[PATCH bpf-next v6 0/2] Introduce a new kfunc of bpf_task_under_cgroup

[PATCH bpf-next v6 0/2] Introduce a new kfunc of bpf_task_under_cgroup

[Feng zhou]

From: Feng Zhou <zhoufeng.zf@bytedance.com>

Trace sched related functions, such as enqueue_task_fair, it is necessary to
specify a task instead of the current task which within a given cgroup.

Feng Zhou (2):
  bpf: Add bpf_task_under_cgroup() kfunc
  selftests/bpf: Add testcase for bpf_task_under_cgroup

Changelog:
v5->v6: Addressed comments from Yonghong Song
- Some code format modifications.
- Add ack-by
Details in here:
https://lore.kernel.org/all/20230504031513.13749-1-zhoufeng.zf@bytedance.com/

v4->v5: Addressed comments from Yonghong Song
- Some code format modifications.
Details in here:
https://lore.kernel.org/all/20230428071737.43849-1-zhoufeng.zf@bytedance.com/

v3->v4: Addressed comments from Yonghong Song
- Modify test cases and test other tasks, not the current task.
Details in here:
https://lore.kernel.org/all/20230427023019.73576-1-zhoufeng.zf@bytedance.com/

v2->v3: Addressed comments from Alexei Starovoitov
- Modify the comment information of the function.
- Narrow down the testcase's hook point
Details in here:
https://lore.kernel.org/all/20230421090403.15515-1-zhoufeng.zf@bytedance.com/

v1->v2: Addressed comments from Alexei Starovoitov
- Add kfunc instead.
Details in here:
https://lore.kernel.org/all/20230420072657.80324-1-zhoufeng.zf@bytedance.com/

 kernel/bpf/helpers.c                          | 20 +++++++
 tools/testing/selftests/bpf/DENYLIST.s390x    |  1 +
 .../bpf/prog_tests/task_under_cgroup.c        | 53 +++++++++++++++++++
 .../bpf/progs/test_task_under_cgroup.c        | 51 ++++++++++++++++++
 4 files changed, 125 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

-- 
2.20.1

[PATCH bpf-next v6 1/2] bpf: Add bpf_task_under_cgroup() kfunc

[Feng zhou]

From: Feng Zhou <zhoufeng.zf@bytedance.com>

Add a kfunc that's similar to the bpf_current_task_under_cgroup.
The difference is that it is a designated task.

When hook sched related functions, sometimes it is necessary to
specify a task instead of the current task.

Signed-off-by: Feng Zhou <zhoufeng.zf@bytedance.com>
Acked-by: Yonghong Song <yhs@fb.com>
---
 kernel/bpf/helpers.c | 20 ++++++++++++++++++++
 1 file changed, 20 insertions(+)

diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
index bb6b4637ebf2..453cbd312366 100644
--- a/kernel/bpf/helpers.c
+++ b/kernel/bpf/helpers.c
@@ -2149,6 +2149,25 @@ __bpf_kfunc struct cgroup *bpf_cgroup_from_id(u64 cgid)
 		return NULL;
 	return cgrp;
 }
+
+/**
+ * bpf_task_under_cgroup - wrap task_under_cgroup_hierarchy() as a kfunc, test
+ * task's membership of cgroup ancestry.
+ * @task: the task to be tested
+ * @ancestor: possible ancestor of @task's cgroup
+ *
+ * Tests whether @task's default cgroup hierarchy is a descendant of @ancestor.
+ * It follows all the same rules as cgroup_is_descendant, and only applies
+ * to the default hierarchy.
+ */
+__bpf_kfunc long bpf_task_under_cgroup(struct task_struct *task,
+				       struct cgroup *ancestor)
+{
+	if (unlikely(!ancestor || !task))
+		return -EINVAL;
+
+	return task_under_cgroup_hierarchy(task, ancestor);
+}
 #endif /* CONFIG_CGROUPS */
 
