[PATCH v4 bpf-next 5/9] bpf: Relax verifier recursion check.

[Alexei Starovoitov <alexei.starovoitov@gmail.com>]

From: Alexei Starovoitov <ast@kernel.org>

In the following bpf subprogram:
static int timer_cb(void *map, void *key, void *value)
{
    bpf_timer_set_callback(.., timer_cb);
}

the 'timer_cb' is a pointer to a function.
ld_imm64 insn is used to carry this pointer.
bpf_pseudo_func() returns true for such ld_imm64 insn.

Unlike bpf_for_each_map_elem() the bpf_timer_set_callback() is asynchronous.
Relax control flow check to allow such "recursion" that is seen as an infinite
loop by check_cfg(). The distinction between bpf_for_each_map_elem() the
bpf_timer_set_callback() is done in the follow up patch.

Signed-off-by: Alexei Starovoitov <ast@kernel.org>
---
 kernel/bpf/verifier.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 62759164759d..45435471192b 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -9465,8 +9465,12 @@ static int visit_func_call_insn(int t, int insn_cnt,
 		init_explored_state(env, t + 1);
 	if (visit_callee) {
 		init_explored_state(env, t);
-		ret = push_insn(t, t + insns[t].imm + 1, BRANCH,
-				env, false);
+		ret = push_insn(t, t + insns[t].imm + 1, BRANCH, env,
+				/* It's ok to allow recursion from CFG point of
+				 * view. __check_func_call() will do the actual
+				 * check.
+				 */
+				bpf_pseudo_func(insns + t));
 	}
 	return ret;
 }
-- 
2.30.2