From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <gcc-bugzilla@gcc.gnu.org>
Received: by sourceware.org (Postfix, from userid 48)
 id 984F3385B835; Thu, 16 Apr 2020 09:42:18 +0000 (GMT)
DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org 984F3385B835
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gcc.gnu.org;
 s=default; t=1587030138;
 bh=bC0jvyEBshoJNcPFgdvlTq+ji8E1M1GiEwlb/BuuN+U=;
 h=From:To:Subject:Date:From;
 b=D7UdfKGzQzTvmxxE/ECL4tCCN0s+BvOVBB1nw+vWaEkcw8bsog1QnaUeeheaUbjVD
 r6/di+GD9LOxyQM0Tlw4R5CMz/phT3JSfULL+ERQE3r5WXrsRa6lJpogz0Bqq+PH9s
 6oHGhGuniNv/sBHnY3LPJaMf8ye9KO0gv7qpUMQk=
From: "allison.karlitskaya at redhat dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c/94615] New: -Wstringop-truncation warns on strncpy() with
 struct lastlog (or utmp)
Date: Thu, 16 Apr 2020 09:42:18 +0000
X-Bugzilla-Reason: CC
X-Bugzilla-Type: new
X-Bugzilla-Watch-Reason: None
X-Bugzilla-Product: gcc
X-Bugzilla-Component: c
X-Bugzilla-Version: 8.0
X-Bugzilla-Keywords: 
X-Bugzilla-Severity: normal
X-Bugzilla-Who: allison.karlitskaya at redhat dot com
X-Bugzilla-Status: UNCONFIRMED
X-Bugzilla-Resolution: 
X-Bugzilla-Priority: P3
X-Bugzilla-Assigned-To: unassigned at gcc dot gnu.org
X-Bugzilla-Target-Milestone: ---
X-Bugzilla-Flags: 
X-Bugzilla-Changed-Fields: bug_id short_desc product version bug_status
 bug_severity priority component assigned_to reporter target_milestone
Message-ID: <bug-94615-4@http.gcc.gnu.org/bugzilla/>
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable
X-Bugzilla-URL: http://gcc.gnu.org/bugzilla/
Auto-Submitted: auto-generated
MIME-Version: 1.0
X-BeenThere: gcc-bugs@gcc.gnu.org
X-Mailman-Version: 2.1.29
Precedence: list
List-Id: Gcc-bugs mailing list <gcc-bugs.gcc.gnu.org>
List-Unsubscribe: <http://gcc.gnu.org/mailman/options/gcc-bugs>,
 <mailto:gcc-bugs-request@gcc.gnu.org?subject=unsubscribe>
List-Archive: <https://gcc.gnu.org/pipermail/gcc-bugs/>
List-Post: <mailto:gcc-bugs@gcc.gnu.org>
List-Help: <mailto:gcc-bugs-request@gcc.gnu.org?subject=help>
List-Subscribe: <http://gcc.gnu.org/mailman/listinfo/gcc-bugs>,
 <mailto:gcc-bugs-request@gcc.gnu.org?subject=subscribe>
X-List-Received-Date: Thu, 16 Apr 2020 09:42:18 -0000

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D94615

            Bug ID: 94615
           Summary: -Wstringop-truncation warns on strncpy() with struct
                    lastlog (or utmp)
           Product: gcc
           Version: 8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: allison.karlitskaya at redhat dot com
  Target Milestone: ---

Consider this code:

{
  struct lastlog entry;

  strncpy (entry.ll_host, rhost, sizeof entry.ll_host);
  /* fill other fields */

  /* write record to the lastlog */
}

strncpy() turns out to be exactly what you want in this case because the
ll_host field in struct lastlog is only *optionally* nul terminated, in the
case of the hostname being shorter than the field width.  If the hostname is
equal to or longer than the field length then you should write the whole th=
ing
in, without including a nul.  Consequently, any reader code needs to be more
careful about checking for both conditions (which it ought to be, in any ca=
se).

GCC 8 gives this warning, though:

/usr/include/bits/string_fortified.h:106:10: warning: '__builtin_strncpy'
specified bound 256 equals destination size [-Wstringop-truncation]

... which on its face seems kinda funny, because the bound *should* equal t=
he
destination size.  Particularly in this case.


Perhaps there is some attribute or other hint that can be added to the stru=
ct
in question that the character array is intended to be only optionally nul
terminated, and in that case, we could skip the warning?  Otherwise, it wou=
ld
be nice if there were a way to avoid this warning that didn't involve a mes=
s of
#if and #pragma.=