Re: [PATCH bpf-next v3 1/2] bpf: Skip bounds adjustment for conditional jumps on same scalar register

On Fri, Oct 31, 2025 at 8:44 AM KaFai Wan kafai.wan@linux.dev wrote:

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@hust.edu.cn Reported-by: Yinhao Hu dddddd@hust.edu.cn Closes: https://lore.kernel.org/all/1881f0f5.300df.199f2576a01.Coremail.kaiyanm@hust... Signed-off-by: KaFai Wan kafai.wan@linux.dev

---

kernel/bpf/verifier.c | 33 +++++++++++++++++++++++++++++++++ 1 file changed, 33 insertions(+)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c index 542e23fb19c7..a571263f4ebe 100644 --- a/kernel/bpf/verifier.c +++ b/kernel/bpf/verifier.c @@ -15995,6 +15995,8 @@ static int is_scalar_branch_taken(struct bpf_reg_state *reg1, struct bpf_reg_sta

    switch (opcode) {
    case BPF_JEQ:

•           if (reg1 == reg2)
  

•                   return 1;
            /* constants, umin/umax and smin/smax checks would be
             * redundant in this case because they all should match
             */
  

@@ -16021,6 +16023,8 @@ static int is_scalar_branch_taken(struct bpf_reg_state *reg1, struct bpf_reg_sta } break; case BPF_JNE:

•           if (reg1 == reg2)
  

•                   return 0;
            /* constants, umin/umax and smin/smax checks would be
             * redundant in this case because they all should match
             */
  

@@ -16047,6 +16051,12 @@ static int is_scalar_branch_taken(struct bpf_reg_state *reg1, struct bpf_reg_sta } break; case BPF_JSET:

•           if (reg1 == reg2) {
  

•                   if (tnum_is_const(t1))
  

•                           return t1.value != 0;
  

•                   else
  

•                           return (smin1 <= 0 && smax1 >= 0) ? -1 : 1;
  

•           }
            if (!is_reg_const(reg2, is_jmp32)) {
                    swap(reg1, reg2);
                    swap(t1, t2);
  

@@ -16059,48 +16069,64 @@ static int is_scalar_branch_taken(struct bpf_reg_state *reg1, struct bpf_reg_sta return 0; break; case BPF_JGT:

•           if (reg1 == reg2)
  

•                   return 0;
            if (umin1 > umax2)
                    return 1;
            else if (umax1 <= umin2)
                    return 0;
            break;
    case BPF_JSGT:
  

•           if (reg1 == reg2)
  

•                   return 0;
  

This is uglier than the previous version. reg1 == reg2 is a syzbot territory. We shouldn't uglify the code everywhere because of it.

pw-bot: cr