Re: [RFC PATCH] mm/slab: Add size validation in kmalloc_array_* functions

[Harry Yoo <harry.yoo@oracle.com>]

On Tue, Sep 23, 2025 at 10:41:39AM +0530, viswanath wrote:
> On Mon, 22 Sept 2025 at 23:30, Harry Yoo <harry.yoo@oracle.com> wrote:
> 
> > When bytes > KKMALLOC_MAX_SIZE (8K on my system), kmalloc redirects allocation
> > to the buddy allocator, which can allocate up to (PAGE_SIZE << MAX_PAGE_ORDER)
> > bytes (4M on my system).
> 
> In include/linux/slab.h,
> KMALLOC_MAX_SIZE is ultimately defined as PAGE_SIZE << MAX_PAGE_ORDER and
> KMALLOC_MAX_CACHE_SIZE as PAGE_SIZE << 1
> 
> I was using those definitions

Err, you're right :) you mentioned KMALLOC_MAX_SIZE, not
KMALLOC_MAX_CACHE_SIZE. Apologies for the confusion.

> > Because allocating a page with order > MAX_PAGE_ORDER page is never
> > supposed to succeed, the caller of kmalloc should be fixed rather than
> > kmalloc itself.
> 
> So, Is it almost never a good idea to add new validation in the allocator code?

Yes, because such allocations will always fail and that's never a good
thing.

> > I think the right fix should be to return -EINVAL in max_vclocks_store()
> > if max * sizeof(int) exceeds PAGE_SIZE << MAX_PAGE_ORDER?
> 
> Thanks, I will go ahead with this approach
> 
> Thanks,
> Viswanath

-- 
Cheers,
Harry / Hyeonggon