Date: Tue, 12 Mar 2024 17:53:21 +0800
From: Wen Gu <guwen@...ux.alibaba.com>
To: Jan Karcher <jaka@...ux.ibm.com>,
 "Gustavo A. R. Silva" <gustavo@...eddedor.com>,
 "Gustavo A. R. Silva" <gustavoars@...nel.org>,
 Wenjia Zhang <wenjia@...ux.ibm.com>, "D. Wythe" <alibuda@...ux.alibaba.com>,
 Tony Lu <tonylu@...ux.alibaba.com>, "David S. Miller" <davem@...emloft.net>,
 Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>,
 Paolo Abeni <pabeni@...hat.com>
Cc: linux-s390@...r.kernel.org, netdev@...r.kernel.org,
 linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org,
 Kees Cook <keescook@...omium.org>
Subject: Re: [PATCH][next] net/smc: Avoid -Wflex-array-member-not-at-end
 warnings



On 2024/3/12 15:54, Jan Karcher wrote:
> 
> 
> On 11/03/2024 11:59, Wen Gu wrote:
>>
>>
>> On 2024/3/8 07:46, Gustavo A. R. Silva wrote:
>>>
>>>
>>> On 3/7/24 02:17, Jan Karcher wrote:
>>>>
>>>>
>>>> On 04/03/2024 10:00, Wen Gu wrote:
>>>>>
>>>>>
>>>>> On 2024/3/2 02:40, Gustavo A. R. Silva wrote:
>>>>>> -Wflex-array-member-not-at-end is coming in GCC-14, and we are getting
>>>>>> ready to enable it globally.
>>>>>>
>>>>>> There are currently a couple of objects in `struct smc_clc_msg_proposal_area`
>>>>>> that contain a couple of flexible structures:
>>>>>>
>>>>
>>>> Thank you Gustavo for the proposal.
>>>> I had to do some reading to better understand what's happening and how your patch solves this.
>>>>
>>>>>> struct smc_clc_msg_proposal_area {
>>>>>>     ...
>>>>>>     struct smc_clc_v2_extension             pclc_v2_ext;
>>>>>>     ...
>>>>>>     struct smc_clc_smcd_v2_extension        pclc_smcd_v2_ext;
>>>>>>     ...
>>>>>> };
>>>>>>
>>>>>> So, in order to avoid ending up with a couple of flexible-array members
>>>>>> in the middle of a struct, we use the `struct_group_tagged()` helper to
>>>>>> separate the flexible array from the rest of the members in the flexible
>>>>>> structure:
>>>>>>
>>>>>> struct smc_clc_smcd_v2_extension {
>>>>>>          struct_group_tagged(smc_clc_smcd_v2_extension_hdr, hdr,
>>>>>>                              u8 system_eid[SMC_MAX_EID_LEN];
>>>>>>                              u8 reserved[16];
>>>>>>          );
>>>>>>          struct smc_clc_smcd_gid_chid gidchid[];
>>>>>> };
>>>>>>
>>>>>> With the change described above, we now declare objects of the type of
>>>>>> the tagged struct without embedding flexible arrays in the middle of
>>>>>> another struct:
>>>>>>
>>>>>> struct smc_clc_msg_proposal_area {
>>>>>>          ...
>>>>>>          struct smc_clc_v2_extension_hdr        pclc_v2_ext;
>>>>>>          ...
>>>>>>          struct smc_clc_smcd_v2_extension_hdr    pclc_smcd_v2_ext;
>>>>>>          ...
>>>>>> };
>>>>>>
>>>>>> We also use `container_of()` when we need to retrieve a pointer to the
>>>>>> flexible structures.
>>>>>>
>>>>>> So, with these changes, fix the following warnings:
>>>>>>
>>>>>> In file included from net/smc/af_smc.c:42:
>>>>>> net/smc/smc_clc.h:186:49: warning: structure containing a flexible array member is not at the end of another 
>>>>>> structure [-Wflex-array-member-not-at-end]
>>>>>>    186 |         struct smc_clc_v2_extension             pclc_v2_ext;
>>>>>>        |                                                 ^~~~~~~~~~~
>>>>>> net/smc/smc_clc.h:188:49: warning: structure containing a flexible array member is not at the end of another 
