diff mbox series

[bpf-next,v4,4/8] bpf: Introduce css open-coded iterator kfuncs

Message ID 20231007124522.34834-5-zhouchuyi@bytedance.com
State New
Headers
Series Add Open-coded task, css_task and css iters |

Commit Message

Chuyi Zhou Oct. 7, 2023, 12:45 p.m. UTC 
  This Patch adds kfuncs bpf_iter_css_{new,next,destroy} which allow
creation and manipulation of struct bpf_iter_css in open-coded iterator
style. These kfuncs actually wrapps css_next_descendant_{pre, post}.
css_iter can be used to:

1) iterating a sepcific cgroup tree with pre/post/up order

2) iterating cgroup_subsystem in BPF Prog, like
for_each_mem_cgroup_tree/cpuset_for_each_descendant_pre in kernel.

The API design is consistent with cgroup_iter. bpf_iter_css_new accepts
parameters defining iteration order and starting css. Here we also reuse
BPF_CGROUP_ITER_DESCENDANTS_PRE, BPF_CGROUP_ITER_DESCENDANTS_POST,
BPF_CGROUP_ITER_ANCESTORS_UP enums.

Signed-off-by: Chuyi Zhou <zhouchuyi@bytedance.com>
Acked-by: Tejun Heo <tj@kernel.org>
---
 kernel/bpf/cgroup_iter.c                      | 59 +++++++++++++++++++
 kernel/bpf/helpers.c                          |  3 +
 .../testing/selftests/bpf/bpf_experimental.h  |  6 ++
 3 files changed, 68 insertions(+)

Comments

kernel test robot Oct. 7, 2023, 4:05 p.m. UTC | #1 
Hi Chuyi,

kernel test robot noticed the following build warnings:

[auto build test WARNING on bpf-next/master]

url:    https://github.com/intel-lab-lkp/linux/commits/Chuyi-Zhou/cgroup-Prepare-for-using-css_task_iter_-in-BPF/20231007-204750
base:   https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf-next.git master
patch link:    https://lore.kernel.org/r/20231007124522.34834-5-zhouchuyi%40bytedance.com
patch subject: [PATCH bpf-next v4 4/8] bpf: Introduce css open-coded iterator kfuncs
config: m68k-allyesconfig (https://download.01.org/0day-ci/archive/20231007/202310072337.CzRlbffm-lkp@intel.com/config)
compiler: m68k-linux-gcc (GCC) 13.2.0
reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20231007/202310072337.CzRlbffm-lkp@intel.com/reproduce)

If you fix the issue in a separate patch/commit (i.e. not just a new version of
the same patch/commit), kindly add following tags
| Reported-by: kernel test robot <lkp@intel.com>
| Closes: https://lore.kernel.org/oe-kbuild-all/202310072337.CzRlbffm-lkp@intel.com/

All warnings (new ones prefixed by >>):

