Re: [PATCH bpf-next v1 8/9] bpf: Introduce cgroup iter

[Yonghong Song <yhs@fb.com>]

On 3/2/22 2:45 PM, Kumar Kartikeya Dwivedi wrote:
> On Sat, Feb 26, 2022 at 05:13:38AM IST, Hao Luo wrote:
>> Introduce a new type of iter prog: cgroup. Unlike other bpf_iter, this
>> iter doesn't iterate a set of kernel objects. Instead, it is supposed to
>> be parameterized by a cgroup id and prints only that cgroup. So one
>> needs to specify a target cgroup id when attaching this iter.
>>
>> The target cgroup's state can be read out via a link of this iter.
>> Typically, we can monitor cgroup creation and deletion using sleepable
>> tracing and use it to create corresponding directories in bpffs and pin
>> a cgroup id parameterized link in the directory. Then we can read the
>> auto-pinned iter link to get cgroup's state. The output of the iter link
>> is determined by the program. See the selftest test_cgroup_stats.c for
>> an example.
>>
>> Signed-off-by: Hao Luo <haoluo@google.com>
>> ---
>>   include/linux/bpf.h            |   1 +
>>   include/uapi/linux/bpf.h       |   6 ++
>>   kernel/bpf/Makefile            |   2 +-
>>   kernel/bpf/cgroup_iter.c       | 141 +++++++++++++++++++++++++++++++++
>>   tools/include/uapi/linux/bpf.h |   6 ++
>>   5 files changed, 155 insertions(+), 1 deletion(-)
>>   create mode 100644 kernel/bpf/cgroup_iter.c
>>
>> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
>> index 759ade7b24b3..3ce9b0b7ed89 100644
>> --- a/include/linux/bpf.h
>> +++ b/include/linux/bpf.h
>> @@ -1595,6 +1595,7 @@ int bpf_obj_get_path(bpfptr_t pathname, int flags);
>>
>>   struct bpf_iter_aux_info {
>>   	struct bpf_map *map;
>> +	u64 cgroup_id;
>>   };
>>
>>   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 a5dbc794403d..855ad80d9983 100644
>> --- a/include/uapi/linux/bpf.h
>> +++ b/include/uapi/linux/bpf.h
>> @@ -91,6 +91,9 @@ union bpf_iter_link_info {
>>   	struct {
>>   		__u32	map_fd;
>>   	} map;
>> +	struct {
>> +		__u64	cgroup_id;
>> +	} cgroup;
>>   };
>>
>>   /* BPF syscall commands, see bpf(2) man-page for more details. */
>> @@ -5887,6 +5890,9 @@ struct bpf_link_info {
>>   				struct {
>>   					__u32 map_id;
>>   				} map;
>> +				struct {
>> +					__u64 cgroup_id;
>> +				} cgroup;
>>   			};
>>   		} iter;
>>   		struct  {
>> diff --git a/kernel/bpf/Makefile b/kernel/bpf/Makefile
>> index c1a9be6a4b9f..52a0e4c6e96e 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
>>   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 000000000000..011d9dcd1d51
>> --- /dev/null
>> +++ b/kernel/bpf/cgroup_iter.c
>> @@ -0,0 +1,141 @@
>> +// 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>
>> +
>> +struct bpf_iter__cgroup {
>> +	__bpf_md_ptr(struct bpf_iter_meta *, meta);
>> +	__bpf_md_ptr(struct cgroup *, cgroup);
>> +};
>> +
>> +static void *cgroup_iter_seq_start(struct seq_file *seq, loff_t *pos)
>> +{
>> +	struct cgroup *cgroup;
>> +	u64 cgroup_id;
>> +
>> +	/* Only one session is supported. */
>> +	if (*pos > 0)
>> +		return NULL;
>> +
>> +	cgroup_id = *(u64 *)seq->private;
>> +	cgroup = cgroup_get_from_id(cgroup_id);
>> +	if (!cgroup)
>> +		return NULL;
>> +
>> +	if (*pos == 0)
>> +		++*pos;
>> +
>> +	return cgroup;
>> +}
>> +
>> +static void *cgroup_iter_seq_next(struct seq_file *seq, void *v, loff_t *pos)
>> +{
>> +	++*pos;
>> +	return NULL;
>> +}
>> +
>> +static int cgroup_iter_seq_show(struct seq_file *seq, void *v)
>> +{
>> +	struct bpf_iter__cgroup ctx;
>> +	struct bpf_iter_meta meta;
>> +	struct bpf_prog *prog;
>> +	int ret = 0;
>> +
>> +	ctx.meta = &meta;
>> +	ctx.cgroup = v;
>> +	meta.seq = seq;
>> +	prog = bpf_iter_get_info(&meta, false);
>> +	if (prog)
>> +		ret = bpf_iter_run_prog(prog, &ctx);
>> +
>> +	return ret;
>> +}
>> +
>> +static void cgroup_iter_seq_stop(struct seq_file *seq, void *v)
>> +{
>> +	if (v)
>> +		cgroup_put(v);
>> +}
> 
> I think in existing iterators, we make a final call to seq_show, with v as NULL,
> is there a specific reason to do it differently for this? There is logic in
> bpf_iter.c to trigger ->stop() callback again when ->start() or ->next() returns
> NULL, to execute BPF program with NULL p, see the comment above stop label.
> 
> If you do add the seq_show call with NULL, you'd also need to change the
> ctx_arg_info PTR_TO_BTF_ID to PTR_TO_BTF_ID_OR_NULL.

