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SubjectRe: [PATCH bpf-next v5 4/8] bpf: Introduce cgroup iter
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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;
> +}
> +
[...]

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