linux-kernel - Re: [PATCH] riscv: bpf: Fix uninitialized symbol 'retval

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Message-ID: <9cbdefd6-a757-44b3-a1db-69ca8117aacb@huawei.com>
Date: Thu, 21 Aug 2025 09:58:20 +0800
From: Pu Lehui <pulehui@...wei.com>
To: Chenghao Duan <duanchenghao@...inos.cn>
CC: <ast@...nel.org>, <bjorn@...nel.org>, <puranjay@...nel.org>,
	<paul.walmsley@...ive.com>, <palmer@...belt.com>, <aou@...s.berkeley.edu>,
	<daniel@...earbox.net>, <andrii@...nel.org>, <martin.lau@...ux.dev>,
	<eddyz87@...il.com>, <song@...nel.org>, <yonghong.song@...ux.dev>,
	<john.fastabend@...il.com>, <kpsingh@...nel.org>, <sdf@...ichev.me>,
	<haoluo@...gle.com>, <jolsa@...nel.org>, <alex@...ti.fr>,
	<bpf@...r.kernel.org>, <linux-riscv@...ts.infradead.org>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] riscv: bpf: Fix uninitialized symbol 'retval_off'



On 2025/8/20 18:35, Chenghao Duan wrote:
> On Wed, Aug 20, 2025 at 06:10:07PM +0800, Pu Lehui wrote:
>>
>>
>> On 2025/8/20 17:26, Chenghao Duan wrote:
>>> On Wed, Aug 20, 2025 at 02:52:01PM +0800, Pu Lehui wrote:
>>>>
>>>>
>>>> On 2025/8/20 14:25, Chenghao Duan wrote:
>>>>> In __arch_prepare_bpf_trampoline(), retval_off is only meaningful when
>>>>> save_ret is true, so the current logic is correct. However, in the

OK, I think we should make commit msg more explicit. Such like the 
follow. wdyt?

`However, in the fmod_ret logic, the compiler is not aware that the 
flags of the fmod_ret prog have set BPF_TRAMP_F_CALL_ORIG, resulting in 
an uninitialized symbol compilation warning.`

>>>>
>>>> lgtm, and same for `ip_off`, pls patch it together.
>>>
>>> I also checked at the time that ip_off is only initialized and assigned
>>> when flags & BPF_TRAMP_F_IP_ARG is true. However, I noticed that the use
>>> of ip_off also requires this condition, so the compiler did not issue a
>>> warning.
>>>
>>> Chenghao
>>>
>>>>
>>>>> original logic, retval_off is only initialized under certain
>>
>> Can you show how to replay this warning? I guess the warning path is as
>> follow. Compiler didn't know fmod_ret prog need BPF_TRAMP_F_CALL_ORIG.
>>
>> ```
>> if (fmod_ret->nr_links) {
>> 	...
>> 	emit_sd(RV_REG_FP, -retval_off, RV_REG_ZERO, ctx);
>> }
>> ```
>>
> 
> Exactly, the compiler sees the unconditional use of retval_off.
> 
> Chenghao
> 
>>>>> conditions, which may cause a build warning.
>>>>>
>>>>> So initialize retval_off unconditionally to fix it.
>>>>>
>>>>> Signed-off-by: Chenghao Duan <duanchenghao@...inos.cn>
>>>>> ---
>>>>>     arch/riscv/net/bpf_jit_comp64.c | 5 ++---
>>>>>     1 file changed, 2 insertions(+), 3 deletions(-)
>>>>>
>>>>> diff --git a/arch/riscv/net/bpf_jit_comp64.c b/arch/riscv/net/bpf_jit_comp64.c
>>>>> index 10e01ff06312..49bbda8372b0 100644
>>>>> --- a/arch/riscv/net/bpf_jit_comp64.c
>>>>> +++ b/arch/riscv/net/bpf_jit_comp64.c
>>>>> @@ -1079,10 +1079,9 @@ static int __arch_prepare_bpf_trampoline(struct bpf_tramp_image *im,
>>>>>     	stack_size += 16;
>>>>>     	save_ret = flags & (BPF_TRAMP_F_CALL_ORIG | BPF_TRAMP_F_RET_FENTRY_RET);
>>>>> -	if (save_ret) {
>>>>> +	if (save_ret)
>>>>>     		stack_size += 16; /* Save both A5 (BPF R0) and A0 */
>>>>> -		retval_off = stack_size;
>>>>> -	}
>>>>> +	retval_off = stack_size;
>>>>>     	stack_size += nr_arg_slots * 8;
>>>>>     	args_off = stack_size;