 /**
@@ -2400,6 +2419,7 @@ BTF_ID_FLAGS(func, bpf_cgroup_acquire, KF_ACQUIRE | KF_RCU | KF_RET_NULL)
 BTF_ID_FLAGS(func, bpf_cgroup_release, KF_RELEASE)
 BTF_ID_FLAGS(func, bpf_cgroup_ancestor, KF_ACQUIRE | KF_RCU | KF_RET_NULL)
 BTF_ID_FLAGS(func, bpf_cgroup_from_id, KF_ACQUIRE | KF_RET_NULL)
+BTF_ID_FLAGS(func, bpf_task_under_cgroup, KF_RCU)
 #endif
 BTF_ID_FLAGS(func, bpf_task_from_pid, KF_ACQUIRE | KF_RET_NULL)
 BTF_SET8_END(generic_btf_ids)
-- 
2.20.1

[PATCH bpf-next v6 2/2] selftests/bpf: Add testcase for bpf_task_under_cgroup

[Feng zhou]

From: Feng Zhou <zhoufeng.zf@bytedance.com>

test_progs:
Tests new kfunc bpf_task_under_cgroup().

The bpf program saves the new task's pid 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 fork syscall, then check if
remote_pid and local_pid are unequal.

Signed-off-by: Feng Zhou <zhoufeng.zf@bytedance.com>
Acked-by: Yonghong Song <yhs@fb.com>
---
 tools/testing/selftests/bpf/DENYLIST.s390x    |  1 +
 .../bpf/prog_tests/task_under_cgroup.c        | 53 +++++++++++++++++++
 .../bpf/progs/test_task_under_cgroup.c        | 51 ++++++++++++++++++
 3 files changed, 105 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..4224727fb364
--- /dev/null
+++ b/tools/testing/selftests/bpf/prog_tests/task_under_cgroup.c
@@ -0,0 +1,53 @@
+// 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;
+	pid_t pid;
+
+	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->bss->remote_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;
+
+	pid = fork();
+	if (pid == 0)
+		exit(0);
+
+	ret = (pid == -1);
+	if (ASSERT_OK(ret, "fork process"))
+		wait(NULL);
+
+	test_task_under_cgroup__detach(skel);
+
+	ASSERT_NEQ(skel->bss->remote_pid, skel->rodata->local_pid,
+		   "test task_under_cgroup");
+
+cleanup:
+	test_task_under_cgroup__destroy(skel);
+	close(foo);
+}
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..56cdc0a553f0
--- /dev/null
+++ b/tools/testing/selftests/bpf/progs/test_task_under_cgroup.c
@@ -0,0 +1,51 @@
+// 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"
+
+struct cgroup *bpf_cgroup_from_id(u64 cgid) __ksym;
+long bpf_task_under_cgroup(struct task_struct *task, struct cgroup *ancestor) __ksym;
+void bpf_cgroup_release(struct cgroup *p) __ksym;
+struct task_struct *bpf_task_acquire(struct task_struct *p) __ksym;
+void bpf_task_release(struct task_struct *p) __ksym;
+
+const volatile int local_pid;
+const volatile __u64 cgid;
+int remote_pid;
+
+SEC("tp_btf/task_newtask")
+int BPF_PROG(handle__task_newtask, struct task_struct *task, u64 clone_flags)
+{
+	struct cgroup *cgrp = NULL;
+	struct task_struct *acquired;
+
+	if (local_pid != (bpf_get_current_pid_tgid() >> 32))
+		return 0;
+
+	acquired = bpf_task_acquire(task);
+	if (!acquired)
+		return 0;
+
+	if (local_pid == acquired->tgid)
+		goto out;
+
+	cgrp = bpf_cgroup_from_id(cgid);
+	if (!cgrp)
+		goto out;
+
+	if (bpf_task_under_cgroup(acquired, cgrp))
+		remote_pid = acquired->tgid;
+
+out:
+	if (cgrp)
+		bpf_cgroup_release(cgrp);
+	bpf_task_release(acquired);
+
+	return 0;
+}
+
+char _license[] SEC("license") = "GPL";
-- 
2.20.1

Re: [PATCH bpf-next v6 1/2] bpf: Add bpf_task_under_cgroup() kfunc

[Hao Luo]

