Re: [PATCH bpf-next v2 4/8] bpf: Introduce cgroup iter

[Yosry Ahmed <yosryahmed@google.com>]

On Mon, Jun 27, 2022 at 9:09 PM Yonghong Song <yhs@fb.com> wrote:
>
>
>
> On 6/10/22 12:44 PM, Yosry Ahmed wrote:
> > From: Hao Luo <haoluo@google.com>
> >
> > Cgroup_iter is a type of bpf_iter. It walks over cgroups in two modes:
> >
> >   - walking a cgroup's descendants.
> >   - walking a cgroup's ancestors.
>
> The implementation has another choice, BPF_ITER_CGROUP_PARENT_UP.
> We should add it here as well.
>

BPF_ITER_CGROUP_PARENT_UP is expressed here, I think what's actually
missing here (and down below where only 2 modes are specified again)
is that walking descendants is broken down into two separate modes,
pre and post order traversals.

> >
> > 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.
>
> Overall looks good to me with a few nits below.
>
> Acked-by: Yonghong Song <yhs@fb.com>
>
> >
> > Signed-off-by: Hao Luo <haoluo@google.com>
> > Signed-off-by: Yosry Ahmed <yosryahmed@google.com>
> > ---
> >   include/linux/bpf.h            |   8 ++
> >   include/uapi/linux/bpf.h       |  21 +++
> >   kernel/bpf/Makefile            |   2 +-
> >   kernel/bpf/cgroup_iter.c       | 235 +++++++++++++++++++++++++++++++++
> >   tools/include/uapi/linux/bpf.h |  21 +++
> >   5 files changed, 286 insertions(+), 1 deletion(-)
> >   create mode 100644 kernel/bpf/cgroup_iter.c
> >
> > diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> > index 8e6092d0ea956..48d8e836b9748 100644
> > --- a/include/linux/bpf.h
> > +++ b/include/linux/bpf.h
> > @@ -44,6 +44,7 @@ struct kobject;
> >   struct mem_cgroup;
> >   struct module;
> >   struct bpf_func_state;
> > +struct cgroup;
> >
> >   extern struct idr btf_idr;
> >   extern spinlock_t btf_idr_lock;
> > @@ -1590,7 +1591,14 @@ 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 f4009dbdf62da..4fd05cde19116 100644
> > --- a/include/uapi/linux/bpf.h
> > +++ b/include/uapi/linux/bpf.h
> > @@ -87,10 +87,27 @@ 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 for walking the
> > +              * descendants; this is the starting cgroup if for walking the ancestors.
> > +              */
> > +             __u32   cgroup_fd;
> > +             __u32   traversal_order;
> > +     } cgroup;
> >   };
> >
> >   /* BPF syscall commands, see bpf(2) man-page for more details. */
> > @@ -6050,6 +6067,10 @@ struct bpf_link_info {
> >                               struct {
> >                                       __u32 map_id;
> >                               } map;
> > +                             struct {
> > +                                     __u32 traversal_order;
> > +                                     __aligned_u64 cgroup_id;
> > +                             } cgroup;
> >                       };
> >               } iter;
> >               struct  {
> > diff --git a/kernel/bpf/Makefile b/kernel/bpf/Makefile
> > index 057ba8e01e70f..9741b9314fb46 100644
> > --- a/kernel/bpf/Makefile
> > +++ b/kernel/bpf/Makefile
> > @@ -8,7 +8,7 @@ CFLAGS_core.o += $(call cc-disable-warning, override-init) $(cflags-nogcse-yy)
> >
> >   obj-$(CONFIG_BPF_SYSCALL) += syscall.o verifier.o inode.o helpers.o tnum.o bpf_iter.o map_iter.o task_iter.o prog_iter.o link_iter.o
> >   obj-$(CONFIG_BPF_SYSCALL) += hashtab.o arraymap.o percpu_freelist.o bpf_lru_list.o lpm_trie.o map_in_map.o bloom_filter.o
> > -obj-$(CONFIG_BPF_SYSCALL) += local_storage.o queue_stack_maps.o ringbuf.o
> > +obj-$(CONFIG_BPF_SYSCALL) += local_storage.o queue_stack_maps.o ringbuf.o cgroup_iter.o
> >   obj-$(CONFIG_BPF_SYSCALL) += bpf_local_storage.o bpf_task_storage.o
> >   obj-${CONFIG_BPF_LSM}         += bpf_inode_storage.o
> >   obj-$(CONFIG_BPF_SYSCALL) += disasm.o
> > diff --git a/kernel/bpf/cgroup_iter.c b/kernel/bpf/cgroup_iter.c
> > new file mode 100644
> > index 0000000000000..88deb655efa71
> > --- /dev/null
> > +++ b/kernel/bpf/cgroup_iter.c
> > @@ -0,0 +1,235 @@
> > +// 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 two modes of traversal to the cgroup hierarchy.
> > + *
> > + *  1. Walk the descendants of a cgroup.
> > + *  2. Walk the ancestors of a cgroup.
>
> three modes here?
>
> > + *
> > + * 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.
> > + */
> > +
> > +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);