>> kernel/bpf/cgroup_iter.c:308:17: warning: no previous prototype for 'bpf_iter_css_new' [-Wmissing-prototypes]
     308 | __bpf_kfunc int bpf_iter_css_new(struct bpf_iter_css *it,
         |                 ^~~~~~~~~~~~~~~~
>> kernel/bpf/cgroup_iter.c:332:41: warning: no previous prototype for 'bpf_iter_css_next' [-Wmissing-prototypes]
     332 | __bpf_kfunc struct cgroup_subsys_state *bpf_iter_css_next(struct bpf_iter_css *it)
         |                                         ^~~~~~~~~~~~~~~~~
>> kernel/bpf/cgroup_iter.c:353:18: warning: no previous prototype for 'bpf_iter_css_destroy' [-Wmissing-prototypes]
     353 | __bpf_kfunc void bpf_iter_css_destroy(struct bpf_iter_css *it)
         |                  ^~~~~~~~~~~~~~~~~~~~
   In file included from <command-line>:
   kernel/bpf/cgroup_iter.c: In function 'bpf_iter_css_new':
   include/linux/compiler_types.h:425:45: error: call to '__compiletime_assert_320' declared with attribute error: BUILD_BUG_ON failed: sizeof(struct bpf_iter_css_kern) != sizeof(struct bpf_iter_css)
     425 |         _compiletime_assert(condition, msg, __compiletime_assert_, __COUNTER__)
         |                                             ^
   include/linux/compiler_types.h:406:25: note: in definition of macro '__compiletime_assert'
     406 |                         prefix ## suffix();                             \
         |                         ^~~~~~
   include/linux/compiler_types.h:425:9: note: in expansion of macro '_compiletime_assert'
     425 |         _compiletime_assert(condition, msg, __compiletime_assert_, __COUNTER__)
         |         ^~~~~~~~~~~~~~~~~~~
   include/linux/build_bug.h:39:37: note: in expansion of macro 'compiletime_assert'
      39 | #define BUILD_BUG_ON_MSG(cond, msg) compiletime_assert(!(cond), msg)
         |                                     ^~~~~~~~~~~~~~~~~~
   include/linux/build_bug.h:50:9: note: in expansion of macro 'BUILD_BUG_ON_MSG'
      50 |         BUILD_BUG_ON_MSG(condition, "BUILD_BUG_ON failed: " #condition)
         |         ^~~~~~~~~~~~~~~~
   kernel/bpf/cgroup_iter.c:313:9: note: in expansion of macro 'BUILD_BUG_ON'
     313 |         BUILD_BUG_ON(sizeof(struct bpf_iter_css_kern) != sizeof(struct bpf_iter_css));
         |         ^~~~~~~~~~~~


vim +/bpf_iter_css_new +308 kernel/bpf/cgroup_iter.c

   307	
 > 308	__bpf_kfunc int bpf_iter_css_new(struct bpf_iter_css *it,
   309			struct cgroup_subsys_state *start, unsigned int flags)
   310	{
   311		struct bpf_iter_css_kern *kit = (void *)it;
   312	
   313		BUILD_BUG_ON(sizeof(struct bpf_iter_css_kern) != sizeof(struct bpf_iter_css));
   314		BUILD_BUG_ON(__alignof__(struct bpf_iter_css_kern) != __alignof__(struct bpf_iter_css));
   315	
   316		kit->start = NULL;
   317		switch (flags) {
   318		case BPF_CGROUP_ITER_DESCENDANTS_PRE:
   319		case BPF_CGROUP_ITER_DESCENDANTS_POST:
   320		case BPF_CGROUP_ITER_ANCESTORS_UP:
   321			break;
   322		default:
   323			return -EINVAL;
   324		}
   325	
   326		kit->start = start;
   327		kit->pos = NULL;
   328		kit->flags = flags;
   329		return 0;
   330	}
   331	
 > 332	__bpf_kfunc struct cgroup_subsys_state *bpf_iter_css_next(struct bpf_iter_css *it)
   333	{
   334		struct bpf_iter_css_kern *kit = (void *)it;
   335	
   336		if (!kit->start)
   337			return NULL;
   338	
   339		switch (kit->flags) {
   340		case BPF_CGROUP_ITER_DESCENDANTS_PRE:
   341			kit->pos = css_next_descendant_pre(kit->pos, kit->start);
   342			break;
   343		case BPF_CGROUP_ITER_DESCENDANTS_POST:
   344			kit->pos = css_next_descendant_post(kit->pos, kit->start);
   345			break;
   346		case BPF_CGROUP_ITER_ANCESTORS_UP:
   347			kit->pos = kit->pos ? kit->pos->parent : kit->start;
   348		}
   349	
   350		return kit->pos;
   351	}
   352	
 > 353	__bpf_kfunc void bpf_iter_css_destroy(struct bpf_iter_css *it)
Chuyi Zhou Oct. 11, 2023, 4:44 a.m. UTC | #2 
在 2023/10/7 20:45, Chuyi Zhou 写道:
> This Patch adds kfuncs bpf_iter_css_{new,next,destroy} which allow
> creation and manipulation of struct bpf_iter_css in open-coded iterator
> style. These kfuncs actually wrapps css_next_descendant_{pre, post}.
> css_iter can be used to:
> 
> 1) iterating a sepcific cgroup tree with pre/post/up order
> 
> 2) iterating cgroup_subsystem in BPF Prog, like
> for_each_mem_cgroup_tree/cpuset_for_each_descendant_pre in kernel.
> 
> The API design is consistent with cgroup_iter. bpf_iter_css_new accepts
> parameters defining iteration order and starting css. Here we also reuse
> BPF_CGROUP_ITER_DESCENDANTS_PRE, BPF_CGROUP_ITER_DESCENDANTS_POST,
> BPF_CGROUP_ITER_ANCESTORS_UP enums.
> 
> Signed-off-by: Chuyi Zhou <zhouchuyi@bytedance.com>
> Acked-by: Tejun Heo <tj@kernel.org>
> ---
>   kernel/bpf/cgroup_iter.c                      | 59 +++++++++++++++++++
>   kernel/bpf/helpers.c                          |  3 +
>   .../testing/selftests/bpf/bpf_experimental.h  |  6 ++
>   3 files changed, 68 insertions(+)
> 
> diff --git a/kernel/bpf/cgroup_iter.c b/kernel/bpf/cgroup_iter.c
> index 810378f04fbc..9c6ad892ae82 100644
> --- a/kernel/bpf/cgroup_iter.c
> +++ b/kernel/bpf/cgroup_iter.c
> @@ -294,3 +294,62 @@ static int __init bpf_cgroup_iter_init(void)
>   }
>   
>   late_initcall(bpf_cgroup_iter_init);
> +
> +struct bpf_iter_css {
> +	__u64 __opaque[3];
> +} __attribute__((aligned(8)));
> +
> +struct bpf_iter_css_kern {
> +	struct cgroup_subsys_state *start;
> +	struct cgroup_subsys_state *pos;
> +	unsigned int flags;
> +} __attribute__((aligned(8)));
> +
> +__bpf_kfunc int bpf_iter_css_new(struct bpf_iter_css *it,
> +		struct cgroup_subsys_state *start, unsigned int flags)
> +{
> +	struct bpf_iter_css_kern *kit = (void *)it;
> +
> +	BUILD_BUG_ON(sizeof(struct bpf_iter_css_kern) != sizeof(struct bpf_iter_css));
> +	BUILD_BUG_ON(__alignof__(struct bpf_iter_css_kern) != __alignof__(struct bpf_iter_css));
> +

