[PATCH v2] mm: module_alloc: check if size is 0

Andrew Morton
2013-06-27T22:23:35Z

On Thu, 27 Jun 2013 11:39:17 +0200 Ralf Baechle <ralf@linux-mips.org> wrote:

> Imho de7d2b567d040e3b67fe7121945982f14343213d [mm/vmalloc.c: report more
> vmalloc failures] is overly strict in that it also reports zero-sized
> allocations. I consider such allocations stupid but legitimiate and often
> better preferrable over having to scatter checks for zero size all over
> place. So maybe something like below patch?
>
> ...
>
> --- a/mm/vmalloc.c
> +++ b/mm/vmalloc.c
> @@ -1679,7 +1679,10 @@ void *__vmalloc_node_range(unsigned long size, unsigned long align,
> unsigned long real_size = size;
>
> size = PAGE_ALIGN(size);
> - if (!size || (size >> PAGE_SHIFT) > totalram_pages)
> + if (unlikely(!size))
> + return NULL;
> +
> + if ((size >> PAGE_SHIFT) > totalram_pages)
> goto fail;
>
> area = __get_vm_area_node(size, align, VM_ALLOC | VM_UNLIST,
> @@ -1711,6 +1714,7 @@ fail:
> warn_alloc_failed(gfp_mask, 0,
> "vmalloc: allocation failure: %lu bytes\n",
> real_size);
> +
> return NULL;
> }

If the caller actually dereferences the returned pointer the kernel
will go oops, which should provide adequate notification of a
programming error ;) But all callers should be checking the return
value. So I worry about the by-far-most-common case where code does

size = some_screwed_up_calculation();
p = vmalloc(size);
if (!p)
return -ENOMEM;

So the mistake gets propagated back to who-knows-where as memory
exhaustion and thereby becomes a lot harder to diagnose.

How many callsites really truly need to be edited to avoid the warning?

Veli-Pekka's original patch would be neater if we were to add a new

void *__vmalloc_node_range_zero_size_ok(<args>)
{
if (size == 0)
return NULL;
return __vmalloc_node_range(<args>);
}

(with a better name than __vmalloc_node_range_zero_size_ok!)