linux-kernel - [PATCH bpf-next v4 1/2] bpf: Skip bounds adjustment for conditional jumps on same scalar register

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Message-ID: <20251103063108.1111764-2-kafai.wan@linux.dev>
Date: Mon,  3 Nov 2025 14:31:07 +0800
From: KaFai Wan <kafai.wan@...ux.dev>
To: ast@...nel.org,
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Cc: Kaiyan Mei <M202472210@...t.edu.cn>,
	Yinhao Hu <dddddd@...t.edu.cn>
Subject: [PATCH bpf-next v4 1/2] 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 <M202472210@...t.edu.cn>
Reported-by: Yinhao Hu <dddddd@...t.edu.cn>
Closes: https://lore.kernel.org/all/1881f0f5.300df.199f2576a01.Coremail.kaiyanm@hust.edu.cn/
Signed-off-by: KaFai Wan <kafai.wan@...ux.dev>
---
 kernel/bpf/verifier.c | 31 +++++++++++++++++++++++++++++++
 1 file changed, 31 insertions(+)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 542e23fb19c7..e4928846e763 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -15993,6 +15993,30 @@ static int is_scalar_branch_taken(struct bpf_reg_state *reg1, struct bpf_reg_sta
 	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
@@ -16439,6 +16463,13 @@ static int reg_set_min_max(struct bpf_verifier_env *env,
 	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 */
 	regs_refine_cond_op(false_reg1, false_reg2, rev_opcode(opcode), is_jmp32);
 	reg_bounds_sync(false_reg1);
-- 
2.43.0