Re: [PATCH bpf] xdp: Prevent overflow in devmap_hash cost calculation for 32-bit builds

[Jakub Kicinski <jakub.kicinski@netronome.com>]

On Fri, 18 Oct 2019 11:15:39 +0200, Toke Høiland-Jørgensen wrote:
> Jakub Kicinski <jakub.kicinski@netronome.com> writes:
> 
> > On Thu, 17 Oct 2019 12:57:02 +0200, Toke Høiland-Jørgensen wrote:  
> >> Tetsuo pointed out that without an explicit cast, the cost calculation for
> >> devmap_hash type maps could overflow on 32-bit builds. This adds the
> >> missing cast.
> >> 
> >> Fixes: 6f9d451ab1a3 ("xdp: Add devmap_hash map type for looking up devices by hashed index")
> >> Reported-by: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
> >> Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
> >> ---
> >>  kernel/bpf/devmap.c | 2 +-
> >>  1 file changed, 1 insertion(+), 1 deletion(-)
> >> 
> >> diff --git a/kernel/bpf/devmap.c b/kernel/bpf/devmap.c
> >> index a0a1153da5ae..e34fac6022eb 100644
> >> --- a/kernel/bpf/devmap.c
> >> +++ b/kernel/bpf/devmap.c
> >> @@ -128,7 +128,7 @@ static int dev_map_init_map(struct bpf_dtab *dtab, union bpf_attr *attr)
> >>  
> >>  		if (!dtab->n_buckets) /* Overflow check */
> >>  			return -EINVAL;
> >> -		cost += sizeof(struct hlist_head) * dtab->n_buckets;
> >> +		cost += (u64) sizeof(struct hlist_head) * dtab->n_buckets;  
> >
> > array_size()?  
> 
> Well, array_size does this:
> 
> 	if (check_mul_overflow(a, b, &bytes))
> 		return SIZE_MAX;
> 
> However, we don't to return SIZE_MAX on overflow, we want the
> calculation itself to be done in 64 bits so it won't overflow... Or?

Note that array_size calculates on size_t, so it should be fine.
But looking at it, it seems all of this code uses the (u64) cast, 
so I guess that's fine. Clean up for another day :)