> > +
> > +     /* support only one session */
> > +     if (*pos > 0)
> > +             return NULL;
> > +
> > +     ++*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;
> > +}
> > +
> > +static int __cgroup_iter_seq_show(struct seq_file *seq,
> > +                               struct cgroup_subsys_state *css, int in_stop)
> > +{
> > +     struct cgroup_iter_priv *p = seq->private;
> > +     struct bpf_iter__cgroup ctx;
> > +     struct bpf_iter_meta meta;
> > +     struct bpf_prog *prog;
> > +     int ret = 0;
> > +
> > +     /* cgroup is dead, skip this element */
> > +     if (css && cgroup_is_dead(css->cgroup))
> > +             return 0;
> > +
> > +     ctx.meta = &meta;
> > +     ctx.cgroup = css ? css->cgroup : NULL;
> > +     meta.seq = seq;
> > +     prog = bpf_iter_get_info(&meta, in_stop);
> > +     if (prog)
> > +             ret = bpf_iter_run_prog(prog, &ctx);
> > +
> > +     /* if prog returns > 0, terminate after this element. */
> > +     if (ret != 0)
> > +             p->terminate = true;
> > +
> > +     return 0;
> > +}
> > +
> > +static int cgroup_iter_seq_show(struct seq_file *seq, void *v)
> > +{
> > +     return __cgroup_iter_seq_show(seq, (struct cgroup_subsys_state *)v,
> > +                                   false);
> > +}
> > +
> > +static const struct seq_operations cgroup_iter_seq_ops = {
> > +     .start  = cgroup_iter_seq_start,
> > +     .next   = cgroup_iter_seq_next,
> > +     .stop   = cgroup_iter_seq_stop,
> > +     .show   = cgroup_iter_seq_show,
> > +};
> > +
> > +BTF_ID_LIST_SINGLE(bpf_cgroup_btf_id, struct, cgroup)
> > +
> > +static int cgroup_iter_seq_init(void *priv, struct bpf_iter_aux_info *aux)
> > +{
> > +     struct cgroup_iter_priv *p = (struct cgroup_iter_priv *)priv;
> > +     struct cgroup *cgrp = aux->cgroup.start;
> > +
> > +     p->start_css = &cgrp->self;
> > +     p->terminate = false;
> > +     p->order = aux->cgroup.order;
> > +     return 0;
> > +}
> > +
> > +static const struct bpf_iter_seq_info cgroup_iter_seq_info = {
> > +     .seq_ops                = &cgroup_iter_seq_ops,
> > +     .init_seq_private       = cgroup_iter_seq_init,
> > +     .seq_priv_size          = sizeof(struct cgroup_iter_priv),
> > +};
> > +
> > +static int bpf_iter_attach_cgroup(struct bpf_prog *prog,
> > +                               union bpf_iter_link_info *linfo,
> > +                               struct bpf_iter_aux_info *aux)
> > +{
> > +     int fd = linfo->cgroup.cgroup_fd;
> > +     struct cgroup *cgrp;
> > +
> > +     if (fd)
> > +             cgrp = cgroup_get_from_fd(fd);
> > +     else /* walk the entire hierarchy by default. */
> > +             cgrp = cgroup_get_from_path("/");
> > +
> > +     if (IS_ERR(cgrp))
> > +             return PTR_ERR(cgrp);
> > +
> > +     aux->cgroup.start = cgrp;
> > +     aux->cgroup.order = linfo->cgroup.traversal_order;
>
> The legality of traversal_order should be checked.
>
> > +     return 0;
> > +}
> > +
> > +static void bpf_iter_detach_cgroup(struct bpf_iter_aux_info *aux)
> > +{
> > +     cgroup_put(aux->cgroup.start);
> > +}
> > +
> > +static void bpf_iter_cgroup_show_fdinfo(const struct bpf_iter_aux_info *aux,
> > +                                     struct seq_file *seq)
> > +{
> > +     char *buf;
> > +
> > +     buf = kzalloc(PATH_MAX, GFP_KERNEL);
> > +     if (!buf) {
> > +             seq_puts(seq, "cgroup_path:\n");
>
> This is a really unlikely case. maybe "cgroup_path:<unknown>"?
>
> > +             goto show_order;
> > +     }
> > +
> > +     /* If cgroup_path_ns() fails, buf will be an empty string, cgroup_path
> > +      * will print nothing.
> > +      *
> > +      * Path is in the calling process's cgroup namespace.
> > +      */
> > +     cgroup_path_ns(aux->cgroup.start, buf, PATH_MAX,
> > +                    current->nsproxy->cgroup_ns);
> > +     seq_printf(seq, "cgroup_path:\t%s\n", buf);
> > +     kfree(buf);
> > +
> > +show_order:
> > +     if (aux->cgroup.order == BPF_ITER_CGROUP_PRE)
> > +             seq_puts(seq, "traversal_order: pre\n");
> > +     else if (aux->cgroup.order == BPF_ITER_CGROUP_POST)
> > +             seq_puts(seq, "traversal_order: post\n");
> > +     else /* BPF_ITER_CGROUP_PARENT_UP */
> > +             seq_puts(seq, "traversal_order: parent_up\n");
> > +}
> > +
> [...]