bpf: Skip bounds adjustment for conditional jumps on same scalar register

When conditional jumps are performed on the same scalar register
(e.g., r0 <= r0, r0 > r0, r0 < r0), the BPF verifier incorrectly
attempts to adjust the register's min/max bounds. This leads to
invalid range bounds and triggers a BUG warning.

The problematic BPF program:
0: call bpf_get_prandom_u32
1: w8 = 0x80000000
2: r0 &= r8
3: if r0 > r0 goto <exit>

The instruction 3 triggers kernel warning:
3: if r0 > r0 goto <exit>
true_reg1: range bounds violation u64=[0x1, 0x0] s64=[0x1, 0x0] u32=[0x1, 0x0] s32=[0x1, 0x0] var_off=(0x0, 0x0)
true_reg2: const tnum out of sync with range bounds u64=[0x0, 0xffffffffffffffff] s64=[0x8000000000000000, 0x7fffffffffffffff] var_off=(0x0, 0x0)

Comparing a register with itself should not change its bounds and
for most comparison operations, comparing a register with itself has
a known result (e.g., r0 == r0 is always true, r0 < r0 is always false).

Fix this by:
1. Enhance is_scalar_branch_taken() to properly handle branch direction
computation for same register comparisons across all BPF jump operations
2. Adds early return in reg_set_min_max() to avoid bounds adjustment
for unknown branch directions (e.g., BPF_JSET) on the same register

The fix ensures that unnecessary bounds adjustments are skipped, preventing
the verifier bug while maintaining correct branch direction analysis.

Reported-by: Kaiyan Mei <[e-mail]>
Reported-by: Yinhao Hu <[e-mail]>
Closes: https://lore.kernel.org/all/[e-mail]/
Signed-off-by: KaFai Wan <[e-mail]>
Acked-by: Eduard Zingerman <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Alexei Starovoitov <[e-mail]>

kernel/bpf/verifier.c

···
 	s64 smin2 = is_jmp32 ? (s64)reg2->s32_min_value : reg2->smin_value;
 	s64 smax2 = is_jmp32 ? (s64)reg2->s32_max_value : reg2->smax_value;
 
+	if (reg1 == reg2) {
+		switch (opcode) {
+		case BPF_JGE:
+		case BPF_JLE:
+		case BPF_JSGE:
+		case BPF_JSLE:
+		case BPF_JEQ:
+			return 1;
+		case BPF_JGT:
+		case BPF_JLT:
+		case BPF_JSGT:
+		case BPF_JSLT:
+		case BPF_JNE:
+			return 0;
+		case BPF_JSET:
+			if (tnum_is_const(t1))
+				return t1.value != 0;
+			else
+				return (smin1 <= 0 && smax1 >= 0) ? -1 : 1;
+		default:
+			return -1;
+		}
+	}
+
 	switch (opcode) {
 	case BPF_JEQ:
 		/* constants, umin/umax and smin/smax checks would be
···
 	 * the same object, but we don't bother with that).
 	 */
 	if (false_reg1->type != SCALAR_VALUE || false_reg2->type != SCALAR_VALUE)
+		return 0;
+
+	/* We compute branch direction for same SCALAR_VALUE registers in
+	 * is_scalar_branch_taken(). For unknown branch directions (e.g., BPF_JSET)
+	 * on the same registers, we don't need to adjust the min/max values.
+	 */
+	if (false_reg1 == false_reg2)
 		return 0;
 
 	/* fallthrough (FALSE) branch */