flexmember: port better to GCC + valgrind

With a char[] flexible array member in a struct with nontrivial
alignment, GCC-generated code can access past the end of the
array, because GCC assumes there are padding bytes to get the
struct aligned.  So the common idiom of malloc (offsetof (struct
s, m), n) does not properly allocate an n-byte trailing member, as
malloc’s argument should be the next multiple of alignof (struct s).
See GCC Bug#66661: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66661
Although C11 apparently permits this GCC optimization (i.e., there
was a bug in Gnulib not in GCC), possibly this is a defect in C11.
See the thread containing:
https://gcc.gnu.org/ml/gcc-patches/2016-09/msg00317.html
* lib/flexmember.h: New file.
* lib/fnmatch.c, lib/fts.c, lib/glob.c, lib/idcache.c:
* lib/localename.c, lib/time_rz.c:
Include flexmember.h.
* lib/fnmatch_loop.c (struct patternlist):
* lib/localename.c (struct hash_node):
Use FLEXIBLE_ARRAY_MEMBER.
* lib/fnmatch_loop.c (EXT):
* lib/fts.c (fts_alloc):
* lib/glob.c (glob_in_dir):
* lib/idcache.c (getuser, getuidbyname, getgroup, getgidbyname):
* lib/localename.c (gl_lock_define_initialized):
* lib/time_rz.c (tzalloc):
Use FLEXSIZEOF instead of offsetof.
* m4/flexmember.m4 (AC_C_FLEXIBLE_ARRAY_MEMBER):
Check that the size of the struct can be taken.
* modules/flexmember (Files): Add lib/flexmember.h.
* modules/fnmatch, modules/glob, modules/localename (Depends-on):
Add flexmember.

1 files changed, 2 insertions, 1 deletions

diff --git a/lib/time_rz.c b/lib/time_rz.c
index 10a006ca2e..8bfe988479 100644
--- a/lib/time_rz.c
+++ b/lib/time_rz.c
@@ -32,6 +32,7 @@
 #include <stdlib.h>
 #include <string.h>
 
+#include "flexmember.h"
 #include "time-internal.h"
 
 #if !HAVE_TZSET
@@ -94,7 +95,7 @@ tzalloc (char const *name)
 {
   size_t name_size = name ? strlen (name) + 1 : 0;
   size_t abbr_size = name_size < ABBR_SIZE_MIN ? ABBR_SIZE_MIN : name_size + 1;
-  timezone_t tz = malloc (offsetof (struct tm_zone, abbrs) + abbr_size);
+  timezone_t tz = malloc (FLEXSIZEOF (struct tm_zone, abbrs, abbr_size));
   if (tz)
     {
       tz->next = NULL;