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

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

On Wed, Apr 29, 2020 at 12:07 AM Yonghong Song <yhs@fb.com> wrote:
>
>
>
> On 4/28/20 11:56 PM, Andrii Nakryiko wrote:
> > 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?
>
> Yes. This is true. The idea is to minimize the target change.
> But maybe this is not a good goal by itself.
>
> You are right, if I intercept all seq_ops(), I do not need the
> above change, I can tailor seq_file private_data right before
> calling target one and restore after the target call.
>
> Originally I only have one interception, show(), now I have
> stop() too to call bpf at the end of iteration. Maybe I can
> interpret all four, I think. This way, I can also get ride
> of target feature.

If the main goal is to minimize target changes and make them exactly
seq_operations implementation, then one easier way to get easy access
to our own metadata in seq_file->private is to set it to point
**after** our metadata, but before target's metadata. Roughly in
pseudo code:

struct bpf_iter_seq_file_meta {} __attribute((aligned(8)));

void *meta = kmalloc(sizeof(struct bpf_iter_seq_file_meta) +
target_private_size);
seq_file->private = meta + sizeof(struct bpf_iter_seq_file_meta);


Then to recover bpf_iter_Seq_file_meta:

struct bpf_iter_seq_file_meta *meta = seq_file->private - sizeof(*meta);

/* voila! */

This doesn't have a benefit of making targets simpler, but will
require no changes to them at all. Plus less indirect calls, so less
performance penalty.

>
> >
> > 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?
> >
> > [...]
> >