[PATCH bpf-next 03/10] bpf: enforce precision of R0 on callback return

[Andrii Nakryiko <andrii@kernel.org>]

Given verifier checks actual value, r0 has to be precise, so we need to
propagate precision properly. r0 also has to be marked as read,
otherwise subsequent state comparisons will ignore such register as
unimportant and precision won't really help here.

Fixes: 69c087ba6225 ("bpf: Add bpf_for_each_map_elem() helper")
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
---
 kernel/bpf/verifier.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index a921dba4f603..b227f23e063d 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -9493,6 +9493,13 @@ static int prepare_func_exit(struct bpf_verifier_env *env, int *insn_idx)
 			verbose(env, "R0 not a scalar value\n");
 			return -EACCES;
 		}
+
+		/* we are going to rely on register's precise value */
+		err = mark_reg_read(env, r0, r0->parent, REG_LIVE_READ64);
+		err = err ?: mark_chain_precision(env, BPF_REG_0);
+		if (err)
+			return err;
+
 		if (!tnum_in(range, r0->var_off)) {
 			verbose_invalid_scalar(env, r0, &range, "callback return", "R0");
 			return -EINVAL;
-- 
2.34.1