This would cause the fail of netdev/build_32bit CI 
(https://netdev.bots.linux.dev/static/nipa/790929/13412333/build_32bit/stderr):

tools/testing/selftests/kvm/settings: warning: ignored by one of the 
.gitignore files
../kernel/bpf/cgroup_iter.c:308:17: warning: no previous prototype for 
‘bpf_iter_css_new’ [-Wmissing-prototypes]
   308 | __bpf_kfunc int bpf_iter_css_new(struct bpf_iter_css *it,
       |                 ^~~~~~~~~~~~~~~~
../kernel/bpf/cgroup_iter.c:332:41: warning: no previous prototype for 
‘bpf_iter_css_next’ [-Wmissing-prototypes]
   332 | __bpf_kfunc struct cgroup_subsys_state 
*bpf_iter_css_next(struct bpf_iter_css *it)
       |                                         ^~~~~~~~~~~~~~~~~
../kernel/bpf/cgroup_iter.c:353:18: warning: no previous prototype for 
‘bpf_iter_css_destroy’ [-Wmissing-prototypes]
   353 | __bpf_kfunc void bpf_iter_css_destroy(struct bpf_iter_css *it)
       |                  ^~~~~~~~~~~~~~~~~~~~
In file included from <command-line>:
../kernel/bpf/cgroup_iter.c: In function ‘bpf_iter_css_new’:
./../include/linux/compiler_types.h:425:45: error: call to 
‘__compiletime_assert_322’ declared with attribute error: BUILD_BUG_ON 
failed: sizeof(struct bpf_iter_css_kern) != sizeof(struct bpf_iter_css)
   425 |         _compiletime_assert(condition, msg, 
__compiletime_assert_, __COUNTER__)
       |                                             ^
./../include/linux/compiler_types.h:406:25: note: in definition of macro 
‘__compiletime_assert’
   406 |                         prefix ## suffix(); 
         \
       |                         ^~~~~~
./../include/linux/compiler_types.h:425:9: note: in expansion of macro 
‘_compiletime_assert’
   425 |         _compiletime_assert(condition, msg, 
__compiletime_assert_, __COUNTER__)
       |         ^~~~~~~~~~~~~~~~~~~
../include/linux/build_bug.h:39:37: note: in expansion of macro 
‘compiletime_assert’
    39 | #define BUILD_BUG_ON_MSG(cond, msg) compiletime_assert(!(cond), 
msg)
       |                                     ^~~~~~~~~~~~~~~~~~
../include/linux/build_bug.h:50:9: note: in expansion of macro 
‘BUILD_BUG_ON_MSG’
    50 |         BUILD_BUG_ON_MSG(condition, "BUILD_BUG_ON failed: " 
#condition)
       |         ^~~~~~~~~~~~~~~~
../kernel/bpf/cgroup_iter.c:313:9: note: in expansion of macro 
‘BUILD_BUG_ON’
   313 |         BUILD_BUG_ON(sizeof(struct bpf_iter_css_kern) != 
sizeof(struct bpf_iter_css));