On Thu, May 4, 2023 at 11:08 PM Feng zhou <zhoufeng.zf@bytedance.com> wrote:
>
<...>
> ---
>  kernel/bpf/helpers.c | 20 ++++++++++++++++++++
>  1 file changed, 20 insertions(+)
>
> diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
> index bb6b4637ebf2..453cbd312366 100644
> --- a/kernel/bpf/helpers.c
> +++ b/kernel/bpf/helpers.c
> @@ -2149,6 +2149,25 @@ __bpf_kfunc struct cgroup *bpf_cgroup_from_id(u64 cgid)
>                 return NULL;
>         return cgrp;
>  }
> +
> +/**
> + * bpf_task_under_cgroup - wrap task_under_cgroup_hierarchy() as a kfunc, test
> + * task's membership of cgroup ancestry.
> + * @task: the task to be tested
> + * @ancestor: possible ancestor of @task's cgroup
> + *
> + * Tests whether @task's default cgroup hierarchy is a descendant of @ancestor.
> + * It follows all the same rules as cgroup_is_descendant, and only applies
> + * to the default hierarchy.
> + */
> +__bpf_kfunc long bpf_task_under_cgroup(struct task_struct *task,
> +                                      struct cgroup *ancestor)
> +{
> +       if (unlikely(!ancestor || !task))
> +               return -EINVAL;
> +
> +       return task_under_cgroup_hierarchy(task, ancestor);
> +}
>  #endif /* CONFIG_CGROUPS */
>

I wonder in what situation a null 'task' or 'ancestor' can be passed.
Please call out in the comment that the returned value can be a
negative error, so that writing if(bpf_task_under_cgroup()) may cause
surprising results.

Hao

Re: [PATCH bpf-next v6 2/2] selftests/bpf: Add testcase for bpf_task_under_cgroup

[Hao Luo]

On Thu, May 4, 2023 at 11:08 PM Feng zhou <zhoufeng.zf@bytedance.com> wrote:
>
> From: Feng Zhou <zhoufeng.zf@bytedance.com>
>
> test_progs:
> Tests new kfunc bpf_task_under_cgroup().
>
> The bpf program saves the new task's pid 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 fork syscall, then check if
> remote_pid and local_pid are unequal.
>
> Signed-off-by: Feng Zhou <zhoufeng.zf@bytedance.com>
> Acked-by: Yonghong Song <yhs@fb.com>
> ---

Hi Feng,

I have a comment about the methodology of the test, but the patch
looks ok to me. Why do we have to test via a tracing program? I think
what we need is just a task and a cgroup. Since we have the kfunc
bpf_task_from_pid() and bpf_cgroup_from_id(), we can write a syscall
program which takes a pid and a cgroup id as input and get the task
and cgroup objects directly in the program.

I like testing via a syscall program because it doesn't depend on the
newtask tracepoint and it should be simpler. But I'm ok with the
current version of the patch, just have some thoughts.

Hao

Re: Re: [PATCH bpf-next v6 1/2] bpf: Add bpf_task_under_cgroup() kfunc

[Feng Zhou]

在 2023/5/5 14:58, Hao Luo 写道:
> On Thu, May 4, 2023 at 11:08 PM Feng zhou <zhoufeng.zf@bytedance.com> wrote:
>>
> <...>
>> ---
>>   kernel/bpf/helpers.c | 20 ++++++++++++++++++++
>>   1 file changed, 20 insertions(+)
>>
>> diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
>> index bb6b4637ebf2..453cbd312366 100644
>> --- a/kernel/bpf/helpers.c
>> +++ b/kernel/bpf/helpers.c
>> @@ -2149,6 +2149,25 @@ __bpf_kfunc struct cgroup *bpf_cgroup_from_id(u64 cgid)
>>                  return NULL;
>>          return cgrp;
>>   }
>> +
>> +/**
>> + * bpf_task_under_cgroup - wrap task_under_cgroup_hierarchy() as a kfunc, test
>> + * task's membership of cgroup ancestry.
>> + * @task: the task to be tested
>> + * @ancestor: possible ancestor of @task's cgroup
>> + *
>> + * Tests whether @task's default cgroup hierarchy is a descendant of @ancestor.
>> + * It follows all the same rules as cgroup_is_descendant, and only applies
>> + * to the default hierarchy.
>> + */
>> +__bpf_kfunc long bpf_task_under_cgroup(struct task_struct *task,
>> +                                      struct cgroup *ancestor)
>> +{
>> +       if (unlikely(!ancestor || !task))
>> +               return -EINVAL;
>> +
>> +       return task_under_cgroup_hierarchy(task, ancestor);
>> +}
>>   #endif /* CONFIG_CGROUPS */
>>
> 
> I wonder in what situation a null 'task' or 'ancestor' can be passed.
> Please call out in the comment that the returned value can be a
> negative error, so that writing if(bpf_task_under_cgroup()) may cause
> surprising results.
> 
> Hao

