Re: [PATCH bpf-next v5 4/8] bpf: Introduce cgroup iter

[Yonghong Song <yhs@fb.com>]

On 7/22/22 10:48 AM, Yosry Ahmed wrote:
> From: Hao Luo <haoluo@google.com>
> 
> Cgroup_iter is a type of bpf_iter. It walks over cgroups in three modes:
> 
>   - walking a cgroup's descendants in pre-order.
>   - walking a cgroup's descendants in post-order.
>   - walking a cgroup's ancestors.
> 
> When attaching cgroup_iter, one can set a cgroup to the iter_link
> created from attaching. This cgroup is passed as a file descriptor and
> serves as the starting point of the walk. If no cgroup is specified,
> the starting point will be the root cgroup.
> 
> For walking descendants, one can specify the order: either pre-order or
> post-order. For walking ancestors, the walk starts at the specified
> cgroup and ends at the root.
> 
> One can also terminate the walk early by returning 1 from the iter
> program.
> 
> Note that because walking cgroup hierarchy holds cgroup_mutex, the iter
> program is called with cgroup_mutex held.
> 
> Currently only one session is supported, which means, depending on the
> volume of data bpf program intends to send to user space, the number
> of cgroups that can be walked is limited. For example, given the current
> buffer size is 8 * PAGE_SIZE, if the program sends 64B data for each
> cgroup, the total number of cgroups that can be walked is 512. This is

PAGE_SIZE needs to be 4KB in order to conclude that the total number of
walked cgroups is 512.

> a limitation of cgroup_iter. If the output data is larger than the
> buffer size, the second read() will signal EOPNOTSUPP. In order to work
> around, the user may have to update their program to reduce the volume
> of data sent to output. For example, skip some uninteresting cgroups.
> In future, we may extend bpf_iter flags to allow customizing buffer
> size.
> 
> Signed-off-by: Hao Luo <haoluo@google.com>
> Signed-off-by: Yosry Ahmed <yosryahmed@google.com>
> Acked-by: Yonghong Song <yhs@fb.com>
> ---
>   include/linux/bpf.h                           |   8 +
>   include/uapi/linux/bpf.h                      |  30 +++
>   kernel/bpf/Makefile                           |   3 +
>   kernel/bpf/cgroup_iter.c                      | 252 ++++++++++++++++++
>   tools/include/uapi/linux/bpf.h                |  30 +++
>   .../selftests/bpf/prog_tests/btf_dump.c       |   4 +-
>   6 files changed, 325 insertions(+), 2 deletions(-)
>   create mode 100644 kernel/bpf/cgroup_iter.c

This patch cannot apply to bpf-next cleanly, so please rebase
and post again.

> 
> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> index a97751d845c9..9061618fe929 100644
> --- a/include/linux/bpf.h
> +++ b/include/linux/bpf.h
> @@ -47,6 +47,7 @@ struct kobject;
>   struct mem_cgroup;
>   struct module;
>   struct bpf_func_state;
> +struct cgroup;
>   
>   extern struct idr btf_idr;
>   extern spinlock_t btf_idr_lock;
> @@ -1717,7 +1718,14 @@ int bpf_obj_get_user(const char __user *pathname, int flags);
>   	int __init bpf_iter_ ## target(args) { return 0; }
>   
>   struct bpf_iter_aux_info {
> +	/* for map_elem iter */
>   	struct bpf_map *map;
> +
> +	/* for cgroup iter */
> +	struct {
> +		struct cgroup *start; /* starting cgroup */
> +		int order;
> +	} cgroup;
>   };
>   
>   typedef int (*bpf_iter_attach_target_t)(struct bpf_prog *prog,
> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> index ffcbf79a556b..fe50c2489350 100644
> --- a/include/uapi/linux/bpf.h
> +++ b/include/uapi/linux/bpf.h
> @@ -87,10 +87,30 @@ struct bpf_cgroup_storage_key {
>   	__u32	attach_type;		/* program attach type (enum bpf_attach_type) */
>   };
>   
> +enum bpf_iter_cgroup_traversal_order {
> +	BPF_ITER_CGROUP_PRE = 0,	/* pre-order traversal */
> +	BPF_ITER_CGROUP_POST,		/* post-order traversal */
> +	BPF_ITER_CGROUP_PARENT_UP,	/* traversal of ancestors up to the root */
> +};
> +
>   union bpf_iter_link_info {
>   	struct {
>   		__u32	map_fd;
>   	} map;
> +
> +	/* cgroup_iter walks either the live descendants of a cgroup subtree, or the
> +	 * ancestors of a given cgroup.
> +	 */
> +	struct {
> +		/* Cgroup file descriptor. This is root of the subtree if walking
> +		 * descendants; it's the starting cgroup if walking the ancestors.
> +		 * If it is left 0, the traversal starts from the default cgroup v2
> +		 * root. For walking v1 hierarchy, one should always explicitly
> +		 * specify the cgroup_fd.
> +		 */

