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Message-ID: <a6d048b8-d017-ea7e-36f0-1c4f88fc4399@fb.com>
Date: Mon, 11 Jul 2022 16:20:08 -0700
From: Yonghong Song <yhs@...com>
To: Yosry Ahmed <yosryahmed@...gle.com>,
Alexei Starovoitov <ast@...nel.org>,
Daniel Borkmann <daniel@...earbox.net>,
Andrii Nakryiko <andrii@...nel.org>,
Martin KaFai Lau <kafai@...com>,
Song Liu <songliubraving@...com>, Hao Luo <haoluo@...gle.com>,
Tejun Heo <tj@...nel.org>, Zefan Li <lizefan.x@...edance.com>
Cc: Johannes Weiner <hannes@...xchg.org>,
Shuah Khan <shuah@...nel.org>,
Michal Hocko <mhocko@...nel.org>,
KP Singh <kpsingh@...nel.org>,
Benjamin Tissoires <benjamin.tissoires@...hat.com>,
John Fastabend <john.fastabend@...il.com>,
Michal Koutný <mkoutny@...e.com>,
Roman Gushchin <roman.gushchin@...ux.dev>,
David Rientjes <rientjes@...gle.com>,
Stanislav Fomichev <sdf@...gle.com>,
Greg Thelen <gthelen@...gle.com>,
Shakeel Butt <shakeelb@...gle.com>,
linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
bpf@...r.kernel.org, cgroups@...r.kernel.org
Subject: Re: [PATCH bpf-next v3 4/8] bpf: Introduce cgroup iter
On 7/10/22 5:19 PM, Yonghong Song wrote:
>
>
> On 7/8/22 5:04 PM, Yosry Ahmed wrote:
>> From: Hao Luo <haoluo@...gle.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@...gle.com>
>> Signed-off-by: Yosry Ahmed <yosryahmed@...gle.com>
>> Acked-by: Yonghong Song <yhs@...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.
Briefly discussed with Alexei, the following structure iter definition
should work. Later on, if we need to addition fields for other iter's,
for a single __u32, the field can be added to either the first or the
second union. If fields are more than __u32, they can be placed
in the second union.
struct {
__aligned_u64 target_name; /* in/out:
target_name buffer ptr */
__u32 target_name_len; /* in/out:
target_name buffer len */
union {
struct {
__u32 map_id;
} map;
};
union {
struct {
__u64 cgroup_id;
__u32 traversal_order;
} cgroup;
};
} iter;
>
>
>> };
>> } 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);
>> +}
>> +
> [...]
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