[Bug c/100401] New: Bogus -Wformat-overflow warning

[Bug c/100401] New: Bogus -Wformat-overflow warning

[lavr at ncbi dot nlm.nih.gov]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100401

            Bug ID: 100401
           Summary: Bogus -Wformat-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 bogus.c
#include <stdio.h>

#define RECLEN  128

struct S {
    int  code;
    char text[0];
};


const void fun(int n)
{
    union {
        struct S s;
        char     rec[RECLEN];
    } x;
    const char* err;

    if (!n) {
        err = "No error";
    } else {
        sprintf(x.s.text, "Error %d", n);
        err = x.s.text;
    }
    printf("%s\n", err);
}


int main(int argc, const char* argv[])
{
    fun(argc);
    return 0;
}


$ gcc -Wall -O2 bogus.c
bogus.c: In function ‘fun’:
bogus.c:22:28: warning: ‘Error ’ directive writing 6 bytes into a region of
size 0 [-Wformat-overflow=]
   22 |         sprintf(x.s.text, "Error %d", n);
      |                            ^~~~~~
bogus.c:22:9: note: ‘sprintf’ output between 8 and 18 bytes into a destination
of size 0
   22 |         sprintf(x.s.text, "Error %d", n);
      |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

[Bug middle-end/100401] Bogus -Wformat-overflow for a trailing zero-length array of a union

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100401

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
      Known to fail|                            |10.2.0, 11.1.0
            Summary|Bogus -Wformat-overflow     |Bogus -Wformat-overflow for
                   |warning                     |a trailing zero-length
                   |                            |array of a union
                 CC|                            |msebor at gcc dot gnu.org
             Status|UNCONFIRMED                 |NEW
          Component|c                           |middle-end
   Last reconfirmed|                            |2021-05-03
     Ever confirmed|0                           |1

--- Comment #1 from Martin Sebor <msebor at gcc dot gnu.org> ---
Confirmed with GCC 10.  The warning is avoided in GCC 11 (since r11-5622).  The
change is too intrusive to backport and it's seems that the warning doesn't
trigger anymore might be incidental to it.  It can still be reproduced on trunk
but only in C++.

I would suggest to expect warnings for tricks like in the test case (writing
past subobject boundaries).  GCC warnings are designed to "report constructions
that are not inherently erroneous but that are risky or suggest there may have
been an error."  Access warnings like -Warray-bounds, -Wformat-overflow, and
-Wstringop-overflow try to accommodate code they have been taught is likely
safe but the logic is secondary to helping find bugs, and not foolproof.

[Bug middle-end/100401] Bogus -Wformat-overflow for a trailing zero-length array of a union

[lavr at ncbi dot nlm.nih.gov]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100401

--- Comment #2 from lavr at ncbi dot nlm.nih.gov ---
> GCC warnings are designed to "report constructions that are not inherently erroneous but that are risky or suggest there may have been an error."

Certainly, but the [0] size trailing member arrays were specifically devised to
be such constructions, TBH.  So the warning in this case is like shooting
yourself in the foot...  Especially when the boundaries are well observed.