Kumar, PTR_TO_BTF_ID should be okay since the show() never takes a 
non-NULL cgroup. But we do have issues for cgroup_iter_seq_stop() which 
I missed earlier.

For cgroup_iter, the following is the current workflow:
    start -> not NULL -> show -> next -> NULL -> stop
or
    start -> NULL -> stop

So for cgroup_iter_seq_stop, the input parameter 'v' will be NULL, so
the cgroup_put() is not actually called, i.e., corresponding cgroup is
not freed.

There are two ways to fix the issue:
   . call cgroup_put() in next() before return NULL. This way,
     stop() will be a noop.
   . put cgroup_get_from_id() and cgroup_put() in
     bpf_iter_attach_cgroup() and bpf_iter_detach_cgroup().

I prefer the second approach as it is cleaner.

> 
>> +
>> +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_data, struct bpf_iter_aux_info *aux)
>> +{
>> +	*(u64 *)priv_data = aux->cgroup_id;
>> +	return 0;
>> +}
>> +
>> +static void cgroup_iter_seq_fini(void *priv_data)
>> +{
>> +}
>> +
>> +static const struct bpf_iter_seq_info cgroup_iter_seq_info = {
>> +	.seq_ops                = &cgroup_iter_seq_ops,
>> +	.init_seq_private       = cgroup_iter_seq_init,
>> +	.fini_seq_private       = cgroup_iter_seq_fini,
>> +	.seq_priv_size          = sizeof(u64),
>> +};
>> +
>> +static int bpf_iter_attach_cgroup(struct bpf_prog *prog,
>> +				  union bpf_iter_link_info *linfo,
>> +				  struct bpf_iter_aux_info *aux)
>> +{
>> +	aux->cgroup_id = linfo->cgroup.cgroup_id;
>> +	return 0;
>> +}
>> +
>> +static void bpf_iter_detach_cgroup(struct bpf_iter_aux_info *aux)
>> +{
>> +}
>> +
>> +void bpf_iter_cgroup_show_fdinfo(const struct bpf_iter_aux_info *aux,
>> +				 struct seq_file *seq)
>> +{
>> +	char buf[64] = {0};
>> +
>> +	cgroup_path_from_kernfs_id(aux->cgroup_id, buf, sizeof(buf));
>> +	seq_printf(seq, "cgroup_id:\t%lu\n", aux->cgroup_id);
>> +	seq_printf(seq, "cgroup_path:\t%s\n", buf);
>> +}
>> +
>> +int bpf_iter_cgroup_fill_link_info(const struct bpf_iter_aux_info *aux,
>> +				   struct bpf_link_info *info)
>> +{
>> +	info->iter.cgroup.cgroup_id = aux->cgroup_id;
>> +	return 0;
>> +}
>> +
>> +DEFINE_BPF_ITER_FUNC(cgroup, struct bpf_iter_meta *meta,
>> +		     struct cgroup *cgroup)
>> +
>> +static struct bpf_iter_reg bpf_cgroup_reg_info = {
>> +	.target			= "cgroup",
>> +	.attach_target		= bpf_iter_attach_cgroup,
>> +	.detach_target		= bpf_iter_detach_cgroup,
>> +	.show_fdinfo		= bpf_iter_cgroup_show_fdinfo,
>> +	.fill_link_info		= bpf_iter_cgroup_fill_link_info,
>> +	.ctx_arg_info_size	= 1,
>> +	.ctx_arg_info		= {
>> +		{ offsetof(struct bpf_iter__cgroup, cgroup),
>> +		  PTR_TO_BTF_ID },
>> +	},
>> +	.seq_info		= &cgroup_iter_seq_info,
>> +};
>> +
>> +static int __init bpf_cgroup_iter_init(void)
>> +{
>> +	bpf_cgroup_reg_info.ctx_arg_info[0].btf_id = bpf_cgroup_btf_id[0];
>> +	return bpf_iter_reg_target(&bpf_cgroup_reg_info);
>> +}
>> +
>> +late_initcall(bpf_cgroup_iter_init);
>> diff --git a/tools/include/uapi/linux/bpf.h b/tools/include/uapi/linux/bpf.h
>> index a5dbc794403d..855ad80d9983 100644
>> --- a/tools/include/uapi/linux/bpf.h
>> +++ b/tools/include/uapi/linux/bpf.h
>> @@ -91,6 +91,9 @@ union bpf_iter_link_info {
>>   	struct {
>>   		__u32	map_fd;
>>   	} map;
>> +	struct {
>> +		__u64	cgroup_id;
>> +	} cgroup;
>>   };
>>
>>   /* BPF syscall commands, see bpf(2) man-page for more details. */
>> @@ -5887,6 +5890,9 @@ struct bpf_link_info {
>>   				struct {
>>   					__u32 map_id;
>>   				} map;
>> +				struct {
>> +					__u64 cgroup_id;
>> +				} cgroup;
>>   			};
>>   		} iter;
>>   		struct  {
>> --
>> 2.35.1.574.g5d30c73bfb-goog
>>
> 
> --
> Kartikeya