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To:     Nathan Chancellor <natechancellor@gmail.com>
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From:   "Gustavo A. R. Silva" <gustavo@embeddedor.com>
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Subject: Re: remaining flexible-array conversions
Message-ID: <f636ad65-b313-90e9-557b-856d44c2e14b@embeddedor.com>
Date:   Fri, 24 Apr 2020 10:17:51 -0500
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Sender: linux-kernel-owner@vger.kernel.org
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Hi Nathan,

On 4/23/20 22:47, Nathan Chancellor wrote:
> Hi Gustavo,
> 
> That patch in -next appears to introduce some warnings with clang when
> CONFIG_UAPI_HEADER_TEST is enabled (allyesconfig/allmodconfig exposed it
> for us with KernelCI [1]):
> 

Thanks a lot for reporting this.

> ./usr/include/rdma/ib_user_verbs.h:436:34: warning: field 'base' with
> variable sized type 'struct ib_uverbs_create_cq_resp' not at the end of
> a struct or class is a GNU extension
> [-Wgnu-variable-sized-type-not-at-end]
>         struct ib_uverbs_create_cq_resp base;
>                                         ^
> ./usr/include/rdma/ib_user_verbs.h:647:34: warning: field 'base' with
> variable sized type 'struct ib_uverbs_create_qp_resp' not at the end of
> a struct or class is a GNU extension
> [-Wgnu-variable-sized-type-not-at-end]
>         struct ib_uverbs_create_qp_resp base;
>                                         ^
> ./usr/include/rdma/ib_user_verbs.h:743:29: warning: field 'base' with
> variable sized type 'struct ib_uverbs_modify_qp' not at the end of a
> struct or class is a GNU extension
> [-Wgnu-variable-sized-type-not-at-end]
>         struct ib_uverbs_modify_qp base;
>                                    ^
> 3 warnings generated.
> 
> I presume this is part of the point of the conversion since you mention
> a compiler warning when the flexible member is not at the end of a
> struct. How should they be fixed? That should probably happen before the
> patch gets merged.
> 
For all the cases above, the solution seems to be to move the declaration
of the "base" member to the end of the corresponding structure, as below:

diff --git a/include/uapi/rdma/ib_user_verbs.h b/include/uapi/rdma/ib_user_verbs.h
index a390a667b3f3..e05538be8b30 100644
--- a/include/uapi/rdma/ib_user_verbs.h
+++ b/include/uapi/rdma/ib_user_verbs.h
@@ -644,9 +644,9 @@ struct ib_uverbs_create_qp_resp {
 };

 struct ib_uverbs_ex_create_qp_resp {
-       struct ib_uverbs_create_qp_resp base;
        __u32 comp_mask;
        __u32 response_length;
+       struct ib_uverbs_create_qp_resp base;
 };

 /*

but I guess this will change the ABI?

Also, notice that:

"A structure containing a flexible array member, or a union containing
such a structure (possibly recursively), may not be a member of a structure
or an element of an array. (However, these uses are permitted by GCC as
extensions.)"[1]

[1] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html

Thanks
--
Gustavo