[RFC PATCH bpf-next 02/11] bpf: Add verifier support for custom callback return range

[Dave Marchevsky <davemarchevsky@xxxxxx>]

Verifier logic to confirm that a callback function returns 0 or 1 was
added in commit 69c087ba6225b ("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 further in this series, 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, 1) instead of using tnum_unknown as a sentinel
value as the latter would prevent the valid range (0, U64_MAX) being
used.

Signed-off-by: Dave Marchevsky <davemarchevsky@xxxxxx>
---
 include/linux/bpf_verifier.h | 1 +
 kernel/bpf/verifier.c        | 4 +++-
 2 files changed, 4 insertions(+), 1 deletion(-)

diff --git a/include/linux/bpf_verifier.h b/include/linux/bpf_verifier.h
index 2e3bad8640dc..9c017575c034 100644
--- a/include/linux/bpf_verifier.h
+++ b/include/linux/bpf_verifier.h
@@ -237,6 +237,7 @@ 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() */
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index c5fbaa9f025a..1f50becce141 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -1745,6 +1745,7 @@ 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, 1);
 	init_reg_state(env, state);
 	mark_verifier_state_scratched(env);
 }
@@ -6880,6 +6881,7 @@ 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;
 }
 
@@ -6907,7 +6909,7 @@ 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");
-- 
2.30.2