Hmm, you are right. As kfunc, the NULL value of the parameter is judged, 
and bpf verify will prompt the developer to add it. There is really no 
need to add this part of the judgment. See other people's opinions.

Re: Re: [PATCH bpf-next v6 2/2] selftests/bpf: Add testcase for bpf_task_under_cgroup

[Feng Zhou]

在 2023/5/5 15:13, Hao Luo 写道:
> On Thu, May 4, 2023 at 11:08 PM Feng zhou <zhoufeng.zf@bytedance.com> wrote:
>>
>> From: Feng Zhou <zhoufeng.zf@bytedance.com>
>>
>> test_progs:
>> Tests new kfunc bpf_task_under_cgroup().
>>
>> The bpf program saves the new task's pid 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 fork syscall, then check if
>> remote_pid and local_pid are unequal.
>>
>> Signed-off-by: Feng Zhou <zhoufeng.zf@bytedance.com>
>> Acked-by: Yonghong Song <yhs@fb.com>
>> ---
> 
> Hi Feng,
> 
> I have a comment about the methodology of the test, but the patch
> looks ok to me. Why do we have to test via a tracing program? I think
> what we need is just a task and a cgroup. Since we have the kfunc
> bpf_task_from_pid() and bpf_cgroup_from_id(), we can write a syscall
> program which takes a pid and a cgroup id as input and get the task
> and cgroup objects directly in the program.
> 
> I like testing via a syscall program because it doesn't depend on the
> newtask tracepoint and it should be simpler. But I'm ok with the
> current version of the patch, just have some thoughts.
> 
> Hao

Yes, your method is also very good. The reason why I did this is because 
of Song's suggestion before, hope that the parameter of the hook point 
will have a task, so I chose this to test.

Re: [PATCH bpf-next v6 1/2] bpf: Add bpf_task_under_cgroup() kfunc

[Yonghong Song]

On 5/5/23 12:18 AM, Feng Zhou wrote:
> 在 2023/5/5 14:58, Hao Luo 写道:
>> On Thu, May 4, 2023 at 11:08 PM Feng zhou <zhoufeng.zf@bytedance.com> 
>> wrote:
>>>
>> <...>
>>> ---
>>>   kernel/bpf/helpers.c | 20 ++++++++++++++++++++
>>>   1 file changed, 20 insertions(+)
>>>
>>> diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
>>> index bb6b4637ebf2..453cbd312366 100644
>>> --- a/kernel/bpf/helpers.c
>>> +++ b/kernel/bpf/helpers.c
>>> @@ -2149,6 +2149,25 @@ __bpf_kfunc struct cgroup 
>>> *bpf_cgroup_from_id(u64 cgid)
>>>                  return NULL;
>>>          return cgrp;
>>>   }
>>> +
>>> +/**
>>> + * bpf_task_under_cgroup - wrap task_under_cgroup_hierarchy() as a 
>>> kfunc, test
>>> + * task's membership of cgroup ancestry.
>>> + * @task: the task to be tested
>>> + * @ancestor: possible ancestor of @task's cgroup
>>> + *
>>> + * Tests whether @task's default cgroup hierarchy is a descendant of 
>>> @ancestor.
>>> + * It follows all the same rules as cgroup_is_descendant, and only 
>>> applies
>>> + * to the default hierarchy.
>>> + */
>>> +__bpf_kfunc long bpf_task_under_cgroup(struct task_struct *task,
>>> +                                      struct cgroup *ancestor)
>>> +{
>>> +       if (unlikely(!ancestor || !task))
>>> +               return -EINVAL;
>>> +
>>> +       return task_under_cgroup_hierarchy(task, ancestor);
>>> +}
>>>   #endif /* CONFIG_CGROUPS */
>>>
>>
>> I wonder in what situation a null 'task' or 'ancestor' can be passed.
>> Please call out in the comment that the returned value can be a
>> negative error, so that writing if(bpf_task_under_cgroup()) may cause
>> surprising results.
>>
>> Hao
> 
> Hmm, you are right. As kfunc, the NULL value of the parameter is judged, 
> and bpf verify will prompt the developer to add it. There is really no 
> need to add this part of the judgment. See other people's opinions.

