bpf: Add verifier support for custom callback return range

Verifier logic to confirm that a callback function returns 0 or 1 was
added in commit 69c087b ("bpf: Add bpf_for_each_map_elem() helper").
At the time, callback return value was only used to continue or stop
iteration.

In order to support callbacks with a broader return value range, such as
those added in rbtree series[0] and others, add a callback_ret_range to
bpf_func_state. Verifier's helpers which set in_callback_fn will also
set the new field, which the verifier will later use to check return
value bounds.

Default to tnum_range(0, 0) instead of using tnum_unknown as a sentinel
value as the latter would prevent the valid range (0, U64_MAX) being
used. Previous global default tnum_range(0, 1) is explicitly set for
extant callback helpers. The change to global default was made after
discussion around this patch in rbtree series [1], goal here is to make
it more obvious that callback_ret_range should be explicitly set.

  [0]: lore.kernel.org/bpf/20220830172759.4069786-1-davemarchevsky@fb.com/
  [1]: lore.kernel.org/bpf/20220830172759.4069786-2-davemarchevsky@fb.com/

Signed-off-by: Dave Marchevsky <davemarchevsky@fb.com>
Reviewed-by: Stanislav Fomichev <sdf@google.com>
Link: https://lore.kernel.org/r/20220908230716.2751723-1-davemarchevsky@fb.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

include/linux/bpf_verifier.h

@@ -248,6 +248,7 @@ struct bpf_func_state {
 	 */
 	u32 async_entry_cnt;
 	bool in_callback_fn;
+	struct tnum callback_ret_range;
 	bool in_async_callback_fn;
 
 	/* The following fields should be last. See copy_func_state() */

kernel/bpf/verifier.c

@@ -1750,6 +1750,7 @@ static void init_func_state(struct bpf_verifier_env *env,
 	state->callsite = callsite;
 	state->frameno = frameno;
 	state->subprogno = subprogno;
+	state->callback_ret_range = tnum_range(0, 0);
 	init_reg_state(env, state);
 	mark_verifier_state_scratched(env);
 }
@@ -6790,6 +6791,7 @@ static int set_map_elem_callback_state(struct bpf_verifier_env *env,
 		return err;
 
 	callee->in_callback_fn = true;
+	callee->callback_ret_range = tnum_range(0, 1);
 	return 0;
 }
 
@@ -6811,6 +6813,7 @@ static int set_loop_callback_state(struct bpf_verifier_env *env,
 	__mark_reg_not_init(env, &callee->regs[BPF_REG_5]);
 
 	callee->in_callback_fn = true;
+	callee->callback_ret_range = tnum_range(0, 1);
 	return 0;
 }
 
@@ -6840,6 +6843,7 @@ static int set_timer_callback_state(struct bpf_verifier_env *env,
 	__mark_reg_not_init(env, &callee->regs[BPF_REG_4]);
 	__mark_reg_not_init(env, &callee->regs[BPF_REG_5]);
 	callee->in_async_callback_fn = true;
+	callee->callback_ret_range = tnum_range(0, 1);
 	return 0;
 }
 
@@ -6867,6 +6871,7 @@ static int set_find_vma_callback_state(struct bpf_verifier_env *env,
 	__mark_reg_not_init(env, &callee->regs[BPF_REG_4]);
 	__mark_reg_not_init(env, &callee->regs[BPF_REG_5]);
 	callee->in_callback_fn = true;
+	callee->callback_ret_range = tnum_range(0, 1);
 	return 0;
 }
 
@@ -6894,7 +6899,7 @@ static int prepare_func_exit(struct bpf_verifier_env *env, int *insn_idx)
 	caller = state->frame[state->curframe];
 	if (callee->in_callback_fn) {
 		/* enforce R0 return value range [0, 1]. */
-		struct tnum range = tnum_range(0, 1);
+		struct tnum range = callee->callback_ret_range;
 
 		if (r0->type != SCALAR_VALUE) {
 			verbose(env, "R0 not a scalar value\n");