Re: [PATCH v2 4/7] slab: Add __alloc_size attributes for better bounds checking

Rasmus Villemoes
2021-08-19T08:27:39Z

On 18/08/2021 23.40, Kees Cook wrote:
> As already done in GrapheneOS, add the __alloc_size attribute for
> regular kmalloc interfaces, to provide additional hinting for better
> bounds checking, assisting CONFIG_FORTIFY_SOURCE and other compiler
> optimizations.
>

> #ifdef CONFIG_NUMA
> +__alloc_size(1)
> void *__kmalloc_node(size_t size, gfp_t flags, int node) __assume_slab_alignment __malloc;

Eh, can we keep all the attributes together instead of having some
before, some after?

I don't necessarily think this is a good idea, but just throwing it out
there: __alloc_size almost always goes along with __malloc, so one could
define __alloc_size in such a way that it implies __malloc, then just
have a "raw" ____alloc_size version to use for krealloc() and similar.
But I guess it's cleaner to keep it this way.

While declared in string.h, kmemdup() is also eligible for alloc_size(2).

Which brings me to an old wishlist item of mine [it's almost christmas]:
that alloc_size could understand more general expressions for the size
of the returned memory, not just the primitive one based on
malloc()/calloc() prototypes. So e.g. kmemdup_nul() returns something of
size $2+1, while it is also very common to have a alloc_foo(void) helper
which returns something of size sizeof(struct foo). Unfortunately I
don't think gcc's attribute parsing machinery can easily be tweaked into
accepting

struct bar *alloc_bars(unsigned count) __new_a_s(count * sizeof(struct bar))

but maybe clang could. If a compiler could understand that kind of
attribute, it would also pave the way for implementing
__attribute__((__buffer_size__(param, size, access)))

e.g.

memchr(src, c, size) __buffer_size(src, size, "r")

clk_bulk_get(struct device *dev, int num_clks, struct clk_bulk_data
*clks) __buffer_size(clks, num_clks * sizeof(*clks), "rw")

which could be used for both static analysis and optional run-time
instrumentation.

Rasmus