bpf: Add kfunc bpf_rcu_read_lock/unlock()

Add two kfunc's bpf_rcu_read_lock() and bpf_rcu_read_unlock(). These two kfunc's
can be used for all program types. The following is an example about how
rcu pointer are used w.r.t. bpf_rcu_read_lock()/bpf_rcu_read_unlock().

  struct task_struct {
    ...
    struct task_struct              *last_wakee;
    struct task_struct __rcu        *real_parent;
    ...
  };

Let us say prog does 'task = bpf_get_current_task_btf()' to get a
'task' pointer. The basic rules are:
  - 'real_parent = task->real_parent' should be inside bpf_rcu_read_lock
    region. This is to simulate rcu_dereference() operation. The
    'real_parent' is marked as MEM_RCU only if (1). task->real_parent is
    inside bpf_rcu_read_lock region, and (2). task is a trusted ptr. So
    MEM_RCU marked ptr can be 'trusted' inside the bpf_rcu_read_lock region.
  - 'last_wakee = real_parent->last_wakee' should be inside bpf_rcu_read_lock
    region since it tries to access rcu protected memory.
  - the ptr 'last_wakee' will be marked as PTR_UNTRUSTED since in general
    it is not clear whether the object pointed by 'last_wakee' is valid or
    not even inside bpf_rcu_read_lock region.

The verifier will reset all rcu pointer register states to untrusted
at bpf_rcu_read_unlock() kfunc call site, so any such rcu pointer
won't be trusted any more outside the bpf_rcu_read_lock() region.

The current implementation does not support nested rcu read lock
region in the prog.

Acked-by: Martin KaFai Lau <martin.lau@kernel.org>
Signed-off-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/r/20221124053217.2373910-1-yhs@fb.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

1 files changed, 3 insertions, 0 deletions

diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
index 9dbfda2b5c6c..bd3369100239 100644
--- a/kernel/bpf/btf.c
+++ b/kernel/bpf/btf.c
@@ -6238,6 +6238,9 @@ error:
 				/* check __percpu tag */
 				if (strcmp(tag_value, "percpu") == 0)
 					tmp_flag = MEM_PERCPU;
+				/* check __rcu tag */
+				if (strcmp(tag_value, "rcu") == 0)
+					tmp_flag = MEM_RCU;
 			}
 
 			stype = btf_type_skip_modifiers(btf, mtype->type, &id);