("fentry/" SYS_PREFIX "sys_getpgid")
> +int sys_getpgid(void *ctx)
> +{
> +	struct cgroup *cgrp;
> +
> +	if (local_pid != (bpf_get_current_pid_tgid() >> 32))
> +		return 0;
> +
> +	cgrp = bpf_cgroup_from_id(cgid);
> +	if (!cgrp)
> +		return 0;
> +
> +	if (!bpf_task_under_cgroup(bpf_get_current_task_btf(), cgrp))
> +		goto out;

The whole point of using bpf_task_under_cgroup() is to test a 
non-current task is under a particular cgroup.

Tracing kernel function enqueue_task_fair() is an option, but it
may be inlined with certain compilers as enqueue_task_fair() is
a static function. Also, the 'task' argument in enqueue_task_fair()
is not trusted, could be null ptr and not rcu safe as well.
We could promote 'task' in enqueue_task_fair() by:
   - checking NULL ptr, if not
   - increase reference count by 1 if current reference count is not 0
     (using bpf_task_acquire kfunc)
   - if reference count can be increased, it becomes trusted, and
     then bpf_task_under_cgroup() can be used.

Could you try this approach or even better if there is another
non-static function also having a task (not the current task)?

> +
> +	remote_pid = local_pid;
> +
> +out:
> +	bpf_cgroup_release(cgrp);
> +	return 0;
> +}
> +
> +char _license[] SEC("license") = "GPL";