I did see how the above cgroup v1/v2 scenarios are enforced.

> +		__u32	cgroup_fd;
> +		__u32	traversal_order;
> +	} cgroup;
>   };
>   
>   /* BPF syscall commands, see bpf(2) man-page for more details. */
> @@ -6136,6 +6156,16 @@ struct bpf_link_info {
>   					__u32 map_id;
>   				} map;
>   			};
> +			union {
> +				struct {
> +					__u64 cgroup_id;
> +					__u32 traversal_order;
> +				} cgroup;
> +			};
> +			/* For new iters, if the first field is larger than __u32,
> +			 * the struct should be added in the second union. Otherwise,
> +			 * it will create holes before map_id, breaking uapi.
> +			 */

Please put the comment above the union. Let us just say, if
the iter specific field is __u32, it can be put in the first or
second union. Otherwise, it is put in second union.

>   		} iter;
>   		struct  {
>   			__u32 netns_ino;
> diff --git a/kernel/bpf/Makefile b/kernel/bpf/Makefile
> index 057ba8e01e70..00e05b69a4df 100644
> --- a/kernel/bpf/Makefile
> +++ b/kernel/bpf/Makefile
> @@ -24,6 +24,9 @@ endif
>   ifeq ($(CONFIG_PERF_EVENTS),y)
>   obj-$(CONFIG_BPF_SYSCALL) += stackmap.o
>   endif
> +ifeq ($(CONFIG_CGROUPS),y)
> +obj-$(CONFIG_BPF_SYSCALL) += cgroup_iter.o
> +endif
>   obj-$(CONFIG_CGROUP_BPF) += cgroup.o
>   ifeq ($(CONFIG_INET),y)
>   obj-$(CONFIG_BPF_SYSCALL) += reuseport_array.o
> diff --git a/kernel/bpf/cgroup_iter.c b/kernel/bpf/cgroup_iter.c
> new file mode 100644
> index 000000000000..1027faed0b8b
> --- /dev/null
> +++ b/kernel/bpf/cgroup_iter.c
> @@ -0,0 +1,252 @@
> +// SPDX-License-Identifier: GPL-2.0-only
> +/* Copyright (c) 2022 Google */
> +#include <linux/bpf.h>
> +#include <linux/btf_ids.h>
> +#include <linux/cgroup.h>
> +#include <linux/kernel.h>
> +#include <linux/seq_file.h>
> +
> +#include "../cgroup/cgroup-internal.h"  /* cgroup_mutex and cgroup_is_dead */
> +
> +/* cgroup_iter provides three modes of traversal to the cgroup hierarchy.
> + *
> + *  1. Walk the descendants of a cgroup in pre-order.
> + *  2. Walk the descendants of a cgroup in post-order.
> + *  2. Walk the ancestors of a cgroup.
> + *
> + * For walking descendants, cgroup_iter can walk in either pre-order or
> + * post-order. For walking ancestors, the iter walks up from a cgroup to
> + * the root.
> + *
> + * The iter program can terminate the walk early by returning 1. Walk
> + * continues if prog returns 0.
> + *
> + * The prog can check (seq->num == 0) to determine whether this is
> + * the first element. The prog may also be passed a NULL cgroup,
> + * which means the walk has completed and the prog has a chance to
> + * do post-processing, such as outputing an epilogue.
> + *
> + * Note: the iter_prog is called with cgroup_mutex held.
> + *
> + * Currently only one session is supported, which means, depending on the
> + * volume of data bpf program intends to send to user space, the number
> + * of cgroups that can be walked is limited. For example, given the current
> + * buffer size is 8 * PAGE_SIZE, if the program sends 64B data for each
> + * cgroup, the total number of cgroups that can be walked is 512. This is

