From: Alexei Starovoitov <ast@kernel.org>
Date: Wed, 14 Jul 2021 17:54:13 -0700
Subject: bpf: Relax verifier recursion check.
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Patch-mainline: v5.15-rc1
Git-commit: 86fc6ee6e246438d394e41bb7cc210b0fe724872
References: jsc#PED-1377

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>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Toke Høiland-Jørgensen <toke@redhat.com>
Link: https://lore.kernel.org/bpf/20210715005417.78572-8-alexei.starovoitov@gmail.com
Acked-by: Shung-Hsi Yu <shung-hsi.yu@suse.com>
---
 kernel/bpf/verifier.c |    8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -9451,8 +9451,12 @@ static int visit_func_call_insn(int t, i
 		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;
 }