[PATCH bpf-next v2 1/4] bpf: Generalize bpf_sk_storage

[KP Singh]

Thanks for your feedback! Apologies it took some time for me
to incorporate this into another revision.

On 18-Jun 23:43, Martin KaFai Lau wrote:
> On Wed, Jun 17, 2020 at 10:29:38PM +0200, KP Singh wrote:
> > From: KP Singh <kpsingh at google.com>
> > 
> > Refactor the functionality in bpf_sk_storage.c so that concept of
> > storage linked to kernel objects can be extended to other objects like
> > inode, task_struct etc.
> > 
> > bpf_sk_storage is updated to be bpf_local_storage with a union that
> > contains a pointer to the owner object. The type of the
> > bpf_local_storage can be determined using the newly added
> > bpf_local_storage_type enum.
> > 
> > Each new local storage will still be a separate map and provide its own
> > set of helpers. This allows for future object specific extensions and
> > still share a lot of the underlying implementation.
> Thanks for taking up this effort to refactor sk_local_storage.
> 
> I took a quick look.  I have some comments and would like to explore
> some thoughts.
> 
> > --- a/net/core/bpf_sk_storage.c
> > +++ b/kernel/bpf/bpf_local_storage.c
> > @@ -1,19 +1,22 @@
> >  // SPDX-License-Identifier: GPL-2.0
> >  /* Copyright (c) 2019 Facebook  */
> > +#include "linux/bpf.h"
> > +#include "asm-generic/bug.h"
> > +#include "linux/err.h"
> "<" ">"
> 
> >  #include <linux/rculist.h>
> >  #include <linux/list.h>
> >  #include <linux/hash.h>
> >  #include <linux/types.h>
> >  #include <linux/spinlock.h>
> >  #include <linux/bpf.h>
> > -#include <net/bpf_sk_storage.h>
> > +#include <linux/bpf_local_storage.h>
> >  #include <net/sock.h>
> >  #include <uapi/linux/sock_diag.h>
> >  #include <uapi/linux/btf.h>
> >  
> >  static atomic_t cache_idx;
> inode local storage and sk local storage probably need a separate
> cache_idx.  An improvement on picking cache_idx has just been
> landed also.

I see, thanks! I rebased and I now see that cache_idx is now a:

  static u64 cache_idx_usage_counts[BPF_STORAGE_CACHE_SIZE];

which tracks the free cache slots rather than using a single atomic
cache_idx. I guess all types of local storage can share this now
right?

> 
> [ ... ]
> 
> > +struct bpf_local_storage {
> > +	struct bpf_local_storage_data __rcu *cache[BPF_STORAGE_CACHE_SIZE];
> >  	return NULL;

[...]

> >  }
> >  
> > -/* sk_storage->lock must be held and selem->sk_storage == sk_storage.
> > +static void __unlink_local_storage(struct bpf_local_storage *local_storage,
> > +				   bool uncharge_omem)
> Nit. indent is off.  There are a few more cases like this.

Thanks, will fix this. (note to self: don't trust the editor's
clang-format blindly).

> 
> > +{
> > +	struct sock *sk;
> > +
> > +	switch (local_storage->stype) {
> Does it need a new bpf_local_storage_type?  Is map_type as good?
> 
> Instead of adding any new member (e.g. stype) to
> "struct bpf_local_storage",  can the smap pointer be directly used
> here instead?
> 
> For example in __unlink_local_storage() here, it should
> have a hold to the selem which then has a hold to smap.

Good point, Updated to using the map->map_type.

> 
> > +	case BPF_LOCAL_STORAGE_SK:
> > +		sk = local_storage->sk;
> > +		if (uncharge_omem)
> > +			atomic_sub(sizeof(struct bpf_local_storage),
> > +				   &sk->sk_omem_alloc);
> > +
> > +		/* After this RCU_INIT, sk may be freed and cannot be used */
> > +		RCU_INIT_POINTER(sk->sk_bpf_storage, NULL);
> > +		local_storage->sk = NULL;
> > +		break;
> > +	}
> Another thought on the stype switch cases.
> 
> Instead of having multiple switches on stype in bpf_local_storage.c which may
> not be scalable soon if we are planning to support a few more kernel objects,
> have you considered putting them into its own "ops".  May be a few new
> ops can be added to bpf_map_ops to do local storage unlink/update/alloc...etc.

Good idea, I was able to refactor this with the following ops:

        /* Functions called by bpf_local_storage maps */
	void (*map_local_storage_unlink)(struct bpf_local_storage *local_storage,
                                         bool uncharge_omem);
	struct bpf_local_storage_elem *(*map_selem_alloc)(
		struct bpf_local_storage_map *smap, void *owner, void *value,
		bool charge_omem);
	struct bpf_local_storage_data *(*map_local_storage_update)(
		void  *owner, struct bpf_map *map, void *value, u64 flags);
	int (*map_local_storage_alloc)(void *owner,
				       struct bpf_local_storage_map *smap,
				       struct bpf_local_storage_elem *elem);

Let me know if you have any particular thoughts/suggestions about
this.

> 
> > +}
> > +
> > +/* local_storage->lock must be held and selem->local_storage == local_storage.
> >   * The caller must ensure selem->smap is still valid to be
> >   * dereferenced for its smap->elem_size and smap->cache_idx.
> > + *
> > + * uncharge_omem is only relevant when:

[...]

> > +	/* bpf_local_storage_map is currently limited to CAP_SYS_ADMIN as
> >  	 * the map_alloc_check() side also does.
> >  	 */
> >  	if (!bpf_capable())
> > @@ -1025,10 +1127,10 @@ bpf_sk_storage_diag_alloc(const struct nlattr *nla_stgs)
> >  }
> >  EXPORT_SYMBOL_GPL(bpf_sk_storage_diag_alloc);
> Would it be cleaner to leave bpf_sk specific function, map_ops, and func_proto
> in net/core/bpf_sk_storage.c?

Sure, I can also keep the sk_clone code their as well for now.

> 
> There is a test in map_tests/sk_storage_map.c, in case you may not notice.

I will try to make it generic as a part of this series. If it takes
too much time, I will send a separate patch for testing
inode_storage_map and till then we have some assurance with
test_local_storage in test_progs.

- KP