Re: [PATCH][next] net/smc: Avoid -Wflex-array-member-not-at-end warnings

["Gustavo A. R. Silva" <gustavo@embeddedor.com>]

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@kernel.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@kernel.org>
Build-tested-by: Gustavo A. R. Silva <gustavoars@kernel.org>

Thanks!
--
Gustavo

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