[Bug c/79010] New: -Wlarger-than ineffective for VLAs, alloca, malloc

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79010

            Bug ID: 79010
           Summary: -Wlarger-than ineffective for VLAs, alloca, malloc
           Product: gcc
           Version: 7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: msebor at gcc dot gnu.org
  Target Milestone: ---

The -Wlarger-than is documented to "Warn whenever an object of larger than len
bytes is defined."  Setting aside the typo ("object of larger than") the option
doesn't has the advertised effect: it fails to warn when either a VLA object of
excessive size is created, or when such an object is created by a call to
alloca or malloc.  (The warning for the unnamed array is also duplicated.)

For alloca there is -Walloca-larger-than, and for malloc and other allocation
functions there is -Walloc-size-larger-than, so -Walrger-than should either be
documented to have a limited effect or the option should also enable the other
two.

Either way, the typo in the manual should be corrected and the duplicate
warning suppressed.

$ cat b.c && gcc -DN=123456 -S -Wall -Wextra -Wpedantic  -Wlarger-than=123 b.c
void sink (void*);

char a[N];

void farray (void)
{
  char a[N];
  sink (a);
}

void funnamed_array (void)
{
  sink ((char[N]){ 0 });
}

void fvla (void)
{
  int n = N;
  char a[n];
  sink (a);
}

void falloca (void)
{
  void *a = __builtin_alloca (N);
  sink (a);
}

void fmalloc (void)
{
  void *a = __builtin_malloc (N);
  sink (a);
}
b.c:3:6: warning: size of ‘a’ is 123456 bytes [-Wlarger-than=]
 char a[N];
      ^
b.c: In function ‘farray’:
b.c:7:8: warning: size of ‘a’ is 123456 bytes [-Wlarger-than=]
   char a[N];
        ^
b.c: In function ‘funnamed_array’:
b.c:13:18: warning: size of ‘({anonymous})’ is 123456 bytes [-Wlarger-than=]
   sink ((char[N]){ 0 });
                  ^
b.c:13:18: warning: size of ‘({anonymous})’ is 123456 bytes [-Wlarger-than=]