[PATCH bpf-next 5/7] bpf: Mark potential spilled loop index variable as precise

[Yonghong Song <yhs@fb.com>]

For a loop, if loop index variable is spilled and between loop
iterations, the only reg/spill state difference is spilled loop
index variable, then verifier may assume an infinite loop which
cause verification failure. In such cases, we should mark
spilled loop index variable as precise to differentiate states
between loop iterations.

Since verifier is not able to accurately identify loop index
variable, add a heuristic such that if both old reg state and
new reg state are consts, mark old reg state as precise which
will trigger constant value comparison later.

Signed-off-by: Yonghong Song <yhs@fb.com>
---
 kernel/bpf/verifier.c | 20 ++++++++++++++++++--
 1 file changed, 18 insertions(+), 2 deletions(-)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index d070943a8ba1..d1aa2c7ae7c0 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -14850,6 +14850,23 @@ static bool stacksafe(struct bpf_verifier_env *env, struct bpf_func_state *old,
 		/* Both old and cur are having same slot_type */
 		switch (old->stack[spi].slot_type[BPF_REG_SIZE - 1]) {
 		case STACK_SPILL:
+			/* sometime loop index variable is spilled and the spill
+			 * is not marked as precise. If only state difference
+			 * between two iterations are spilled loop index, the
+			 * "infinite loop detected at insn" error will be hit.
+			 * Mark spilled constant as precise so it went through value
+			 * comparison.
+			 */
+			old_reg = &old->stack[spi].spilled_ptr;
+			cur_reg = &cur->stack[spi].spilled_ptr;
+			if (!old_reg->precise) {
+				if (old_reg->type == SCALAR_VALUE &&
+				    cur_reg->type == SCALAR_VALUE &&
+				    tnum_is_const(old_reg->var_off) &&
+				    tnum_is_const(cur_reg->var_off))
+					old_reg->precise = true;
+			}
+
 			/* when explored and current stack slot are both storing
 			 * spilled registers, check that stored pointers types
 			 * are the same as well.
@@ -14860,8 +14877,7 @@ static bool stacksafe(struct bpf_verifier_env *env, struct bpf_func_state *old,
 			 * such verifier states are not equivalent.
 			 * return false to continue verification of this path
 			 */
-			if (!regsafe(env, &old->stack[spi].spilled_ptr,
-				     &cur->stack[spi].spilled_ptr, idmap))
+			if (!regsafe(env, old_reg, cur_reg, idmap))
 				return false;
 			break;
 		case STACK_DYNPTR:
-- 
2.34.1