Again, let us specify PAGE_SIZE = 4KB here.

> + * a limitation of cgroup_iter. If the output data is larger than the
> + * buffer size, the second read() will signal EOPNOTSUPP. In order to work
> + * around, the user may have to update their program to reduce the volume
> + * of data sent to output. For example, skip some uninteresting cgroups.
> + */
> +
> +struct bpf_iter__cgroup {
> +	__bpf_md_ptr(struct bpf_iter_meta *, meta);
> +	__bpf_md_ptr(struct cgroup *, cgroup);
> +};
> +
> +struct cgroup_iter_priv {
> +	struct cgroup_subsys_state *start_css;
> +	bool terminate;
> +	int order;
> +};
> +
> +static void *cgroup_iter_seq_start(struct seq_file *seq, loff_t *pos)
> +{
> +	struct cgroup_iter_priv *p = seq->private;
> +
> +	mutex_lock(&cgroup_mutex);
> +
> +	/* cgroup_iter doesn't support read across multiple sessions. */
> +	if (*pos > 0)
> +		return ERR_PTR(-EOPNOTSUPP);

This is not quite right. Let us say, the number of cgroups is 1,
after bpf program run, pos = 1, and the control return to user
space. Now the second read() will return -EOPNOTSUPP which is not
right. -EOPNOTSUPP should be returned ONLY if the previous cgroup
iterations do not traverse all cgroups.
So you might need to record additional information in cgroup_iter_priv
to record such information.

> +
> +	++*pos;
> +	p->terminate = false;
> +	if (p->order == BPF_ITER_CGROUP_PRE)
> +		return css_next_descendant_pre(NULL, p->start_css);
> +	else if (p->order == BPF_ITER_CGROUP_POST)
> +		return css_next_descendant_post(NULL, p->start_css);
> +	else /* BPF_ITER_CGROUP_PARENT_UP */
> +		return p->start_css;
> +}
> +
> +static int __cgroup_iter_seq_show(struct seq_file *seq,
> +				  struct cgroup_subsys_state *css, int in_stop);
> +
> +static void cgroup_iter_seq_stop(struct seq_file *seq, void *v)
> +{
> +	/* pass NULL to the prog for post-processing */
> +	if (!v)
> +		__cgroup_iter_seq_show(seq, NULL, true);
> +	mutex_unlock(&cgroup_mutex);
> +}
> +
> +static void *cgroup_iter_seq_next(struct seq_file *seq, void *v, loff_t *pos)
> +{
> +	struct cgroup_subsys_state *curr = (struct cgroup_subsys_state *)v;
> +	struct cgroup_iter_priv *p = seq->private;
> +
> +	++*pos;
> +	if (p->terminate)
> +		return NULL;
> +
> +	if (p->order == BPF_ITER_CGROUP_PRE)
> +		return css_next_descendant_pre(curr, p->start_css);
> +	else if (p->order == BPF_ITER_CGROUP_POST)
> +		return css_next_descendant_post(curr, p->start_css);
> +	else
> +		return curr->parent;
> +}
> +
[...]