Re: [RFC][PATCH 0/4] slab: Allow for type introspection during allocation

Roman Gushchin
2024-07-09T16:57:47Z

On Mon, Jul 08, 2024 at 12:18:34PM -0700, Kees Cook wrote:
> Hi,
>
> This is an RFC for some changes I'd like to make to the kernel's
> allocators (starting with slab) that allow for type introspection, which
> has been a long-time gap in potential analysis capabilities available
> at compile-time. The changes here are just a "first step" example that
> updates kmalloc() and kzalloc() to show what I'm thinking we can do,
> and shows an example conversion within the fs/pstore tree.
>
> Repeating patch 3's commit log here:
>
> There is currently no way for the slab to know what type is being
> allocated, and this hampers the development of any logic that would need
> this information including basic type checking, alignment need analysis,
> etc.
>
> Allow the size argument to optionally be a variable, from which the
> type (and there by the size, alignment, or any other features) can be
> determined at compile-time. This allows for the incremental replacement
> of the classic code pattern:
>
> obj = kmalloc(sizeof(*obj), gfp);
>
> into:
>
> obj = kmalloc(obj, gfp);
>
> As an additional build-time safety feature, the return value of kmalloc()
> also becomes typed so that the assignment and first argument cannot drift,
> doing away with the other, more fragile, classic code pattern:
>
> obj = kmalloc(sizeof(struct the_object), gfp);
>
> into:
>
> obj = kmalloc(obj, gfp);

I like the idea, however it's not as simple and straightforward because
it's common for structures to have a variable part (usually at the end)
and also allocate more than one structure at once.

There are many allocations which look like
kmalloc(sizeof(my_struct) * 2 + SOME_MAGIC_LENGTH, GFP_...)
or something like this, which you can't easily convert to your scheme.

The only option I see is to introduce the new set of functions/macros,
something like kmalloc_obj() or kmalloc_struct(). Or maybe tmalloc()?
(t for typed)

Thanks!