[Bug libc/23037] New: initialize msg_flags to zero for sendmmsg() calls

["guenther at gmail dot com" <sourceware-bugzilla at sourceware dot org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=23037

            Bug ID: 23037
           Summary: initialize msg_flags to zero for sendmmsg() calls
           Product: glibc
           Version: 2.27
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: libc
          Assignee: unassigned at sourceware dot org
          Reporter: guenther at gmail dot com
                CC: drepper.fsp at gmail dot com
  Target Milestone: ---

The BSD sendmsg(2) interface has a flags argument; the msg_flags member of the
supplied msghdr structure is totally ignored by that API.

It appears that in recent versions of the Linux kernel, that is *not* the case
for the sendmmsg(2) API and instead it uses at least the MSG_EOR flag from the
msg_flags member of each supplied msghdr structure: __sys_sendmmsg() invokes
___sys_sendmsg(), passing allowed_msghdr_flags=MSG_EOR, which is used to
indicate which flags from msg_flags to or-info the flags used for processing:
        flags |= (msg_sys->msg_flags & allowed_msghdr_flags);


Since that looks to have been an intentional change on the kernel side, glibc
should initialize the msg_flags member for the calls it makes, particularly in
resolv/res_send.c

The sendmmsg(2) manpage should also be updated to document the need to
initialize the msg_flags members and perhaps call out that this is a difference
in behavior between sendmsg(2) and sendmmsg(2)

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