Re: [PATCH bpf-next v1 07/19] bpf: create anonymous bpf iterator

Andrii Nakryiko
2020-04-29T06:56:26Z

On Mon, Apr 27, 2020 at 1:19 PM Yonghong Song <yhs@fb.com> wrote:
>
> A new bpf command BPF_ITER_CREATE is added.
>
> The anonymous bpf iterator is seq_file based.
> The seq_file private data are referenced by targets.
> The bpf_iter infrastructure allocated additional space
> at seq_file->private after the space used by targets
> to store some meta data, e.g.,
> prog: prog to run
> session_id: an unique id for each opened seq_file
> seq_num: how many times bpf programs are queried in this session
> has_last: indicate whether or not bpf_prog has been called after
> all valid objects have been processed
>
> A map between file and prog/link is established to help
> fops->release(). When fops->release() is called, just based on
> inode and file, bpf program cannot be located since target
> seq_priv_size not available. This map helps retrieve the prog
> whose reference count needs to be decremented.
>
> Signed-off-by: Yonghong Song <yhs@fb.com>
> ---
> include/linux/bpf.h | 3 +
> include/uapi/linux/bpf.h | 6 ++
> kernel/bpf/bpf_iter.c | 162 ++++++++++++++++++++++++++++++++-
> kernel/bpf/syscall.c | 27 ++++++
> tools/include/uapi/linux/bpf.h | 6 ++
> 5 files changed, 203 insertions(+), 1 deletion(-)
>
> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> index 4fc39d9b5cd0..0f0cafc65a04 100644
> --- a/include/linux/bpf.h
> +++ b/include/linux/bpf.h
> @@ -1112,6 +1112,8 @@ struct bpf_link *bpf_link_get_from_fd(u32 ufd);
> int bpf_obj_pin_user(u32 ufd, const char __user *pathname);
> int bpf_obj_get_user(const char __user *pathname, int flags);
>
> +#define BPF_DUMP_SEQ_NET_PRIVATE BIT(0)
> +
> struct bpf_iter_reg {
> const char *target;
> const char *target_func_name;
> @@ -1133,6 +1135,7 @@ int bpf_iter_run_prog(struct bpf_prog *prog, void *ctx);
> int bpf_iter_link_attach(const union bpf_attr *attr, struct bpf_prog *prog);
> int bpf_iter_link_replace(struct bpf_link *link, struct bpf_prog *old_prog,
> struct bpf_prog *new_prog);
> +int bpf_iter_new_fd(struct bpf_link *link);
>
> int bpf_percpu_hash_copy(struct bpf_map *map, void *key, void *value);
> int bpf_percpu_array_copy(struct bpf_map *map, void *key, void *value);
> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> index f39b9fec37ab..576651110d16 100644
> --- a/include/uapi/linux/bpf.h
> +++ b/include/uapi/linux/bpf.h
> @@ -113,6 +113,7 @@ enum bpf_cmd {
> BPF_MAP_DELETE_BATCH,
> BPF_LINK_CREATE,
> BPF_LINK_UPDATE,
> + BPF_ITER_CREATE,
> };
>
> enum bpf_map_type {
> @@ -590,6 +591,11 @@ union bpf_attr {
> __u32 old_prog_fd;
> } link_update;
>
> + struct { /* struct used by BPF_ITER_CREATE command */
> + __u32 link_fd;
> + __u32 flags;
> + } iter_create;
> +
> } __attribute__((aligned(8)));
>
> /* The description below is an attempt at providing documentation to eBPF
> diff --git a/kernel/bpf/bpf_iter.c b/kernel/bpf/bpf_iter.c
> index fc1ce5ee5c3f..1f4e778d1814 100644
> --- a/kernel/bpf/bpf_iter.c
> +++ b/kernel/bpf/bpf_iter.c
> @@ -2,6 +2,7 @@
> /* Copyright (c) 2020 Facebook */
>
> #include <linux/fs.h>
> +#include <linux/anon_inodes.h>
> #include <linux/filter.h>
> #include <linux/bpf.h>
>
> @@ -19,6 +20,19 @@ struct bpf_iter_link {
> struct bpf_iter_target_info *tinfo;
> };
>
> +struct extra_priv_data {
> + struct bpf_prog *prog;
> + u64 session_id;
> + u64 seq_num;
> + bool has_last;
> +};
> +
> +struct anon_file_prog_assoc {
> + struct list_head list;
> + struct file *file;
> + struct bpf_prog *prog;
> +};
> +
> static struct list_head targets;
> static struct mutex targets_mutex;
> static bool bpf_iter_inited = false;
> @@ -26,6 +40,50 @@ static bool bpf_iter_inited = false;
> /* protect bpf_iter_link.link->prog upddate */
> static struct mutex bpf_iter_mutex;
>
> +/* Since at anon seq_file release function, the prog cannot
> + * be retrieved since target seq_priv_size is not available.
> + * Keep a list of <anon_file, prog> mapping, so that
> + * at file release stage, the prog can be released properly.
> + */
> +static struct list_head anon_iter_info;
> +static struct mutex anon_iter_info_mutex;
> +
> +/* incremented on every opened seq_file */
> +static atomic64_t session_id;
> +
> +static u32 get_total_priv_dsize(u32 old_size)
> +{
> + return roundup(old_size, 8) + sizeof(struct extra_priv_data);
> +}
> +
> +static void *get_extra_priv_dptr(void *old_ptr, u32 old_size)
> +{
> + return old_ptr + roundup(old_size, 8);
> +}
> +
> +static int anon_iter_release(struct inode *inode, struct file *file)
> +{
> + struct anon_file_prog_assoc *finfo;
> +
> + mutex_lock(&anon_iter_info_mutex);
> + list_for_each_entry(finfo, &anon_iter_info, list) {
> + if (finfo->file == file) {

I'll look at this and other patches more thoroughly tomorrow with
clear head, but this iteration to find anon_file_prog_assoc is really
unfortunate.

I think the problem is that you are allowing seq_file infrastructure
to call directly into target implementation of seq_operations without
intercepting them. If you change that and put whatever extra info is
necessary into seq_file->private in front of target's private state,
then you shouldn't need this, right?

This would also make each target's logic a bit simpler because you can:
- centralize creation and initialization of bpf_iter_meta (session_id,
seq, seq_num will be set up once in this generic code);
- loff_t pos increments;
- you can extract and centralize bpf_iter_get_prog() call in show()
implementation as well.

I think with that each target's logic will be simpler and you won't
need to maintain anon_file_prog_assocs.

Are there complications I'm missing? …