Fix string/tester.c build with GCC 8.

GCC 8 warns about more cases of string functions truncating their
output or not copying a trailing NUL byte.

This patch fixes testsuite build failures caused by such warnings in
string/tester.c.  In general, the warnings are disabled around the
relevant calls using DIAG_* macros, since the relevant cases are being
deliberately tested.  In one case, the warning is with
-Wstringop-overflow= instead of -Wstringop-truncation; in that case,
the conditional is __GNUC_PREREQ (7, 0) (being the version where
-Wstringop-overflow= was introduced), to allow the conditional to be
removed sooner, since it's harmless to disable the warning for a
GCC version where it doesn't actually occur.  In the case of warnings
for strncpy calls in test_memcmp, the calls in question are changed to
use memcpy, as they don't copy a trailing NUL and the point of that
code is to test memcmp rather than strncpy.

Tested (compilation) with GCC 8 for x86_64-linux-gnu with
build-many-glibcs.py (in conjunction with Martin's patch to allow
glibc to build).

	* string/tester.c (test_stpncpy): Disable -Wstringop-truncation
	for stpncpy calls for GCC 8.
	(test_strncat): Disable -Wstringop-truncation warning for strncat
	calls for GCC 8.  Disable -Wstringop-overflow= warning for one
	strncat call for GCC 7.
	(test_strncpy): Disable -Wstringop-truncation warning for strncpy
	calls for GCC 8.
	(test_memcmp): Use memcpy instead of strncpy for calls not copying
	trailing NUL.

2 files changed, 54 insertions, 2 deletions

diff --git a/ChangeLog b/ChangeLog
index 579185d23e..3930fd1ca4 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,5 +1,15 @@
 2017-11-14  Joseph Myers  <joseph@codesourcery.com>
 
+	* string/tester.c (test_stpncpy): Disable -Wstringop-truncation
+	for stpncpy calls for GCC 8.
+	(test_strncat): Disable -Wstringop-truncation warning for strncat
+	calls for GCC 8.  Disable -Wstringop-overflow= warning for one
+	strncat call for GCC 7.
+	(test_strncpy): Disable -Wstringop-truncation warning for strncpy
+	calls for GCC 8.
+	(test_memcmp): Use memcpy instead of strncpy for calls not copying
+	trailing NUL.
+
 	* string/bug-strncat1.c: Include <libc-diag.h>.
 	(main): Disable -Wstringop-truncation for strncat call for GCC 8.
 
diff --git a/string/tester.c b/string/tester.c
index 4b928b4f5e..66b11515b0 100644
--- a/string/tester.c
+++ b/string/tester.c
@@ -264,8 +264,15 @@ test_stpncpy (void)
 {
   it = "stpncpy";
   memset (one, 'x', sizeof (one));
+  DIAG_PUSH_NEEDS_COMMENT;
+#if __GNUC_PREREQ (8, 0)
+  /* GCC 8 warns about stpncpy truncating output; this is deliberately
+     tested here.  */
+  DIAG_IGNORE_NEEDS_COMMENT (8, "-Wstringop-truncation");
+#endif
   check (stpncpy (one, "abc", 2) == one + 2, 1);
   check (stpncpy (one, "abc", 3) == one + 3, 2);
+  DIAG_POP_NEEDS_COMMENT;
   check (stpncpy (one, "abc", 4) == one + 3, 3);
   check (one[3] == '\0' && one[4] == 'x', 4);
   check (stpncpy (one, "abcd", 5) == one + 4, 5);
@@ -420,13 +427,27 @@ test_strncat (void)
   equal (one, "cd", 9);
 
   (void) strcpy (one, "ab");
+  DIAG_PUSH_NEEDS_COMMENT;
+#if __GNUC_PREREQ (8, 0)
+  /* GCC 8 warns about strncat truncating output; this is deliberately
+     tested here.  */
+  DIAG_IGNORE_NEEDS_COMMENT (8, "-Wstringop-truncation");
+#endif
   (void) strncat (one, "cdef", 2);
+  DIAG_POP_NEEDS_COMMENT;
   equal (one, "abcd", 10);			/* Count-limited. */
 
   (void) strncat (one, "gh", 0);
   equal (one, "abcd", 11);			/* Zero count. */
 
+  DIAG_PUSH_NEEDS_COMMENT;
+#if __GNUC_PREREQ (7, 0)
+  /* GCC 8 warns about strncat bound equal to source length; this is
+     deliberately tested here.  */
+  DIAG_IGNORE_NEEDS_COMMENT (8, "-Wstringop-overflow=");
+#endif
   (void) strncat (one, "gh", 2);
+  DIAG_POP_NEEDS_COMMENT;
   equal (one, "abcdgh", 12);		/* Count and length equal. */
 
   DIAG_PUSH_NEEDS_COMMENT;
@@ -523,11 +544,25 @@ test_strncpy (void)
   equal (one, "abc", 2);			/* Did the copy go right? */
 
   (void) strcpy (one, "abcdefgh");
+  DIAG_PUSH_NEEDS_COMMENT;
+#if __GNUC_PREREQ (8, 0)
+  /* GCC 8 warns about strncpy truncating output; this is deliberately
+     tested here.  */
+  DIAG_IGNORE_NEEDS_COMMENT (8, "-Wstringop-truncation");
+#endif
   (void) strncpy (one, "xyz", 2);
+  DIAG_POP_NEEDS_COMMENT;
   equal (one, "xycdefgh", 3);			/* Copy cut by count. */
 
   (void) strcpy (one, "abcdefgh");
+  DIAG_PUSH_NEEDS_COMMENT;
+#if __GNUC_PREREQ (8, 0)
+  /* GCC 8 warns about strncpy truncating output; this is deliberately
+     tested here.  */
+  DIAG_IGNORE_NEEDS_COMMENT (8, "-Wstringop-truncation");
+#endif
   (void) strncpy (one, "xyz", 3);		/* Copy cut just before NUL. */
+  DIAG_POP_NEEDS_COMMENT;
   equal (one, "xyzdefgh", 4);
 
   (void) strcpy (one, "abcdefgh");
@@ -542,7 +577,14 @@ test_strncpy (void)
   equal (one+5, "fgh", 9);
 
   (void) strcpy (one, "abc");
+  DIAG_PUSH_NEEDS_COMMENT;
+#if __GNUC_PREREQ (8, 0)
+  /* GCC 8 warns about strncpy truncating output; this is deliberately
+     tested here.  */
+  DIAG_IGNORE_NEEDS_COMMENT (8, "-Wstringop-truncation");
+#endif
   (void) strncpy (one, "xyz", 0);		/* Zero-length copy. */
+  DIAG_POP_NEEDS_COMMENT;
   equal (one, "abc", 10);
 
   (void) strncpy (one, "", 2);		/* Zero-length source. */
@@ -1151,8 +1193,8 @@ test_memcmp (void)
     {
       char *a = one + i;
       char *b = two + i;
-      strncpy(a, "--------11112222", 16);
-      strncpy(b, "--------33334444", 16);
+      memcpy(a, "--------11112222", 16);
+      memcpy(b, "--------33334444", 16);
       check(memcmp(b, a, 16) > 0, cnt++);
       check(memcmp(a, b, 16) < 0, cnt++);
     }