Re: [PATCH v3 4/9] s390x: Avoid variable size warning in ipl.h

[Cornelia Huck <cohuck@redhat.com>]

On Thu,  5 Nov 2020 17:19:00 -0500
Daniele Buono <dbuono@linux.vnet.ibm.com> wrote:

> S390IPLState contains two IplParameterBlock, which may in turn have
> either a IPLBlockPV or a IplBlockFcp, both ending with a variable
> sized field (an array).
> 
> This causes a warning with clang 11 or greater, which checks that
> variable sized type are only allocated at the end of the struct:
> 
> In file included from ../qemu-cfi-v3/target/s390x/diag.c:21:
> ../qemu-cfi-v3/hw/s390x/ipl.h:161:23: error: field 'iplb' with variable sized type 'IplParameterBlock' (aka 'union IplParameterBlock') not at the end of a struct or class is a GNU extension [-Werror,-Wgnu-variable-sized-type-not-at-end]
>     IplParameterBlock iplb;
>                       ^
> ../qemu-cfi-v3/hw/s390x/ipl.h:162:23: error: field 'iplb_pv' with variable sized type 'IplParameterBlock' (aka 'union IplParameterBlock') not at the end of a struct or class is a GNU extension [-Werror,-Wgnu-variable-sized-type-not-at-end]
>     IplParameterBlock iplb_pv;
> 
> In this case, however, the warning is a false positive, because
> IPLBlockPV and IplBlockFcp are allocated in a union wrapped at 4K,
> making the union non-variable sized.
> 
> Fix the warning by turning the two variable sized arrays into arrays
> of size 0. This avoids the compiler error and should produce the
> same code.
> 
> Signed-off-by: Daniele Buono <dbuono@linux.vnet.ibm.com>
> ---
> There is the possibility of removing  IplBlockFcp from
> IplParameterBlock, since it is not actually used.
> This would also allow to entirely remove the definition of
> IplBlockFcp, but we may want to keep it for completeness.

We can easily do that in the future clean-up round, if we want that.

> 
>  hw/s390x/ipl.h | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/hw/s390x/ipl.h b/hw/s390x/ipl.h
> index 9e90169695..dfc6dfd89c 100644
> --- a/hw/s390x/ipl.h
> +++ b/hw/s390x/ipl.h
> @@ -32,7 +32,7 @@ struct IPLBlockPV {
>      uint32_t num_comp;          /* 0x74 */
>      uint64_t pv_header_addr;    /* 0x78 */
>      uint64_t pv_header_len;     /* 0x80 */
> -    struct IPLBlockPVComp components[];
> +    struct IPLBlockPVComp components[0];
>  } QEMU_PACKED;
>  typedef struct IPLBlockPV IPLBlockPV;
>  
> @@ -63,7 +63,7 @@ struct IplBlockFcp {
>      uint64_t br_lba;
>      uint32_t scp_data_len;
>      uint8_t  reserved6[260];
> -    uint8_t  scp_data[];
> +    uint8_t  scp_data[0];
>  } QEMU_PACKED;
>  typedef struct IplBlockFcp IplBlockFcp;
>  

For now, this is a nicely small patch.

Acked-by: Cornelia Huck <cohuck@redhat.com>