[tz] [PATCH] add attribute nonstring

Paul Eggert eggert at cs.ucla.edu
Sun Nov 12 09:06:43 UTC 2017

Thanks for mentioning the problem with GCC 8. If the intent of 
__attribute__((__nonstring__)) is to consistently mark char arrays that are not 
necessarily null-terminated strings, then lots of data structures in the tzdb 
code would need to be marked with this attribute. For example, every member of 
tzfile.h's 'struct tzhead' would have the attribute, because they are all char 
arrays that might not be null-terminated.

Looking at the proposed glibc patch, it seems its intent is more modest. All 
it's trying to do is to pacify GCC 8 so that GCC doesn't complain about calls to 
strncpy and strncat. In that case, a simpler and cleaner fix for tzdb is to stop 
using strncpy and strncat. (I suspect that the main reason tzdb uses strncpy is 
that its code is so old that it predates the widespread availability of memcpy.) 
So I installed the attached patch into tzdb's zic.c and it should be part of the 
next tzdb release.

Perhaps you can this idea elsewhere in the rest of the glibc patch too. It would 
be a lot of work to consistently change glibc to use 
__attribute__((__nonstring__)) for its documented purpose. If glibc can avoid 
calling strncpy and strncat, it won't need __NONSTRING; if not, then I suggest 
changing this:

/* Describes a char array that is not necessarily a NUL-terminated
    string.  */
# define __NONSTRING __attribute__ ((__nonstring__))

to something more like this:

/* Describes a char array whose address can safely be passed as the first
    argument to strncpy and strncat, as the char array is not necessarily
    a NUL-terminated string.  */
# define __NONSTRING __attribute__ ((__nonstring__))

in sys/cdefs.h, so that the more-modest goal of the patch is clearer to the reader.

I'll CC: this to tz at iana.org as a heads-up there.
-------------- next part --------------
From 641f2c48b10dec5ab6dd90f560b66e061aa660ba Mon Sep 17 00:00:00 2001
From: Paul Eggert <eggert at cs.ucla.edu>
Date: Sun, 12 Nov 2017 00:57:19 -0800
Subject: [PROPOSED] Port to GCC 8 -Wstringop-truncation

Problem reported by Martin Sebor in:
* NEWS: Mention this.
* zic.c (writezone): Use memcpy, not strncpy.
 NEWS  | 3 +++
 zic.c | 2 +-
 2 files changed, 4 insertions(+), 1 deletion(-)

diff --git a/NEWS b/NEWS
index 966f9b6..7881936 100644
--- a/NEWS
+++ b/NEWS
@@ -29,6 +29,9 @@ Unreleased, experimental changes
     Diagnostics and commentary now distinguish UT from UTC more
     carefully; see theory.html for more information about UT vs UTC.
+    zic has been ported to GCC 8's -Wstringop-truncation option.
+    (Problem reported by Martin Sebor.)
 Release 2017c - 2017-10-20 14:49:34 -0700
diff --git a/zic.c b/zic.c
index 82e653d..e36301f 100644
--- a/zic.c
+++ b/zic.c
@@ -1960,7 +1960,7 @@ writezone(const char *const name, const char *const string, char version)
 #define DO(field)	fwrite(tzh.field, sizeof tzh.field, 1, fp)
 		tzh = tzh0;
-		strncpy(tzh.tzh_magic, TZ_MAGIC, sizeof tzh.tzh_magic);
+		memcpy(tzh.tzh_magic, TZ_MAGIC, sizeof tzh.tzh_magic);
 		tzh.tzh_version[0] = version;
 		convert(thistypecnt, tzh.tzh_ttisgmtcnt);
 		convert(thistypecnt, tzh.tzh_ttisstdcnt);

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