[Bug middle-end/94004] New: missing -Walloca on calls to alloca due to -Wno-system-headers

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94004

            Bug ID: 94004
           Summary: missing -Walloca on calls to alloca due to
                    -Wno-system-headers
           Product: gcc
           Version: 10.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: middle-end
          Assignee: unassigned at gcc dot gnu.org
          Reporter: msebor at gcc dot gnu.org
  Target Milestone: ---

When alloca is a macro defined in a system header like <alloca.h> (for example
in Glibc), the -Walloca and -Walloca-larger-than= don't point out offending
calls to alloca unless -Wsystem-headers is also set.

$ cat t.c && gcc -S -O2 -Walloca-larger-than=8 t.c

#include <alloca.h>

void f (void*, ...);

void g (int n)
{
  if (n < 88888)
    n = 88888;
  char *p = (char*) __builtin_alloca (n);   // warning (good)
  f (p, 0);
} 

void h (int n)
{
  if (n < 99999)
    n = 99999;
  char *p = (char*) alloca (n);   // missing warning
  f (p, 1);
}
t.c: In function ‘g’:
t.c:9:21: warning: argument to ‘alloca’ may be too large
[-Walloca-larger-than=]
    9 |   char *p = (char*) __builtin_alloca (n);
      |                     ^~~~~~~~~~~~~~~~~~~~
t.c:9:21: note: limit is 8 bytes, but argument may be as large as 2147483647


When -Wsystem-headers is also set on the command line GCC diagnoses both calls
as expected:

t.c:9:21: warning: argument to ‘alloca’ may be too large
[-Walloca-larger-than=]
    9 |   char *p = (char*) __builtin_alloca (n);
      |                     ^~~~~~~~~~~~~~~~~~~~
t.c:9:21: note: limit is 8 bytes, but argument may be as large as 2147483647
In file included from t.c:1:
t.c: In function ‘h’:
t.c:17:21: warning: argument to ‘alloca’ may be too large
[-Walloca-larger-than=]
   17 |   char *p = (char*) alloca (n);
      |                     ^~~~~~
t.c:17:21: note: limit is 8 bytes, but argument may be as large as 2147483647