[2.26 COMMITTED] Avoid use of strlen in getlogin_r (bug 22447).

[fweimer@redhat.com (Florian Weimer)]

From: Joseph Myers <joseph@codesourcery.com>

Building glibc with current mainline GCC fails, among other reasons,
because of an error for use of strlen on the nonstring ut_user field.
This patch changes the problem code in getlogin_r to use __strnlen
instead.  It also needs to set the trailing NUL byte of the result
explicitly, because of the case where ut_user does not have such a
trailing NUL byte (but the result should always have one).

Tested for x86_64.  Also tested that, in conjunction with
<https://sourceware.org/ml/libc-alpha/2017-11/msg00797.html>, it fixes
the build for arm with mainline GCC.

	[BZ #22447]
	* sysdeps/unix/getlogin_r.c (__getlogin_r): Use __strnlen not
	strlen to compute length of ut_user and set trailing NUL byte of
	result explicitly.

(cherry picked from commit 4bae615022cb5a5da79ccda83cc6c9ba9f2d479c)

2017-11-22  Joseph Myers  <joseph@codesourcery.com>

	[BZ #22447]
	* sysdeps/unix/getlogin_r.c (__getlogin_r): Use __strnlen not
	strlen to compute length of ut_user and set trailing NUL byte of
	result explicitly.

diff --git a/NEWS b/NEWS
index 33bc5196c8..5569e8d8d2 100644
--- a/NEWS
+++ b/NEWS
@@ -127,6 +127,7 @@ The following bugs are resolved with this release:
   [22375] malloc returns pointer from tcache instead of NULL (CVE-2017-17426)
   [22377] Provide a C++ version of iseqsig
   [22442] if_nametoindex: Check length of ifname before copying it
+  [22447] Avoid use of strlen in getlogin_r
   [22627] $ORIGIN in $LD_LIBRARY_PATH is substituted twice
   [22636] PTHREAD_STACK_MIN is too small on x86-64
   [22637] nptl: Fix stack guard size accounting
diff --git a/sysdeps/unix/getlogin_r.c b/sysdeps/unix/getlogin_r.c
index 4a6a40eeb2..ad8e9111f6 100644
--- a/sysdeps/unix/getlogin_r.c
+++ b/sysdeps/unix/getlogin_r.c
@@ -80,7 +80,7 @@ __getlogin_r (char *name, size_t name_len)
 
   if (result == 0)
     {
-      size_t needed = strlen (ut->ut_user) + 1;
+      size_t needed = __strnlen (ut->ut_user, UT_NAMESIZE) + 1;
 
       if (needed > name_len)
 	{
@@ -89,7 +89,8 @@ __getlogin_r (char *name, size_t name_len)
 	}
       else
 	{
-	  memcpy (name, ut->ut_user, needed);
+	  memcpy (name, ut->ut_user, needed - 1);
+	  name[needed - 1] = 0;
 	  result = 0;
 	}
     }