The reason seems on 32-bit machine, sizeof(struct bpf_iter_css) is 24 
and sizeof(struct bpf_iter_css_kern) is 16.

I was wondering whether the BUILD_BUG_ON check is necessary. Looking at 
the struct bpf_list_node and struct bpf_list_node_kern wich are very 
similay to bpf_iter_css, I didn't see the BUILD_BUG_ON check when 
convert from (struct bpf_list_node *) to (struct bpf_list_node_kern *)

/* Non-opaque version of bpf_list_node in uapi/linux/bpf.h */
struct bpf_list_node_kern {
	struct list_head list_head;
	void *owner;
} __attribute__((aligned(8)));

struct bpf_list_node {
	__u64 :64;
	__u64 :64;
	__u64 :64;
} __attribute__((aligned(8)));

__bpf_kfunc int bpf_list_push_back_impl(struct bpf_list_head *head,
					struct bpf_list_node *node,
					void *meta__ign, u64 off)
{
	struct bpf_list_node_kern *n = (void *)node;

}
Chuyi Zhou Oct. 11, 2023, 5:32 a.m. UTC | #3 
在 2023/10/11 12:44, Chuyi Zhou 写道:
> 
> 
> 在 2023/10/7 20:45, Chuyi Zhou 写道:
>> This Patch adds kfuncs bpf_iter_css_{new,next,destroy} which allow
>> creation and manipulation of struct bpf_iter_css in open-coded iterator
>> style. These kfuncs actually wrapps css_next_descendant_{pre, post}.
>> css_iter can be used to:
>>
>> 1) iterating a sepcific cgroup tree with pre/post/up order
>>
>> 2) iterating cgroup_subsystem in BPF Prog, like
>> for_each_mem_cgroup_tree/cpuset_for_each_descendant_pre in kernel.
>>
>> The API design is consistent with cgroup_iter. bpf_iter_css_new accepts
>> parameters defining iteration order and starting css. Here we also reuse
>> BPF_CGROUP_ITER_DESCENDANTS_PRE, BPF_CGROUP_ITER_DESCENDANTS_POST,
>> BPF_CGROUP_ITER_ANCESTORS_UP enums.
>>
>> Signed-off-by: Chuyi Zhou <zhouchuyi@bytedance.com>
>> Acked-by: Tejun Heo <tj@kernel.org>
>> ---
>>   kernel/bpf/cgroup_iter.c                      | 59 +++++++++++++++++++
>>   kernel/bpf/helpers.c                          |  3 +
>>   .../testing/selftests/bpf/bpf_experimental.h  |  6 ++
>>   3 files changed, 68 insertions(+)
>>
>> diff --git a/kernel/bpf/cgroup_iter.c b/kernel/bpf/cgroup_iter.c
>> index 810378f04fbc..9c6ad892ae82 100644
>> --- a/kernel/bpf/cgroup_iter.c
>> +++ b/kernel/bpf/cgroup_iter.c
>> @@ -294,3 +294,62 @@ static int __init bpf_cgroup_iter_init(void)
>>   }
>>   late_initcall(bpf_cgroup_iter_init);
>> +
>> +struct bpf_iter_css {
>> +    __u64 __opaque[3];
>> +} __attribute__((aligned(8)));
>> +
>> +struct bpf_iter_css_kern {
>> +    struct cgroup_subsys_state *start;
>> +    struct cgroup_subsys_state *pos;
>> +    unsigned int flags;
>> +} __attribute__((aligned(8)));
>> +
>> +__bpf_kfunc int bpf_iter_css_new(struct bpf_iter_css *it,
>> +        struct cgroup_subsys_state *start, unsigned int flags)
>> +{
>> +    struct bpf_iter_css_kern *kit = (void *)it;
>> +
>> +    BUILD_BUG_ON(sizeof(struct bpf_iter_css_kern) != sizeof(struct 
>> bpf_iter_css));
>> +    BUILD_BUG_ON(__alignof__(struct bpf_iter_css_kern) != 
>> __alignof__(struct bpf_iter_css));
>> +
> 
> This would cause the fail of netdev/build_32bit CI 
> (https://netdev.bots.linux.dev/static/nipa/790929/13412333/build_32bit/stderr):
> 
> tools/testing/selftests/kvm/settings: warning: ignored by one of the 
> .gitignore files
