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

["Michael Kerrisk (man-pages)" <mtk dot manpages at gmail dot com>]

Hi Szabolcs,

On 22 April 2016 at 12:25, Szabolcs Nagy <szabolcs.nagy@arm.com> wrote:
> 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.)

So, the code in the cmsg(3) man page seems okay then?

Cheers,

Michael

>> [[
>>     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
>>
>



-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/