Re: [PATCH bpf-next v2 11/12] tools: libbpf: allow map reuse

[Jakub Kicinski <jakub.kicinski@netronome.com>]

On Mon, 9 Jul 2018 13:22:54 -0700, Andrey Ignatov wrote:
> Jakub Kicinski <jakub.kicinski@netronome.com> [Mon, 2018-07-09 11:01 -0700]:
> > More advanced applications may want to only replace programs without
> > destroying associated maps.  Allow libbpf users to achieve that.
> > Instead of always creating all of the maps at load time, expose to
> > users an API to reconstruct the map object from already existing
> > map.
> > 
> > The map parameters are read from the kernel and replace the parameters
> > of the ELF map.  libbpf does not restrict the map replacement, i.e.
> > the reused map does not have to be compatible with the ELF map
> > definition.  We relay on the verifier for checking the compatibility
> > between maps and programs.  The ELF map definition is completely
> > overwritten by the information read from the kernel, to make sure
> > libbpf's view of map object corresponds to the actual map.  
> 
> Thanks for working on this Jakub! I encountered this shortcoming of
> libbpf as well and was planning to fix it, but you beat me to it :)

Ah!  I wish I didn't! :)

> > Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
> > Reviewed-by: Quentin Monnet <quentin.monnet@netronome.com>
> > ---
> >  tools/lib/bpf/libbpf.c | 35 +++++++++++++++++++++++++++++++++++
> >  tools/lib/bpf/libbpf.h |  1 +
> >  2 files changed, 36 insertions(+)
> > 
> > diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> > index b653dbb266c7..c80033fe66c3 100644
> > --- a/tools/lib/bpf/libbpf.c
> > +++ b/tools/lib/bpf/libbpf.c
> > @@ -215,6 +215,7 @@ struct bpf_map {
> >  	int fd;
> >  	char *name;
> >  	size_t offset;
> > +	bool fd_preset;  
> 
> Any reason not to use map->fd itself to identify if fd is present?

Note: pre-set, not present.

> fd of every map is set to -1 in bpf_object__init_maps() that, in turn, is
> called from __bpf_object__open():
> 
> 	for (i = 0; i < nr_maps; i++)
> 		obj->maps[i].fd = -1;
> 
> Later it will either contain valid fd that is >= 0, or that same -1, what
> should be enough to identify fd presence.

I thought it to be cleaner to indicate the fd has been pre-set, in case
things get more complicated in the future and fd >= 0 becomes ambiguous.

But no strong preference, should I change?

> >  	int map_ifindex;
> >  	struct bpf_map_def def;
> >  	uint32_t btf_key_type_id;
> > @@ -1082,6 +1083,34 @@ static int bpf_map_find_btf_info(struct bpf_map *map, const struct btf *btf)
> >  	return 0;
> >  }
> >  
> > +int bpf_map__reuse_fd(struct bpf_map *map, int fd)
> > +{
> > +	struct bpf_map_info info = {};
> > +	__u32 len = sizeof(info);
> > +	int err;
> > +
> > +	err = bpf_obj_get_info_by_fd(fd, &info, &len);
> > +	if (err)
> > +		return err;
> > +  
> 
> Should there be a check that map->fd doesn't contain any valid fd (>= 0)
> before rewriting it so that if it does (e.g. because the function is
> called after bpf_object__load() by mistake), current map->fd won't be
> leaked?

Hm.  In my first implementation libbpf just took the passed fd and
didn't do a dup(), the lifetime of the fd remained with the caller.
Having a check will prevent changing the descriptor unless we add some
from of "un-reuse" as well.  Perhaps I should just add a close() in
case fd >= 0?  Or do you prefer a hard error?

> > +	map->fd = dup(fd);  
> 
> Unfortunately, new descriptor created by dup(2) will not have O_CLOEXEC set, in
> contrast to original fd returned by kernel on map creation.
> 
> libbpf has other interface shortcomings where it comes up. E.g. struct
> bpf_object owns all descriptors it contains (progs, maps) and closes them in
> bpf_object__close(). if one wants to open/load ELF, then close it but
> keep, say, prog fd to attach it to cgroup some time later, then fd
> should be duplicated as well to get a new one not owned by bpf_object.
> 
> Currently I use this workaround to avoid time when new fd doesn't have
> O_CLOEXEC:
> 
> 	int new_prog_fd = open("/dev/null", O_RDONLY | O_CLOEXEC);
> 	if (new_prog_fd < 0 ||
> 	    dup3(bpf_program__fd(prog), new_prog_fd, O_CLOEXEC) == -1) {
> 		/* .. handle error .. */
> 		close(new_prog_fd);
> 	}
> 	/* .. use new_prog_fd with O_CLOEXEC set */
> 
> Not sure how to simplify it. dup2() has same problem with regard to
> O_CLOEXEC.
> 
> Use-case: standalone server application that uses libbpf and does
> fork()/execve() a lot.

Good point!  I have no better ideas.  Although being slightly paranoid
I would perhaps use "/" instead of "/dev/null"?  Shouldn't matter?

> > +	if (map->fd < 0)
> > +		return map->fd;
> > +	map->fd_preset = true;
> > +
> > +	free(map->name);
> > +	map->name = strdup(info.name);
> > +	map->def.type = info.type;
> > +	map->def.key_size = info.key_size;
> > +	map->def.value_size = info.value_size;
> > +	map->def.max_entries = info.max_entries;
> > +	map->def.map_flags = info.map_flags;
> > +	map->btf_key_type_id = info.btf_key_type_id;
> > +	map->btf_value_type_id = info.btf_value_type_id;
> > +
> > +	return 0;
> > +}
> > +
> >  static int
> >  bpf_object__create_maps(struct bpf_object *obj)
> >  {
> > @@ -1094,6 +1123,12 @@ bpf_object__create_maps(struct bpf_object *obj)
> >  		struct bpf_map_def *def = &map->def;
> >  		int *pfd = &map->fd;
> >  
> > +		if (map->fd_preset) {
> > +			pr_debug("skip map create (preset) %s: fd=%d\n",
> > +				 map->name, map->fd);
> > +			continue;
> > +		}
> > +
> >  		create_attr.name = map->name;
> >  		create_attr.map_ifindex = map->map_ifindex;
> >  		create_attr.map_type = def->type;
> > diff --git a/tools/lib/bpf/libbpf.h b/tools/lib/bpf/libbpf.h
> > index 60593ac44700..8e709a74f47c 100644
> > --- a/tools/lib/bpf/libbpf.h
> > +++ b/tools/lib/bpf/libbpf.h
> > @@ -261,6 +261,7 @@ typedef void (*bpf_map_clear_priv_t)(struct bpf_map *, void *);
> >  int bpf_map__set_priv(struct bpf_map *map, void *priv,
> >  		      bpf_map_clear_priv_t clear_priv);
> >  void *bpf_map__priv(struct bpf_map *map);
> > +int bpf_map__reuse_fd(struct bpf_map *map, int fd);
> >  bool bpf_map__is_offload_neutral(struct bpf_map *map);
> >  void bpf_map__set_ifindex(struct bpf_map *map, __u32 ifindex);
> >  int bpf_map__pin(struct bpf_map *map, const char *path);
> > -- 
> > 2.17.1
> >   
>