Re: [PATCH bpf v3] net/xdp: fix zero-size allocation warning in xskq_create()

[Martin KaFai Lau <martin.lau@linux.dev>]

On 10/6/23 4:24 PM, Andrew Kanner wrote:
>> Thanks for the explanation, so iiuc it means it will overflow the
>> struct_size() first because of the is_power_of_2(nentries) requirement?
>> Could you help adding some comment to explain? Thanks.
>>
> The overflow happens because there's no upper limit for nentries
> (userspace input). Let me add more context, e.g. from net/xdp/xsk.c:
> 
> static int xsk_setsockopt(struct socket *sock, int level, int optname,
>                            sockptr_t optval, unsigned int optlen)
> {
> [...]
>                  if (copy_from_sockptr(&entries, optval, sizeof(entries)))
>                          return -EFAULT;
> [...]
>                  err = xsk_init_queue(entries, q, false);
> [...]
> }
> 
> 'entries' is passed to xsk_init_queue() and there're 2 checks: for 0
> and is_power_of_2() only, no upper bound check:
> 
> static int xsk_init_queue(u32 entries, struct xsk_queue **queue,
>                            bool umem_queue)
> {
>          struct xsk_queue *q;
> 
>          if (entries == 0 || *queue || !is_power_of_2(entries))
>                  return -EINVAL;
> 
>          q = xskq_create(entries, umem_queue);
>          if (!q)
>                  return -ENOMEM;
> [...]
> }
> 
> The 'entries' value is next passed to struct_size() in
> net/xdp/xsk_queue.c. If it's large enough - SIZE_MAX will be returned.

All make sense. I was mostly asking to add a comment at the "if (unlikely(size 
== SIZE_MAX)" check to explain this details on why checking SIZE_MAX is enough.