> ../kernel/bpf/cgroup_iter.c:308:17: warning: no previous prototype for 
> ‘bpf_iter_css_new’ [-Wmissing-prototypes]
>    308 | __bpf_kfunc int bpf_iter_css_new(struct bpf_iter_css *it,
>        |                 ^~~~~~~~~~~~~~~~
> ../kernel/bpf/cgroup_iter.c:332:41: warning: no previous prototype for 
> ‘bpf_iter_css_next’ [-Wmissing-prototypes]
>    332 | __bpf_kfunc struct cgroup_subsys_state 
> *bpf_iter_css_next(struct bpf_iter_css *it)
>        |                                         ^~~~~~~~~~~~~~~~~
> ../kernel/bpf/cgroup_iter.c:353:18: warning: no previous prototype for 
> ‘bpf_iter_css_destroy’ [-Wmissing-prototypes]
>    353 | __bpf_kfunc void bpf_iter_css_destroy(struct bpf_iter_css *it)
>        |                  ^~~~~~~~~~~~~~~~~~~~
> In file included from <command-line>:
> ../kernel/bpf/cgroup_iter.c: In function ‘bpf_iter_css_new’:
> ./../include/linux/compiler_types.h:425:45: error: call to 
> ‘__compiletime_assert_322’ declared with attribute error: BUILD_BUG_ON 
> failed: sizeof(struct bpf_iter_css_kern) != sizeof(struct bpf_iter_css)
>    425 |         _compiletime_assert(condition, msg, 
> __compiletime_assert_, __COUNTER__)
>        |                                             ^
> ./../include/linux/compiler_types.h:406:25: note: in definition of macro 
> ‘__compiletime_assert’
>    406 |                         prefix ## suffix();         \
>        |                         ^~~~~~
> ./../include/linux/compiler_types.h:425:9: note: in expansion of macro 
> ‘_compiletime_assert’
>    425 |         _compiletime_assert(condition, msg, 
> __compiletime_assert_, __COUNTER__)
>        |         ^~~~~~~~~~~~~~~~~~~
> ../include/linux/build_bug.h:39:37: note: in expansion of macro 
> ‘compiletime_assert’
>     39 | #define BUILD_BUG_ON_MSG(cond, msg) compiletime_assert(!(cond), 
> msg)
>        |                                     ^~~~~~~~~~~~~~~~~~
> ../include/linux/build_bug.h:50:9: note: in expansion of macro 
> ‘BUILD_BUG_ON_MSG’
>     50 |         BUILD_BUG_ON_MSG(condition, "BUILD_BUG_ON failed: " 
> #condition)
>        |         ^~~~~~~~~~~~~~~~
> ../kernel/bpf/cgroup_iter.c:313:9: note: in expansion of macro 
> ‘BUILD_BUG_ON’
>    313 |         BUILD_BUG_ON(sizeof(struct bpf_iter_css_kern) != 
> sizeof(struct bpf_iter_css));
> 
> 
> The reason seems on 32-bit machine, sizeof(struct bpf_iter_css) is 24 
> and sizeof(struct bpf_iter_css_kern) is 16.
> 
> I was wondering whether the BUILD_BUG_ON check is necessary. Looking at 
> the struct bpf_list_node and struct bpf_list_node_kern wich are very 
> similay to bpf_iter_css, I didn't see the BUILD_BUG_ON check when 
> convert from (struct bpf_list_node *) to (struct bpf_list_node_kern *)
> 
> /* Non-opaque version of bpf_list_node in uapi/linux/bpf.h */
> struct bpf_list_node_kern {
>      struct list_head list_head;
>      void *owner;
> } __attribute__((aligned(8)));
> 
> struct bpf_list_node {
>      __u64 :64;
>      __u64 :64;
>      __u64 :64;
> } __attribute__((aligned(8)));
> 
> __bpf_kfunc int bpf_list_push_back_impl(struct bpf_list_head *head,
>                      struct bpf_list_node *node,
>                      void *meta__ign, u64 off)
> {
>      struct bpf_list_node_kern *n = (void *)node;
> 
> }
> 

