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

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Message-ID: <39af9321-fb9b-4cee-84f1-77248a375e85@linux.dev>
Date: Wed, 22 Oct 2025 11:14:56 -0700
From: Yonghong Song <yonghong.song@...ux.dev>
To: KaFai Wan <kafai.wan@...ux.dev>, ast@...nel.org, daniel@...earbox.net,
 john.fastabend@...il.com, andrii@...nel.org, martin.lau@...ux.dev,
 eddyz87@...il.com, song@...nel.org, kpsingh@...nel.org, sdf@...ichev.me,
 haoluo@...gle.com, jolsa@...nel.org, shuah@...nel.org,
 paul.chaignon@...il.com, m.shachnai@...il.com, luis.gerhorst@....de,
 colin.i.king@...il.com, harishankar.vishwanathan@...il.com,
 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 1/2] bpf: Skip bounds adjustment for conditional
 jumps on same register



On 10/22/25 9:44 AM, 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: 93 at kernel/bpf/verifier.c:2731 reg_bounds_sanity_check+0x163/0x220
> Modules linked in:
> CPU: 0 UID: 0 PID: 93 Comm: repro-x-3 Tainted: G        W           6.18.0-rc1-ge7586577b75f-dirty #218 PREEMPT(full)
> Tainted: [W]=WARN
> 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.part.0+0x1b1/0x360
>   check_cond_jmp_op+0x1195/0x1a60
>   do_check_common+0x33ac/0x33c0
>   ...
>
> The issue occurs in reg_set_min_max() function where bounds adjustment logic
> is applied even when both registers being compared are the same. Comparing a
> register with itself should not change its bounds since the comparison result
> is always known (e.g., r0 == r0 is always true, r0 < r0 is always false).
>
> Fix this by adding an early return in reg_set_min_max() when false_reg1 and
> false_reg2 point to the same register, skipping the unnecessary bounds
> adjustment that leads to the verifier bug.
>
> 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 | 4 ++++
>   1 file changed, 4 insertions(+)
>
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 6d175849e57a..420ad512d1af 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -16429,6 +16429,10 @@ static int reg_set_min_max(struct bpf_verifier_env *env,
>   	if (false_reg1->type != SCALAR_VALUE || false_reg2->type != SCALAR_VALUE)
>   		return 0;
>   
> +	/* If conditional jumps on the same register, skip the adjustment */
> +	if (false_reg1 == false_reg2)
> +		return 0;

Your change looks good. But this is a special case and it should not
happen for any compiler generated code. So could you investigate
why regs_refine_cond_op() does not work? Since false_reg1 and false_reg2
is the same, so register refinement should keep the same. Probably
some minor change in regs_refine_cond_op(...) should work?

> +
>   	/* fallthrough (FALSE) branch */
>   	regs_refine_cond_op(false_reg1, false_reg2, rev_opcode(opcode), is_jmp32);
>   	reg_bounds_sync(false_reg1);