[Bug middle-end/102810] [11/12 Regression] Bogus Wstringop-overread passing a smaller array to an array parameter without a bound

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102810

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |msebor at gcc dot gnu.org
     Ever confirmed|0                           |1
   Last reconfirmed|                            |2021-10-18
          Component|c                           |middle-end
            Summary|Bogus Wstringop-overread    |[11/12 Regression] Bogus
                   |warning when special        |Wstringop-overread passing
                   |(integer) pointer values    |a smaller array to an array
                   |passed to array parameter   |parameter without a bound
                   |of a function               |
             Status|UNCONFIRMED                 |NEW

--- Comment #1 from Martin Sebor <msebor at gcc dot gnu.org> ---
GCC issues the -Wstringop-xxx warnings in this context only because there isn't
a more appropriate option yet.  One should be added.

The warning for argument 2 is a bug.  With -Warray-parameter enabled, for the
purposes of out-of-bounds access detection, GCC treats function parameters
declared using the array form (as in void f (int a[2]);) as an indication that
the function expects an array argument with at least as many elements.  A bug
in the code applies the same logic to an array parameter declared with no
bounds, as in the example.  I confirm this report for this problem.

With the following snippet, a read access warning should only be expected for
the third argument:

extern int foo(const int *a, const int b[], const int c[1]);

int main (void)
{
  foo ((int*)2, (int*)2, (int*2));
}

The warning in this instance is issued because functions that take const array
parameters with non-zero bound are assumed to read as many elements from the
parameters as the bound indicates.  Because (int*)2 is not a pointer to an
array with at least two elements (or a valid pointer at all), the warning
triggers.

(Note that using invalid pointers like (int*)2 in any expression, including
assigning them to function parameters, is undefined and may be diagnosed in the
future regardless of the context they're used in, including in in arguments 1
and 2 above.)