sysdeps/unix/sysv/linux/cmsg_nxthdr.c

   1 /* Return point to next ancillary data entry in message header.
   2    Copyright (C) 1997-2024 Free Software Foundation, Inc.
   3    This file is part of the GNU C Library.
   4
   5    The GNU C Library is free software; you can redistribute it and/or
   6    modify it under the terms of the GNU Lesser General Public
   7    License as published by the Free Software Foundation; either
   8    version 2.1 of the License, or (at your option) any later version.
   9
  10    The GNU C Library is distributed in the hope that it will be useful,
  11    but WITHOUT ANY WARRANTY; without even the implied warranty of
  12    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  13    Lesser General Public License for more details.
  14
  15    You should have received a copy of the GNU Lesser General Public
  16    License along with the GNU C Library; if not, see
  17    <https://www.gnu.org/licenses/>.  */
  18
  19 #include <stddef.h>
  20 #include <sys/socket.h>
  21
  22
  23 struct cmsghdr *
  24 __cmsg_nxthdr (struct msghdr *mhdr, struct cmsghdr *cmsg)
  25 {
  26   /* We may safely assume that cmsg lies between mhdr->msg_control and
  27      mhdr->msg_controllen because the user is required to obtain the first
  28      cmsg via CMSG_FIRSTHDR, set its length, then obtain subsequent cmsgs
  29      via CMSG_NXTHDR, setting lengths along the way.  However, we don't yet
  30      trust the value of cmsg->cmsg_len and therefore do not use it in any
  31      pointer arithmetic until we check its value.  */
  32
  33   unsigned char * msg_control_ptr = (unsigned char *) mhdr->msg_control;
  34   unsigned char * cmsg_ptr = (unsigned char *) cmsg;
  35
  36   size_t size_needed = sizeof (struct cmsghdr)
  37                        + __CMSG_PADDING (cmsg->cmsg_len);
  38
  39   /* The current header is malformed, too small to be a full header.  */
  40   if ((size_t) cmsg->cmsg_len < sizeof (struct cmsghdr))
  41     return (struct cmsghdr *) 0;
  42
  43   /* There isn't enough space between cmsg and the end of the buffer to
  44   hold the current cmsg *and* the next one.  */
  45   if (((size_t)
  46          (msg_control_ptr + mhdr->msg_controllen - cmsg_ptr)
  47        < size_needed)
  48       || ((size_t)
  49             (msg_control_ptr + mhdr->msg_controllen - cmsg_ptr
  50              - size_needed)
  51           < cmsg->cmsg_len))
  52
  53     return (struct cmsghdr *) 0;
  54
  55   /* Now, we trust cmsg_len and can use it to find the next header.  */
  56   cmsg = (struct cmsghdr *) ((unsigned char *) cmsg
  57                              + CMSG_ALIGN (cmsg->cmsg_len));
  58   return cmsg;
  59 }
  60 libc_hidden_def (__cmsg_nxthdr)