linux-kernel - Re: [PATCH bpf-next v2 1/2] bpf: Skip bounds adjustment for conditional jumps on same register

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Message-ID: <51769170ba3cf9eb4007fb0fc22f2434302d9aa5.camel@gmail.com>
Date: Mon, 27 Oct 2025 13:09:13 -0700
From: Eduard Zingerman <eddyz87@...il.com>
To: KaFai Wan <kafai.wan@...ux.dev>, ast@...nel.org, daniel@...earbox.net, 
	john.fastabend@...il.com, andrii@...nel.org, martin.lau@...ux.dev,
 song@...nel.org, 	yonghong.song@...ux.dev, kpsingh@...nel.org,
 sdf@...ichev.me, haoluo@...gle.com, 	jolsa@...nel.org, shuah@...nel.org,
 paul.chaignon@...il.com, m.shachnai@...il.com, 
	harishankar.vishwanathan@...il.com, colin.i.king@...il.com,
 luis.gerhorst@....de, 	bpf@...r.kernel.org, linux-kernel@...r.kernel.org, 
	linux-kselftest@...r.kernel.org
Cc: Kaiyan Mei <M202472210@...t.edu.cn>, Yinhao Hu <dddddd@...t.edu.cn>
Subject: Re: [PATCH bpf-next v2 1/2] bpf: Skip bounds adjustment for
 conditional jumps on same register

On Sat, 2025-10-25 at 13:30 +0800, KaFai Wan wrote:
> When conditional jumps are performed on the same register (e.g., r0 <= r0,
> r0 > r0, r0 < r0) where the register holds a scalar with range, the verifier
> incorrectly attempts to adjust the register's min/max bounds. This leads to
> invalid range bounds and triggers a BUG warning:
> 
> verifier bug: REG INVARIANTS VIOLATION (true_reg1): range bounds violation u64=[0x1, 0x0] s64=[0x1, 0x0] u32=[0x1, 0x0] s32=[0x1, 0x0] var_off=(0x0, 0x0)
> WARNING: CPU: 0 PID: 92 at kernel/bpf/verifier.c:2731 reg_bounds_sanity_check+0x163/0x220
> Hardware name: QEMU Ubuntu 24.04 PC (i440FX + PIIX, 1996), BIOS 1.16.3-debian-1.16.3-2 04/01/2014
> RIP: 0010:reg_bounds_sanity_check+0x163/0x220
> Call Trace:
>  <TASK>
>  reg_set_min_max+0xf7/0x1d0
>  check_cond_jmp_op+0x57b/0x1730
>  ? print_bpf_insn+0x3d5/0xa50
>  do_check_common+0x33ac/0x33c0
>  ...
> 
> The root cause is in regs_refine_cond_op() where BPF_JLT/BPF_JSLT operations
> adjust both min/max bounds on the same register, causing invalid bounds.
> 
> Since comparing a register with itself should not change its bounds (the
> comparison result is always known: r0 == r0 is always true, r0 < r0 is
> always false), the bounds adjustment is unnecessary.
> 
> Fix this by:
> 1. Enhance is_branch_taken() and is_scalar_branch_taken() to properly
>    handle branch direction computation for same register comparisons
>    across all BPF jump operations
> 2. For unknown branch directions (e.g., BPF_JSET), add early return in
>    reg_set_min_max() to avoid bounds adjustment 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/
> Fixes: 0df1a55afa83 ("bpf: Warn on internal verifier errors")
> Signed-off-by: KaFai Wan <kafai.wan@...ux.dev>
> ---
>  kernel/bpf/verifier.c | 32 ++++++++++++++++++++++++++++++++
>  1 file changed, 32 insertions(+)
> 
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 6d175849e57a..653fa96ed0df 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -16037,6 +16037,12 @@ 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;
> +		}

I think this logic is fine, but it needs tests for multiple cases.

>  		if (!is_reg_const(reg2, is_jmp32)) {
>  			swap(reg1, reg2);
>  			swap(t1, t2);
> @@ -16172,6 +16178,25 @@ static int is_pkt_ptr_branch_taken(struct bpf_reg_state *dst_reg,
>  static int is_branch_taken(struct bpf_reg_state *reg1, struct bpf_reg_state *reg2,
>  			   u8 opcode, bool is_jmp32)
>  {
> +	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;
> +		default:
> +			break;
> +		}
> +	}
> +

I think Alexei was against my suggestion to put it in
is_branch_taken() and preferred is_scalar_branch_taken() instead.

>  	if (reg_is_pkt_pointer_any(reg1) && reg_is_pkt_pointer_any(reg2) && !is_jmp32)
>  		return is_pkt_ptr_branch_taken(reg1, reg2, opcode);
>  
> @@ -16429,6 +16454,13 @@ 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 registers in is_branch_taken() and
> +	 * is_scalar_branch_taken(). For unknown branch directions (e.g., BPF_JSET)
> +	 * on the same registers, we don't need to adjusts 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);