msghdr msg_flags is ignored by the sendmsg syscall. It is the flags on the received message. But sendmsg doesn't recieve messages, it only sends them.

Example code snippet:

  // Create msg_hdr. Note that msg_flags isn't being set.
  msg.msg_name = NULL;
  msg.msg_namelen = 0;
  iov[0].iov_base = "one";
  iov[0].iov_len = 3;
  iov[1].iov_base = "two";
  iov[1].iov_len = 3;
  msg.msg_iov = &iov;
  msg.msg_iovlen = 2;
  msg.msg_control = NULL;
  msg.msg_controllen = 0;

  ssize_t s = sendmsg (fd, &msg, 0);

$ valgrind -q ./sendmsg
==2313== Syscall param socketcall.sendmsg(msg) points to uninitialised byte(s)
==2313==    at 0x34F0EE90A0: __sendmsg_nocancel (syscall-template.S:82)
==2313==    by 0x400771: main (sendmsg.c:46)
==2313==  Address 0x7ff000000 is on thread 1's stack
==2313== 

The same holds true for the new sendmmsg () syscall. Newer glibc (2.16) now use that systemcall (without setting the msg_flags field).

==8435== Syscall param sendmsg(mmsg[0].msg_hdr) points to uninitialised byte(s)
==8435==    at 0x34EFAF3D19: sendmmsg (sendmmsg.c:32)
==8435==    by 0x34F1A0B4D6: __libc_res_nsend (res_send.c:1132)
==8435==    by 0x34F1A08BDF: __libc_res_nquery (res_query.c:226)
==8435==    by 0x34F1A097B9: __libc_res_nsearch (res_query.c:582)
==8435==    by 0x13EDCAC7: _nss_dns_gethostbyname4_r (dns-host.c:309)


Reproducible: Always




Both the sendmsg and sendmmsg syscall use pre_mem_read_sendmsg for their checks (through 
generic_PRE_sys_sendmsg).

generic_PRE_sys_recvmsg () and pre_mem_write_recvmsg () does get this right:

static
void pre_mem_write_recvmsg ( ThreadId tid, Bool read,
                             const HChar *msg, Addr base, SizeT size )
{
   HChar *outmsg = strdupcat ( "di.syswrap.pmwr.1",
                               "recvmsg", msg, VG_AR_CORE );
   if ( read )
      PRE_MEM_READ( outmsg, base, size );
   else
      PRE_MEM_WRITE( outmsg, base, size );
   VG_(arena_free) ( VG_AR_CORE, outmsg );
}

But pre_mem_red_sendmsg does:

static
void pre_mem_read_sendmsg ( ThreadId tid, Bool read,
                            const HChar *msg, Addr base, SizeT size )
{
   HChar *outmsg = strdupcat ( "di.syswrap.pmrs.1",
                               "sendmsg", msg, VG_AR_CORE );
 
   PRE_MEM_READ( outmsg, base, size );
   VG_(arena_free) ( VG_AR_CORE, outmsg );
}

It should be:

static
void pre_mem_read_sendmsg ( ThreadId tid, Bool read,
                            const HChar *msg, Addr base, SizeT size )
{
   HChar *outmsg = strdupcat ( "di.syswrap.pmrs.1",
                               "sendmsg", msg, VG_AR_CORE );
   if ( read)
      PRE_MEM_READ( outmsg, base, size );
   VG_(arena_free) ( VG_AR_CORE, outmsg );
}