Re: [PATCH 2/3] network: recvmsg and sendmsg standard compliance (BZ#16919)

[Szabolcs Nagy <szabolcs dot nagy at arm dot com>]

On 28/03/16 15:34, Adhemerval Zanella wrote:
>  /* Structure used for storage of ancillary data object information.  */
>  struct cmsghdr
>    {
> -    size_t cmsg_len;		/* Length of data in cmsg_data plus length
> -				   of cmsghdr structure.
> -				   !! The type should be socklen_t but the
> -				   definition of the kernel is incompatible
> -				   with this.  */
> +#if __BYTE_ORDER == __BIG_ENDIAN
> +    int __glibc_reserved1;	/* Pad toadjust Linux size to POSIX defined
> +				   size for cmsg_len.  */
> +    socklen_t cmsg_len;		/* Length of data in cmsg_data plus length
> +				   of cmsghdr structure.  */
> +#else
> +    socklen_t cmsg_len;
> +    int __glibc_reserved1;
> +#endif
>      int cmsg_level;		/* Originating protocol.  */
>      int cmsg_type;		/* Protocol specific type.  */

i think #if __WORDSIZE == 64 is missing here.

but even in that case there is a subtle issue:
if the size_t member is removed all other
members have 4byte alignment, so the struct
alignment changes from 8byte to 4byte.

it is not clear from the standard how the
msg_control buffer may be allocated (since
only CMSG_* macros can access it), on linux
the kernel makes a copy so it does not care
about alignment in user-space, but the struct
alignment is still visible in the c and c++ abi.

msg_control usage should be probably documented
in the linux man-page: glibc sunrpc sometimes
uses plain char[], nscd uses a union with struct
cmsghdr, i think neither of them makes a

  CMSG_FIRSTHDR (&msg)->cmsg_len

access strictly iso c confrom, but the later at
least uses correct alignment.

maybe a posix issue should be filed to the
austin group.