or we can change the BUILD_BUG_ON check, like bpf_timer_kern in 
bpf_timer_init:

--- a/kernel/bpf/cgroup_iter.c
+++ b/kernel/bpf/cgroup_iter.c
@@ -310,7 +310,7 @@ __bpf_kfunc int bpf_iter_css_new(struct bpf_iter_css 
*it,
  {
         struct bpf_iter_css_kern *kit = (void *)it;

-       BUILD_BUG_ON(sizeof(struct bpf_iter_css_kern) != sizeof(struct 
bpf_iter_css));
+       BUILD_BUG_ON(sizeof(struct bpf_iter_css_kern) > sizeof(struct 
bpf_iter_css));
         BUILD_BUG_ON(__alignof__(struct bpf_iter_css_kern) != 
__alignof__(struct bpf_iter_css));

         kit->start = NULL;
diff --git a/kernel/bpf/task_iter.c b/kernel/bpf/task_iter.c
index 773be9a221f5..0772545568f1 100644
--- a/kernel/bpf/task_iter.c
+++ b/kernel/bpf/task_iter.c
@@ -877,7 +877,7 @@ __bpf_kfunc int bpf_iter_task_new(struct 
bpf_iter_task *it,
  {
         struct bpf_iter_task_kern *kit = (void *)it;

-       BUILD_BUG_ON(sizeof(struct bpf_iter_task_kern) != sizeof(struct 
bpf_iter_task));
+       BUILD_BUG_ON(sizeof(struct bpf_iter_task_kern) > sizeof(struct 
bpf_iter_task));
         BUILD_BUG_ON(__alignof__(struct bpf_iter_task_kern) !=
                                         __alignof__(struct bpf_iter_task));
diff mbox series

Patch

diff --git a/kernel/bpf/cgroup_iter.c b/kernel/bpf/cgroup_iter.c
index 810378f04fbc..9c6ad892ae82 100644
--- a/kernel/bpf/cgroup_iter.c
+++ b/kernel/bpf/cgroup_iter.c
@@ -294,3 +294,62 @@  static int __init bpf_cgroup_iter_init(void)
 }
 
