[Bug network/28846] New: CMSG_NXTHDR may trigger -Wstrict-overflow warning

["fweimer at redhat dot com" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=28846

            Bug ID: 28846
           Summary: CMSG_NXTHDR may trigger -Wstrict-overflow warning
           Product: glibc
           Version: unspecified
            Status: NEW
          Severity: normal
          Priority: P2
         Component: network
          Assignee: unassigned at sourceware dot org
          Reporter: fweimer at redhat dot com
  Target Milestone: ---
             Flags: security-

Downstream bug report from Andreas Schneider:

“
This is actually a bug in /usr/include/bits/socket.h with
-Werror=strict-overflow.


The compiler complains about the CMSG_NXTHDR() macro.


In file included from /usr/include/sys/socket.h:33,
                 from
/builddir/build/BUILD/socket_wrapper-1.3.3/src/socket_wrapper.c:50:
In function '__cmsg_nxthdr',
    inlined from 'test_sendmsg_cmsg' at
/builddir/build/BUILD/socket_wrapper-1.3.3/tests/test_swrap_unit.c:73:9:
/usr/include/bits/socket.h:322:6: error: assuming pointer wraparound does not
occur when comparing P +- C1 with P +- C2 [-Werror=strict-overflow]
  322 |   if ((unsigned char *) (__cmsg + 1) > ((unsigned char *)
__mhdr->msg_control
      |      ^


The line in test_swrap_unit.c:73 is:

73  »·······cmsg = CMSG_NXTHDR(&msg, cmsg);
”


This also impacts the out-of-line internal implementation in
sysdeps/unix/sysv/linux/cmsg_nxthdr.c.

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