bpf: fix control-flow graph checking in privileged mode

When BPF program is verified in privileged mode, BPF verifier allows
bounded loops. This means that from CFG point of view there are
definitely some back-edges. Original commit adjusted check_cfg() logic
to not detect back-edges in control flow graph if they are resulting
from conditional jumps, which the idea that subsequent full BPF
verification process will determine whether such loops are bounded or
not, and either accept or reject the BPF program. At least that's my
reading of the intent.

Unfortunately, the implementation of this idea doesn't work correctly in
all possible situations. Conditional jump might not result in immediate
back-edge, but just a few unconditional instructions later we can arrive
at back-edge. In such situations check_cfg() would reject BPF program
even in privileged mode, despite it might be bounded loop. Next patch
adds one simple program demonstrating such scenario.

To keep things simple, instead of trying to detect back edges in
privileged mode, just assume every back edge is valid and let subsequent
BPF verification prove or reject bounded loops.

Note a few test changes. For unknown reason, we have a few tests that
are specified to detect a back-edge in a privileged mode, but looking at
their code it seems like the right outcome is passing check_cfg() and
letting subsequent verification to make a decision about bounded or not
bounded looping.

Bounded recursion case is also interesting. The example should pass, as
recursion is limited to just a few levels and so we never reach maximum
number of nested frames and never exhaust maximum stack depth. But the
way that max stack depth logic works today it falsely detects this as
exceeding max nested frame count. This patch series doesn't attempt to
fix this orthogonal problem, so we just adjust expected verifier failure.

Suggested-by: Alexei Starovoitov <[e-mail]>
Fixes: 2589726d12a1 ("bpf: introduce bounded loops")
Reported-by: Hao Sun <[e-mail]>
Signed-off-by: Andrii Nakryiko <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Alexei Starovoitov <[e-mail]>

kernel/bpf/verifier.c

···
  * w - next instruction
  * e - edge
  */
-static int push_insn(int t, int w, int e, struct bpf_verifier_env *env,
-		     bool loop_ok)
+static int push_insn(int t, int w, int e, struct bpf_verifier_env *env)
 {
 	int *insn_stack = env->cfg.insn_stack;
 	int *insn_state = env->cfg.insn_state;
···
 		insn_stack[env->cfg.cur_stack++] = w;
 		return KEEP_EXPLORING;
 	} else if ((insn_state[w] & 0xF0) == DISCOVERED) {
-		if (loop_ok && env->bpf_capable)
+		if (env->bpf_capable)
 			return DONE_EXPLORING;
 		verbose_linfo(env, t, "%d: ", t);
 		verbose_linfo(env, w, "%d: ", w);
···
 	int ret, insn_sz;
 
 	insn_sz = bpf_is_ldimm64(&insns[t]) ? 2 : 1;
-	ret = push_insn(t, t + insn_sz, FALLTHROUGH, env, false);
+	ret = push_insn(t, t + insn_sz, FALLTHROUGH, env);
 	if (ret)
 		return ret;
 
···
 
 	if (visit_callee) {
 		mark_prune_point(env, t);
-		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));
+		ret = push_insn(t, t + insns[t].imm + 1, BRANCH, env);
 	}
 	return ret;
 }
···
 	if (BPF_CLASS(insn->code) != BPF_JMP &&
 	    BPF_CLASS(insn->code) != BPF_JMP32) {
 		insn_sz = bpf_is_ldimm64(insn) ? 2 : 1;
-		return push_insn(t, t + insn_sz, FALLTHROUGH, env, false);
+		return push_insn(t, t + insn_sz, FALLTHROUGH, env);
 	}
 
 	switch (BPF_OP(insn->code)) {
···
 			off = insn->imm;
 
 		/* unconditional jump with single edge */
-		ret = push_insn(t, t + off + 1, FALLTHROUGH, env,
-				true);
+		ret = push_insn(t, t + off + 1, FALLTHROUGH, env);
 		if (ret)
 			return ret;
 
···
 		/* conditional jump with two edges */
 		mark_prune_point(env, t);
 
-		ret = push_insn(t, t + 1, FALLTHROUGH, env, true);
+		ret = push_insn(t, t + 1, FALLTHROUGH, env);
 		if (ret)
 			return ret;
 
-		return push_insn(t, t + insn->off + 1, BRANCH, env, true);
+		return push_insn(t, t + insn->off + 1, BRANCH, env);
 	}
 }
 

tools/testing/selftests/bpf/progs/verifier_loops1.c

···
 "	::: __clobber_all);
 }
 
-SEC("tracepoint")
+SEC("socket")
 __description("bounded loop, start in the middle")
-__failure __msg("back-edge")
+__success
+__failure_unpriv __msg_unpriv("back-edge")
 __naked void loop_start_in_the_middle(void)
 {
 	asm volatile ("					\
···
 
 SEC("tracepoint")
 __description("bounded recursion")
-__failure __msg("back-edge")
+__failure
+/* verifier limitation in detecting max stack depth */
+__msg("the call stack of 8 frames is too deep !")
 __naked void bounded_recursion(void)
 {
 	asm volatile ("					\

tools/testing/selftests/bpf/verifier/calls.c

···
 	BPF_EXIT_INSN(),
 	},
 	.prog_type = BPF_PROG_TYPE_TRACEPOINT,
-	.errstr = "back-edge from insn 0 to 0",
+	.errstr = "the call stack of 9 frames is too deep",
 	.result = REJECT,
 },
 {
···
 	BPF_EXIT_INSN(),
 	},
 	.prog_type = BPF_PROG_TYPE_TRACEPOINT,
-	.errstr = "back-edge",
+	.errstr = "the call stack of 9 frames is too deep",
 	.result = REJECT,
 },
 {
···
 	BPF_EXIT_INSN(),
 	},
 	.prog_type = BPF_PROG_TYPE_TRACEPOINT,
-	.errstr = "back-edge",
+	.errstr = "the call stack of 9 frames is too deep",
 	.result = REJECT,
 },
 {