Thanks for pointing out Hou.

Currently, bpf_task_under_cgroup() is marked as KF_RCU.

Per documentation:
2.4.7 KF_RCU flag
-----------------

The KF_RCU flag is a weaker version of KF_TRUSTED_ARGS. The kfuncs 
marked with
KF_RCU expect either PTR_TRUSTED or MEM_RCU arguments. The verifier 
guarantees
that the objects are valid and there is no use-after-free. The pointers 
are not
NULL, but the object's refcount could have reached zero. The kfuncs need to
consider doing refcnt != 0 check, especially when returning a KF_ACQUIRE
pointer. Note as well that a KF_ACQUIRE kfunc that is KF_RCU should very 
likely
also be KF_RET_NULL.


The pointer cannot be NULL, so the following line of code can be removed:
 >>> +       if (unlikely(!ancestor || !task))
 >>> +               return -EINVAL;

I think we do not need to check refcnt != 0 case since ancestor and
task won't go away.

In the example of second patch, both arguments are TRUSTED arguments
which is stronger than RCU, so the test itself is okay.
I am considering whether we should enforce arguments of the kfunc
to be KF_TRUSTED_ARGS, but I think esp. in some cases, cgroup
might be RCU protected e.g., task->cgroup->dfl_cgrp. So leaving argument
requirement as KF_RCU should be better.

> 
>

Re: [PATCH bpf-next v6 2/2] selftests/bpf: Add testcase for bpf_task_under_cgroup

[Yonghong Song]

On 5/5/23 12:24 AM, Feng Zhou wrote:
> 在 2023/5/5 15:13, Hao Luo 写道:
>> On Thu, May 4, 2023 at 11:08 PM Feng zhou <zhoufeng.zf@bytedance.com> 
>> wrote:
>>>
>>> From: Feng Zhou <zhoufeng.zf@bytedance.com>
>>>
>>> test_progs:
>>> Tests new kfunc bpf_task_under_cgroup().
>>>
>>> The bpf program saves the new task's pid 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 fork syscall, then check if
>>> remote_pid and local_pid are unequal.
>>>
>>> Signed-off-by: Feng Zhou <zhoufeng.zf@bytedance.com>
>>> Acked-by: Yonghong Song <yhs@fb.com>
>>> ---
>>
>> Hi Feng,
>>
>> I have a comment about the methodology of the test, but the patch
>> looks ok to me. Why do we have to test via a tracing program? I think
>> what we need is just a task and a cgroup. Since we have the kfunc
>> bpf_task_from_pid() and bpf_cgroup_from_id(), we can write a syscall
>> program which takes a pid and a cgroup id as input and get the task
>> and cgroup objects directly in the program.
>>
>> I like testing via a syscall program because it doesn't depend on the
>> newtask tracepoint and it should be simpler. But I'm ok with the
>> current version of the patch, just have some thoughts.
>>
>> Hao
> 
> Yes, your method is also very good. The reason why I did this is because 
> of Song's suggestion before, hope that the parameter of the hook point 
> will have a task, so I chose this to test.

The motivation of this patch is:
   Trace sched related functions, such as enqueue_task_fair, it is 
necessary to
   specify a task instead of the current task which within a given cgroup.

So I think it is okay to have a test related to sched.