 late_initcall(bpf_cgroup_iter_init);
+
+struct bpf_iter_css {
+	__u64 __opaque[3];
+} __attribute__((aligned(8)));
+
+struct bpf_iter_css_kern {
+	struct cgroup_subsys_state *start;
+	struct cgroup_subsys_state *pos;
+	unsigned int flags;
+} __attribute__((aligned(8)));
+
+__bpf_kfunc int bpf_iter_css_new(struct bpf_iter_css *it,
+		struct cgroup_subsys_state *start, unsigned int flags)
+{
+	struct bpf_iter_css_kern *kit = (void *)it;
+
+	BUILD_BUG_ON(sizeof(struct bpf_iter_css_kern) != sizeof(struct bpf_iter_css));
+	BUILD_BUG_ON(__alignof__(struct bpf_iter_css_kern) != __alignof__(struct bpf_iter_css));
+
+	kit->start = NULL;
+	switch (flags) {
+	case BPF_CGROUP_ITER_DESCENDANTS_PRE:
+	case BPF_CGROUP_ITER_DESCENDANTS_POST:
+	case BPF_CGROUP_ITER_ANCESTORS_UP:
+		break;
+	default:
+		return -EINVAL;
+	}
+
+	kit->start = start;
+	kit->pos = NULL;
+	kit->flags = flags;
+	return 0;
+}
+
+__bpf_kfunc struct cgroup_subsys_state *bpf_iter_css_next(struct bpf_iter_css *it)
+{
+	struct bpf_iter_css_kern *kit = (void *)it;
+
+	if (!kit->start)
+		return NULL;
+
+	switch (kit->flags) {
+	case BPF_CGROUP_ITER_DESCENDANTS_PRE:
+		kit->pos = css_next_descendant_pre(kit->pos, kit->start);
+		break;
+	case BPF_CGROUP_ITER_DESCENDANTS_POST:
+		kit->pos = css_next_descendant_post(kit->pos, kit->start);
+		break;
+	case BPF_CGROUP_ITER_ANCESTORS_UP:
+		kit->pos = kit->pos ? kit->pos->parent : kit->start;
+	}
+
+	return kit->pos;
+}
+
+__bpf_kfunc void bpf_iter_css_destroy(struct bpf_iter_css *it)
+{
+}
diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
index 8111437a999e..7596e83d6715 100644
--- a/kernel/bpf/helpers.c
+++ b/kernel/bpf/helpers.c
@@ -2555,6 +2555,9 @@  BTF_ID_FLAGS(func, bpf_iter_css_task_destroy, KF_ITER_DESTROY)
 BTF_ID_FLAGS(func, bpf_iter_task_new, KF_ITER_NEW | KF_TRUSTED_ARGS)
 BTF_ID_FLAGS(func, bpf_iter_task_next, KF_ITER_NEXT | KF_RET_NULL)
 BTF_ID_FLAGS(func, bpf_iter_task_destroy, KF_ITER_DESTROY)
+BTF_ID_FLAGS(func, bpf_iter_css_new, KF_ITER_NEW | KF_TRUSTED_ARGS)
+BTF_ID_FLAGS(func, bpf_iter_css_next, KF_ITER_NEXT | KF_RET_NULL)
+BTF_ID_FLAGS(func, bpf_iter_css_destroy, KF_ITER_DESTROY)
 BTF_ID_FLAGS(func, bpf_dynptr_adjust)
 BTF_ID_FLAGS(func, bpf_dynptr_is_null)
 BTF_ID_FLAGS(func, bpf_dynptr_is_rdonly)
diff --git a/tools/testing/selftests/bpf/bpf_experimental.h b/tools/testing/selftests/bpf/bpf_experimental.h
index 1ec82997cce7..9aab609f6edd 100644
--- a/tools/testing/selftests/bpf/bpf_experimental.h
+++ b/tools/testing/selftests/bpf/bpf_experimental.h
@@ -463,4 +463,10 @@  extern int bpf_iter_task_new(struct bpf_iter_task *it,
 extern struct task_struct *bpf_iter_task_next(struct bpf_iter_task *it) __weak __ksym;
 extern void bpf_iter_task_destroy(struct bpf_iter_task *it) __weak __ksym;
 
+struct bpf_iter_css;
+extern int bpf_iter_css_new(struct bpf_iter_css *it,
+				struct cgroup_subsys_state *start, unsigned int flags) __weak __ksym;
+extern struct cgroup_subsys_state *bpf_iter_css_next(struct bpf_iter_css *it) __weak __ksym;
+extern void bpf_iter_css_destroy(struct bpf_iter_css *it) __weak __ksym;
+
 #endif