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

[Szabolcs Nagy <szabolcs.nagy@arm.com>]

On 22/04/16 09:04, Michael Kerrisk (man-pages) wrote:
> On 21 April 2016 at 16:01, Szabolcs Nagy <szabolcs.nagy@arm.com> wrote:
>> 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.
> 
> Below are the current snippets of code shown in the cmsg(3) man page.
> Could you please tell me more exactly what you think needs fixing?
> 

thanks, this is what i was looking for.
(i missed cmsg(3), only looked at recvmsg(2) and sendmsg(2).)

this means some code in glibc and iproute2 should be fixed
to use such a union. (what a nasty interface.)

> [[
>     EXAMPLE
>        This code looks for the IP_TTL option in a  received  ancillary
>        buffer:
> 
>            struct msghdr msgh;
>            struct cmsghdr *cmsg;
>            int *ttlptr;
>            int received_ttl;
> 
>            /* Receive auxiliary data in msgh */
> 
>            for (cmsg = CMSG_FIRSTHDR(&msgh); cmsg != NULL;
>                    cmsg = CMSG_NXTHDR(&msgh, cmsg)) {
>                if (cmsg->cmsg_level == IPPROTO_IP
>                        && cmsg->cmsg_type == IP_TTL) {
>                    ttlptr = (int *) CMSG_DATA(cmsg);
>                    received_ttl = *ttlptr;
>                    break;
>                }
>            }
> 
>            if (cmsg == NULL) {
>                /* Error: IP_TTL not enabled or small buffer or I/O error */
>            }
> 
>        The  code below passes an array of file descriptors over a UNIX
>        domain socket using SCM_RIGHTS:
> 
>            struct msghdr msg = {0};
>            struct cmsghdr *cmsg;
>            int myfds[NUM_FD];  /* Contains the file descriptors to pass */
>            int *fdptr;
>            union {         /* Ancillary data buffer, wrapped in a union
>                               in order to ensure it is suitably aligned */
>                char buf[CMSG_SPACE(sizeof myfds)];
>                struct cmsghdr align;
>            } u;
> 
>            msg.msg_control = u.buf;
>            msg.msg_controllen = sizeof u.buf;
>            cmsg = CMSG_FIRSTHDR(&msg);
>            cmsg->cmsg_level = SOL_SOCKET;
>            cmsg->cmsg_type = SCM_RIGHTS;
>            cmsg->cmsg_len = CMSG_LEN(sizeof(int) * NUM_FD);
>            fdptr = (int *) CMSG_DATA(cmsg);    /* Initialize the payload */
>            memcpy(fdptr, myfds, NUM_FD * sizeof(int));
> ]]
> 
> Cheers,
> 
> Michael
>