>>>>>> structure [-Wflex-array-member-not-at-end]
>>>>>>    188 |         struct smc_clc_smcd_v2_extension pclc_smcd_v2_ext;
>>>>>>        | ^~~~~~~~~~~~~~~~
>>>>>>
>>>>>> Signed-off-by: Gustavo A. R. Silva <gustavoars@...nel.org>
>>>>>> ---
>>>>>>   net/smc/smc_clc.c |  5 +++--
>>>>>>   net/smc/smc_clc.h | 24 ++++++++++++++----------
>>>>>>   2 files changed, 17 insertions(+), 12 deletions(-)
>>>>>>
>>>>>> diff --git a/net/smc/smc_clc.c b/net/smc/smc_clc.c
>>>>>> index e55026c7529c..3094cfa1c458 100644
>>>>>> --- a/net/smc/smc_clc.c
>>>>>> +++ b/net/smc/smc_clc.c
>>>>>> @@ -853,8 +853,9 @@ int smc_clc_send_proposal(struct smc_sock *smc, struct smc_init_info *ini)
>>>>>>       pclc_smcd = &pclc->pclc_smcd;
>>>>>>       pclc_prfx = &pclc->pclc_prfx;
>>>>>>       ipv6_prfx = pclc->pclc_prfx_ipv6;
>>>>>> -    v2_ext = &pclc->pclc_v2_ext;
>>>>>> -    smcd_v2_ext = &pclc->pclc_smcd_v2_ext;
>>>>>> +    v2_ext = container_of(&pclc->pclc_v2_ext, struct smc_clc_v2_extension, _hdr);
>>>>>> +    smcd_v2_ext = container_of(&pclc->pclc_smcd_v2_ext,
>>>>>> +                   struct smc_clc_smcd_v2_extension, hdr);
>>>>>>       gidchids = pclc->pclc_gidchids;
>>>>>>       trl = &pclc->pclc_trl;
>>>>>> diff --git a/net/smc/smc_clc.h b/net/smc/smc_clc.h
>>>>>> index 7cc7070b9772..5b91a1947078 100644
>>>>>> --- a/net/smc/smc_clc.h
>>>>>> +++ b/net/smc/smc_clc.h
>>>>>> @@ -134,12 +134,14 @@ struct smc_clc_smcd_gid_chid {
>>>>>>                */
>>>>>>   struct smc_clc_v2_extension {
>>>>>> -    struct smc_clnt_opts_area_hdr hdr;
>>>>>> -    u8 roce[16];        /* RoCEv2 GID */
>>>>>> -    u8 max_conns;
>>>>>> -    u8 max_links;
>>>>>> -    __be16 feature_mask;
>>>>>> -    u8 reserved[12];
>>>>>> +    struct_group_tagged(smc_clc_v2_extension_hdr, _hdr,
>>>>>> +        struct smc_clnt_opts_area_hdr hdr;
>>>>>> +        u8 roce[16];        /* RoCEv2 GID */
>>>>>> +        u8 max_conns;
>>>>>> +        u8 max_links;
>>>>>> +        __be16 feature_mask;
>>>>>> +        u8 reserved[12];
>>>>>> +    );
>>>>>>       u8 user_eids[][SMC_MAX_EID_LEN];
>>>>>>   };
>>>>>> @@ -159,8 +161,10 @@ struct smc_clc_msg_smcd {    /* SMC-D GID information */
>>>>>>   };
>>>>>>   struct smc_clc_smcd_v2_extension {
>>>>>> -    u8 system_eid[SMC_MAX_EID_LEN];
>>>>>> -    u8 reserved[16];
>>>>>> +    struct_group_tagged(smc_clc_smcd_v2_extension_hdr, hdr,
>>>>>> +        u8 system_eid[SMC_MAX_EID_LEN];
>>>>>> +        u8 reserved[16];
>>>>>> +    );
>>>>>>       struct smc_clc_smcd_gid_chid gidchid[];
>>>>>>   };
>>>>>> @@ -183,9 +187,9 @@ struct smc_clc_msg_proposal_area {
>>>>>>       struct smc_clc_msg_smcd            pclc_smcd;
>>>>>>       struct smc_clc_msg_proposal_prefix    pclc_prfx;
>>>>>>       struct smc_clc_ipv6_prefix pclc_prfx_ipv6[SMC_CLC_MAX_V6_PREFIX];
>>>>>> -    struct smc_clc_v2_extension        pclc_v2_ext;
>>>>>> +    struct smc_clc_v2_extension_hdr        pclc_v2_ext;
>>>>>>       u8            user_eids[SMC_CLC_MAX_UEID][SMC_MAX_EID_LEN];
>>>>>> -    struct smc_clc_smcd_v2_extension    pclc_smcd_v2_ext;
>>>>>> +    struct smc_clc_smcd_v2_extension_hdr    pclc_smcd_v2_ext;
>>>>>>       struct smc_clc_smcd_gid_chid
>>>>>>                   pclc_gidchids[SMCD_CLC_MAX_V2_GID_ENTRIES];
>>>>>>       struct smc_clc_msg_trail        pclc_trl;
>>>>>
>>>>> Thank you! Gustavo. This patch can fix this warning well, just the name
