Re: [PATCH bpf-next v3 4/8] bpf: Introduce cgroup iter

Yonghong Song
2022-07-11T00:19:50Z

On 7/8/22 5:04 PM, 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.
>
> 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 | 21 ++
> kernel/bpf/Makefile | 3 +
> kernel/bpf/cgroup_iter.c | 242 ++++++++++++++++++
> tools/include/uapi/linux/bpf.h | 21 ++
> .../selftests/bpf/prog_tests/btf_dump.c | 4 +-
> 6 files changed, 297 insertions(+), 2 deletions(-)
> create mode 100644 kernel/bpf/cgroup_iter.c
>
> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> index 2b21f2a3452ff..5de9de06e2551 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;
> @@ -1714,7 +1715,14 @@ 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 379e68fb866fc..6f5979e221927 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.

Adding comment that cgroup_fd 0 means starting from root cgroup?
Also, if I understand correctly, cgroup v1 is also supported here,
right? If this is the case, for cgroup v1 which root cgroup will be
used for cgroup_fd? It would be good to clarify here too.

> + */
> + __u32 cgroup_fd;
> + __u32 traversal_order;
> + } cgroup;
> };
>
> /* BPF syscall commands, see bpf(2) man-page for more details. */
> @@ -6134,6 +6151,10 @@ struct bpf_link_info {
> struct {
> __u32 map_id;
> } map;
> + struct {
> + __u32 traversal_order;
> + __aligned_u64 cgroup_id;
> + } cgroup;

We actually has a problem here although I don't have a solution yet.

Without this patch, for bpf_link_info structure, the output of pahole,

struct {

__u64 target_name
__attribute__((__aligned__(8))); /* 0 8 */

__u32 target_name_len; /*
8 4 */

union {

struct {

__u32 map_id; /*
12 4 */

} map; /*
12 4 */
}; /*
12 4 */
union {
struct {
__u32 map_id;
/* 0 4 */
} map;
/* 0 4 */
};

} iter;

You can see map_id has the offset 12 from the beginning of 'iter' structure.

With this patch,

struct {
__u64 target_name
__attribute__((__aligned__(8))); /* 0 8 */
__u32 target_name_len; /*
8 4 */

/* XXX 4 bytes hole, try to pack */

union {
struct {
__u32 map_id; /*
16 4 */
} map; /*
16 4 */
struct {
__u32 traversal_order; /*
16 4 */

/* XXX 4 bytes hole, try to pack */

__u64 cgroup_id; /*
24 8 */
} cgroup; /*
16 16 */
}; /*
16 16 */
union {
struct {
__u32 map_id;
/* 0 4 */
} map;
/* 0 4 */
struct {
__u32 traversal_order;
/* 0 4 */

/* XXX 4 bytes hole, try to pack */

__u64 cgroup_id;
/* 8 8 */
} cgroup;
/* 0 16 */
};

} iter;

There is a 4 byte hole after member 'target_name_len'. So map_id will
have a offset 16 from the start of structure 'iter'.

This will break uapi. We probably won't be able to change the existing
uapi with adding a ':32' after member 'target_name_len'. I don't have
a good solution yet, but any suggestion is welcome.

Also, for '__aligned_u64 cgroup_id', '__u64 cgroup_id' is enough.
'__aligned_u64' mostly used for pointers.

> };
> } iter;
> struct {
> diff --git a/kernel/bpf/Makefile b/kernel/bpf/Makefile
> index 057ba8e01e70f..00e05b69a4df1 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 0000000000000..8f50b326016e6
> --- /dev/null
> +++ b/kernel/bpf/cgroup_iter.c
> @@ -0,0 +1,242 @@
> +// 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.
> + */
> +
> +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;

This might be okay. But want to check what is
the practical upper limit for cgroups in a system
and whether we may miss some cgroups. If this
happens, it will be a surprise to the user.

> +
> + ++*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);
> +}
> + …