bpf: enhance BPF_JEQ/BPF_JNE is_branch_taken logic

Use 32-bit subranges to prune some 64-bit BPF_JEQ/BPF_JNE conditions
that otherwise would be "inconclusive" (i.e., is_branch_taken() would
return -1). This can happen, for example, when registers are initialized
as 64-bit u64/s64, then compared for inequality as 32-bit subregisters,
and then followed by 64-bit equality/inequality check. That 32-bit
inequality can establish some pattern for lower 32 bits of a register
(e.g., s< 0 condition determines whether the bit #31 is zero or not),
while overall 64-bit value could be anything (according to a value range
representation).

This is not a fancy quirky special case, but actually a handling that's
necessary to prevent correctness issue with BPF verifier's range
tracking: set_range_min_max() assumes that register ranges are
non-overlapping, and if that condition is not guaranteed by
is_branch_taken() we can end up with invalid ranges, where min > max.

  [0] https://lore.kernel.org/bpf/CACkBjsY2q1_fUohD7hRmKGqv1MV=eP2f6XK8kjkYNw7BaiF8iQ@mail.gmail.com/Acked-by: Shung-Hsi Yu <shung-hsi.yu@suse.com>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20231112010609.848406-4-andrii@kernel.orgSigned-off-by: Alexei Starovoitov <ast@kernel.org>

kernel/bpf/verifier.c

@@ -14283,6 +14283,18 @@ static int is_scalar_branch_taken(struct bpf_reg_state *reg1, struct bpf_reg_sta
 			return 0;
 		if (smin1 > smax2 || smax1 < smin2)
 			return 0;
+		if (!is_jmp32) {
+			/* if 64-bit ranges are inconclusive, see if we can
+			 * utilize 32-bit subrange knowledge to eliminate
+			 * branches that can't be taken a priori
+			 */
+			if (reg1->u32_min_value > reg2->u32_max_value ||
+			    reg1->u32_max_value < reg2->u32_min_value)
+				return 0;
+			if (reg1->s32_min_value > reg2->s32_max_value ||
+			    reg1->s32_max_value < reg2->s32_min_value)
+				return 0;
+		}
 		break;
 	case BPF_JNE:
 		/* constants, umin/umax and smin/smax checks would be
@@ -14295,6 +14307,18 @@ static int is_scalar_branch_taken(struct bpf_reg_state *reg1, struct bpf_reg_sta
 			return 1;
 		if (smin1 > smax2 || smax1 < smin2)
 			return 1;
+		if (!is_jmp32) {
+			/* if 64-bit ranges are inconclusive, see if we can
+			 * utilize 32-bit subrange knowledge to eliminate
+			 * branches that can't be taken a priori
+			 */
+			if (reg1->u32_min_value > reg2->u32_max_value ||
+			    reg1->u32_max_value < reg2->u32_min_value)
+				return 1;
+			if (reg1->s32_min_value > reg2->s32_max_value ||
+			    reg1->s32_max_value < reg2->s32_min_value)
+				return 1;
+		}
 		break;
 	case BPF_JSET:
 		if (!is_reg_const(reg2, is_jmp32)) {