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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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