Re: [PATCH bpf-next v6 1/2] bpf: Add bpf_task_under_cgroup() kfunc

[Alexei Starovoitov]

On Fri, May 5, 2023 at 11:44 AM Yonghong Song <yhs@meta.com> wrote:
>
>
>
> On 5/5/23 12:18 AM, Feng Zhou wrote:
> > 在 2023/5/5 14:58, Hao Luo 写道:
> >> On Thu, May 4, 2023 at 11:08 PM Feng zhou <zhoufeng.zf@bytedance.com>
> >> wrote:
> >>>
> >> <...>
> >>> ---
> >>>   kernel/bpf/helpers.c | 20 ++++++++++++++++++++
> >>>   1 file changed, 20 insertions(+)
> >>>
> >>> diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
> >>> index bb6b4637ebf2..453cbd312366 100644
> >>> --- a/kernel/bpf/helpers.c
> >>> +++ b/kernel/bpf/helpers.c
> >>> @@ -2149,6 +2149,25 @@ __bpf_kfunc struct cgroup
> >>> *bpf_cgroup_from_id(u64 cgid)
> >>>                  return NULL;
> >>>          return cgrp;
> >>>   }
> >>> +
> >>> +/**
> >>> + * bpf_task_under_cgroup - wrap task_under_cgroup_hierarchy() as a
> >>> kfunc, test
> >>> + * task's membership of cgroup ancestry.
> >>> + * @task: the task to be tested
> >>> + * @ancestor: possible ancestor of @task's cgroup
> >>> + *
> >>> + * Tests whether @task's default cgroup hierarchy is a descendant of
> >>> @ancestor.
> >>> + * It follows all the same rules as cgroup_is_descendant, and only
> >>> applies
> >>> + * to the default hierarchy.
> >>> + */
> >>> +__bpf_kfunc long bpf_task_under_cgroup(struct task_struct *task,
> >>> +                                      struct cgroup *ancestor)
> >>> +{
> >>> +       if (unlikely(!ancestor || !task))
> >>> +               return -EINVAL;
> >>> +
> >>> +       return task_under_cgroup_hierarchy(task, ancestor);
> >>> +}
> >>>   #endif /* CONFIG_CGROUPS */
> >>>
> >>
> >> I wonder in what situation a null 'task' or 'ancestor' can be passed.
> >> Please call out in the comment that the returned value can be a
> >> negative error, so that writing if(bpf_task_under_cgroup()) may cause
> >> surprising results.
> >>
> >> Hao
> >
> > Hmm, you are right. As kfunc, the NULL value of the parameter is judged,
> > and bpf verify will prompt the developer to add it. There is really no
> > need to add this part of the judgment. See other people's opinions.
>
> Thanks for pointing out Hou.
>
> Currently, bpf_task_under_cgroup() is marked as KF_RCU.
>
> Per documentation:
> 2.4.7 KF_RCU flag
> -----------------
>
> The KF_RCU flag is a weaker version of KF_TRUSTED_ARGS. The kfuncs
> marked with
> KF_RCU expect either PTR_TRUSTED or MEM_RCU arguments. The verifier
> guarantees
> that the objects are valid and there is no use-after-free. The pointers
> are not
> NULL, but the object's refcount could have reached zero. The kfuncs need to
> consider doing refcnt != 0 check, especially when returning a KF_ACQUIRE
> pointer. Note as well that a KF_ACQUIRE kfunc that is KF_RCU should very
> likely
> also be KF_RET_NULL.
>
>
> The pointer cannot be NULL, so the following line of code can be removed:
>  >>> +       if (unlikely(!ancestor || !task))
>  >>> +               return -EINVAL;

Right. With KF_RCU the verifier guarantees != NULL.
Let's get rid of this check.
This will make the return value clean.

> I think we do not need to check refcnt != 0 case since ancestor and
> task won't go away.

correct.

> In the example of second patch, both arguments are TRUSTED arguments
> which is stronger than RCU, so the test itself is okay.
> I am considering whether we should enforce arguments of the kfunc
> to be KF_TRUSTED_ARGS, but I think esp. in some cases, cgroup
> might be RCU protected e.g., task->cgroup->dfl_cgrp. So leaving argument
> requirement as KF_RCU should be better.

+1