[Bug c/100395] New: Bogus -Wstringop-overflow warning

[Bug c/100395] New: Bogus -Wstringop-overflow warning

[lavr at ncbi dot nlm.nih.gov]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100395

            Bug ID: 100395
           Summary: Bogus -Wstringop-overflow warning
           Product: gcc
           Version: 10.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: lavr at ncbi dot nlm.nih.gov
  Target Milestone: ---

$ gcc --version
gcc (GCC) 10.2.0

$ cat test.c
#include <stdio.h>
#include <string.h>

int main(int argc, const char* argv[])
{
    size_t len0 = strlen(argv[0]), len1 = strlen(argv[1]);
    char buf[444];
    char* s;

    s = buf + sizeof(buf) - len0;
    memcpy(s, argv[0], len0);
    s -= len1;
    memcpy(--s, argv[1], len1);
    s[len1++] = ' ';
    printf("%.*s\n", (int)(len0 + len1), s);
    return 0;
}

$ gcc -O2 -Wall test.c
test.c: In function ‘main’:
test.c:14:15: warning: writing 1 byte into a region of size 0
[-Wstringop-overflow=]
   14 |     s[len1++] = ' ';
      |     ~~~~~~~~~~^~~~~
test.c:7:10: note: at offset -1 to object ‘buf’ with size 444 declared here
    7 |     char buf[444];
      |          ^~~

The offset shown in the message is wrong, which most likely was the reason to
emit the warning altogether.

The test code above mocks up a situation where the lengths of both strings,
which are being copied in the reverse order into "buf", are known and small,
i.e. both fit (a few times, actually) together into the buffer, including the
separator and terminator.

[Bug middle-end/100395] Bogus -Wstringop-overflow warning

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100395

Martin Sebor <msebor at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Blocks|                            |88443
          Component|c                           |middle-end
         Resolution|---                         |FIXED
      Known to fail|                            |10.3.0
           Keywords|                            |diagnostic
                 CC|                            |msebor at gcc dot gnu.org
   Last reconfirmed|                            |2021-5-3
      Known to work|                            |11.1.0, 9.3.0
             Status|UNCONFIRMED                 |RESOLVED

--- Comment #1 from Martin Sebor <msebor at gcc dot gnu.org> ---
Fixed in r11-1183.  The fix is not suitable for backporting to GCC 10.


Referenced Bugs:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88443
[Bug 88443] [meta-bug] bogus/missing -Wstringop-overflow warnings