[Bug other/94647] New: [10 Regression] wrong diagnostic with -Werror=format-security

["doko at debian dot org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94647

            Bug ID: 94647
           Summary: [10 Regression] wrong diagnostic with
                    -Werror=format-security
           Product: gcc
           Version: 10.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: other
          Assignee: unassigned at gcc dot gnu.org
          Reporter: doko at debian dot org
  Target Milestone: ---

[forwarded from https://bugs.debian.org/958062]

seen with trunk 20200417, works with the gcc-9 branch.

$ cat testcase.c
/* Compile with gcc-10 -O2 -c testcase.c -Wall -Wformat -Werror=format-security
*/

#include <string.h>

struct a
{
        int pad;
        char string[512];
};

struct b
{
        int pad;
        char string[256];
};

int f(struct a *d, struct b *s)
{
        int l;

        /* No warning here, so GCC 10 assumes that d->string is properly
         * null terminated. */
        l = strlen(d->string);

        /* Warning here, GCC 10 assumes that d->string is *not* properly
         * null terminated */
        strncpy(d->string, s->string, sizeof(d->string));

        return l;
}


$ gcc-10 -O2 -c testcase.c -Wall -Wformat -Werror=format-security
In file included from /usr/include/string.h:495,
                 from testcase.c:3:
In function ‘strncpy’,
    inlined from ‘f’ at testcase.c:27:2:
/usr/include/x86_64-linux-gnu/bits/string_fortified.h:106:10: warning:
‘__builtin_strncpy’ offset [260, 511] from the object at ‘s’ is out of the
bounds of referenced subobject ‘string’ with type ‘char[256]’ at offset 4
[-Warray-bounds]
  106 |   return __builtin___strncpy_chk (__dest, __src, __len, __bos
(__dest));
      |          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
testcase.c: In function ‘f’:
testcase.c:14:7: note: subobject ‘string’ declared here
   14 |  char string[256];
      |       ^~~~~~