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

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

On 22/04/16 14:19, Michael Kerrisk (man-pages) wrote:
> 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?
> 

yes

(i originally thought the cast in CMSG_FIRSTHDR
from the union* to struct cmsghdr* would be
invalid on the sending side, but it's ok.

on the receiving side the dereference through the
(int*) cast is debatable, but should work in practice.)

> 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
>>>
>>
> 
> 
>