[PING][PATCH] extend -Wstringop-overflow to allocated objects (PR 91582)

[Martin Sebor <msebor@gmail.com>]

Ping: https://gcc.gnu.org/ml/gcc-patches/2019-11/msg00652.html

On 11/8/19 3:11 PM, Martin Sebor wrote:
> Unless it's used with _FORTIFY_SOURCE, -Wstringop-overflow
> doesn't consider out-of-bounds accesses to objects allocated
> by alloca, malloc, other functions declared with attribute
> alloc_size, or even VLAs with variable bounds.  This was
> a known limitation of the checks (done just before expansion)
> relying on the the object size pass when they were introduced
> in GCC 7.
> 
> But since its introduction in GCC 7, the warning has evolved
> beyond some of the limitations of the object size pass.  Unlike
> it, the warning considers non-constant offsets and stores with
> non-constant sizes.  Attached is a simple enhancement that
> (finally) adds the ability to also detect overflow in allocated
> objects to the warning.
> 
> With the patch GCC detects the overflow in code like this:
> 
>    char* f (void)
>    {
>      char s[] = "12345";
>      char *p = malloc (strlen (s));
>      strcpy (p, s);   // warning here
>      return p;
>    }
> 
> but not (yet) in something like this:
> 
>    char* g (const char *s)
>    {
>      char *p = malloc (strlen (s));
>      strcpy (p, s);   // no warning (yet)
>      return p;
>    }
> 
> and quite a few other examples.  Doing better requires extending
> the strlen pass.  I'm working on this extension and expect to
> submit a patch before stage 1 ends.
> 
> Martin
> 
> PS I was originally planning to do all the allocation checking
> in the strlen pass but it occurred to me that by also enhancing
> the compute_objsize function, all warnings that use it will
> benefit.  Besides -Wstringop-overflow this includes a subset
> of -Warray-bounds, -Wformat-overflow, and -Wrestrict.  It's
> nice when a small enhancement has such a broad positive effect.