bpf: Implement cgroup storage available to non-cgroup-attached bpf progs

Similar to sk/inode/task storage, implement similar cgroup local storage.

There already exists a local storage implementation for cgroup-attached
bpf programs.  See map type BPF_MAP_TYPE_CGROUP_STORAGE and helper
bpf_get_local_storage(). But there are use cases such that non-cgroup
attached bpf progs wants to access cgroup local storage data. For example,
tc egress prog has access to sk and cgroup. It is possible to use
sk local storage to emulate cgroup local storage by storing data in socket.
But this is a waste as it could be lots of sockets belonging to a particular
cgroup. Alternatively, a separate map can be created with cgroup id as the key.
But this will introduce additional overhead to manipulate the new map.
A cgroup local storage, similar to existing sk/inode/task storage,
should help for this use case.

The life-cycle of storage is managed with the life-cycle of the
cgroup struct.  i.e. the storage is destroyed along with the owning cgroup
with a call to bpf_cgrp_storage_free() when cgroup itself
is deleted.

The userspace map operations can be done by using a cgroup fd as a key
passed to the lookup, update and delete operations.

Typically, the following code is used to get the current cgroup:
    struct task_struct *task = bpf_get_current_task_btf();
    ... task->cgroups->dfl_cgrp ...
and in structure task_struct definition:
    struct task_struct {
        ....
        struct css_set __rcu            *cgroups;
        ....
    }
With sleepable program, accessing task->cgroups is not protected by rcu_read_lock.
So the current implementation only supports non-sleepable program and supporting
sleepable program will be the next step together with adding rcu_read_lock
protection for rcu tagged structures.

Since map name BPF_MAP_TYPE_CGROUP_STORAGE has been used for old cgroup local
storage support, the new map name BPF_MAP_TYPE_CGRP_STORAGE is used
for cgroup storage available to non-cgroup-attached bpf programs. The old
cgroup storage supports bpf_get_local_storage() helper to get the cgroup data.
The new cgroup storage helper bpf_cgrp_storage_get() can provide similar
functionality. While old cgroup storage pre-allocates storage memory, the new
mechanism can also pre-allocate with a user space bpf_map_update_elem() call
to avoid potential run-time memory allocation failure.
Therefore, the new cgroup storage can provide all functionality w.r.t.
the old one. So in uapi bpf.h, the old BPF_MAP_TYPE_CGROUP_STORAGE is alias to
BPF_MAP_TYPE_CGROUP_STORAGE_DEPRECATED to indicate the old cgroup storage can
be deprecated since the new one can provide the same functionality.

Acked-by: David Vernet <void@manifault.com>
Signed-off-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/r/20221026042850.673791-1-yhs@fb.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

1 files changed, 7 insertions, 0 deletions

diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index a5dbac8f5aba..9fd68b0b3e9c 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -2041,6 +2041,7 @@ struct bpf_link *bpf_link_by_id(u32 id);
 
 const struct bpf_func_proto *bpf_base_func_proto(enum bpf_func_id func_id);
 void bpf_task_storage_free(struct task_struct *task);
+void bpf_cgrp_storage_free(struct cgroup *cgroup);
 bool bpf_prog_has_kfunc_call(const struct bpf_prog *prog);
 const struct btf_func_model *
 bpf_jit_find_kfunc_model(const struct bpf_prog *prog,
@@ -2295,6 +2296,10 @@ static inline bool has_current_bpf_ctx(void)
 static inline void bpf_prog_inc_misses_counter(struct bpf_prog *prog)
 {
 }
+
+static inline void bpf_cgrp_storage_free(struct cgroup *cgroup)
+{
+}
 #endif /* CONFIG_BPF_SYSCALL */
 
 void __bpf_free_used_btfs(struct bpf_prog_aux *aux,
@@ -2535,6 +2540,8 @@ extern const struct bpf_func_proto bpf_copy_from_user_task_proto;
 extern const struct bpf_func_proto bpf_set_retval_proto;
 extern const struct bpf_func_proto bpf_get_retval_proto;
 extern const struct bpf_func_proto bpf_user_ringbuf_drain_proto;
+extern const struct bpf_func_proto bpf_cgrp_storage_get_proto;
+extern const struct bpf_func_proto bpf_cgrp_storage_delete_proto;
 
 const struct bpf_func_proto *tracing_prog_func_proto(
   enum bpf_func_id func_id, const struct bpf_prog *prog);