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

[Roman Gushchin]

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!