>>>>> '*_hdr' might not be very accurate, but I don't have a good idea ATM.
>>>>
>>>> I agree. Should we chose this option we should come up for a better name.
>>>>
>>>>>
>>>>> Besides, I am wondering if this can be fixed by moving
>>>>> user_eids of smc_clc_msg_proposal_area into smc_clc_v2_extension,
>>>>> and
>>>>> pclc_gidchids of smc_clc_msg_proposal_area into smc_clc_smcd_v2_extension.
>>>>>
>>>>> so that we can avoid to use the flexible-array in smc_clc_v2_extension
>>>>> and smc_clc_smcd_v2_extension.
>>>>
>>>> I like the idea and put some thought into it. The only thing that is not perfectly clean IMO is the following:
>>>> By the current definition it is easily visible that we are dealing with a variable sized array. If we move them into 
>>>> the structs one could think they are always at their MAX size which they are not.
>>>> E.g.: An incoming proposal can have 0 UEIDs indicated by the eid_cnt.
>>>> That said nothing a comment can't fix.
>>>>
>>>>  From what i have seen the offset and length calculations regarding the "real" size of those structs is fine with 
>>>> your proposal.
>>>>
>>>> Can you verify that your changes also resolve the warnings?
>>>
>>> I can confirm that the changes Wen Gu is proposing also resolve the warnings.
>>>
>>> Wen,
>>>
>>> If you send a proper patch, you can include the following tags:
>>>
>>> Reviewed-by: Gustavo A. R. Silva <gustavoars@...nel.org>
>>> Build-tested-by: Gustavo A. R. Silva <gustavoars@...nel.org>
>>>
>>
>> Hi Gustavo, thank you for the confirmation that my proposal can fix the warning.
>>
>> But I found that I may have something missed in my proposal when I think further.
>> My proposal changed the sizes of struct smc_clc_v2_extension and smc_clc_smcd_v2_extension,
>> and some places in SMC need them, such as the fill of kvec in smc_clc_send_proposal().
>>
>> So my proposal may involve more changes to current SMC code, and I think it is
>> not as clean as your solution. So I perfer yours now.
> 
> Hi Wen Gu,
> 
> you're right. I missed that the offset calculation is broken with your proposal since the full size of the array is 
> already included in this case which means we would have to subtract the empty slots instead of adding the full ones.
> My bad. Thinking about adding a testcase to sxplicit check the size of the CLC Messages send in the future.
> 

That's OK. I am the one who brings this mistake.
Sometimes the details only become clear when start writing the code.

>>
>> And as for the name, I think maybe we can use '*_elems' as a suffix, at least it
>> is unambiguous. So it will be smc_clc_v2_extension_elems and smc_clc_smcd_v2_extension_elems.
>>
>>
>> Jan, what do you think of the name '*_elems' ?
> 
> Hmm... I think it is way better than priv. One more proposal from my side would be *_fixed since this is the fixed 
> content and not variable. I'm open for both.
> 
> Which one would you prefer more?
> 

'*_fixed' is better, thank you!


Hi Gustavo,

Sorry to complicate things. Could you please post a v2 with the new name updated (avoid using 'hdr') ?

Thank you!

>>
>> Thanks!
>>
>>> Thanks!
>>> -- 
>>> Gustavo
>>>
>>>>
>>>> [...]
>>>>
>>>>>   };
>>>>>
>>>>>
>>>>> Thanks!
>>>>> Wen Gu
>>>>
>>>> Thanks you
>>>> - Jan

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