Re: [PATCH bpf-next v2 1/2] xsk: store struct xdp_sock as a flexible array member of the XSKMAP

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

On Mon, 28 Oct 2019 23:11:50 +0100, Björn Töpel wrote:
> On Mon, 28 Oct 2019 at 18:55, Jakub Kicinski
> <jakub.kicinski@netronome.com> wrote:
> >
> > On Fri, 25 Oct 2019 09:18:40 +0200, Björn Töpel wrote:  
> > > From: Björn Töpel <bjorn.topel@intel.com>
> > >
> > > Prior this commit, the array storing XDP socket instances were stored
> > > in a separate allocated array of the XSKMAP. Now, we store the sockets
> > > as a flexible array member in a similar fashion as the arraymap. Doing
> > > so, we do less pointer chasing in the lookup.
> > >
> > > Signed-off-by: Björn Töpel <bjorn.topel@intel.com>  
> >
> > Thanks for the re-spin.
> >  
> > > diff --git a/kernel/bpf/xskmap.c b/kernel/bpf/xskmap.c
> > > index 82a1ffe15dfa..a83e92fe2971 100644
> > > --- a/kernel/bpf/xskmap.c
> > > +++ b/kernel/bpf/xskmap.c  
> >  
> > > @@ -92,44 +93,35 @@ static struct bpf_map *xsk_map_alloc(union bpf_attr *attr)
> > >           attr->map_flags & ~(BPF_F_NUMA_NODE | BPF_F_RDONLY | BPF_F_WRONLY))
> > >               return ERR_PTR(-EINVAL);
> > >
> > > -     m = kzalloc(sizeof(*m), GFP_USER);
> > > -     if (!m)
> > > +     numa_node = bpf_map_attr_numa_node(attr);
> > > +     size = struct_size(m, xsk_map, attr->max_entries);
> > > +     cost = size + array_size(sizeof(*m->flush_list), num_possible_cpus());  
> >
> > Now we didn't use array_size() previously because the sum here may
> > overflow.
> >
> > We could use __ab_c_size() here, the name is probably too ugly to use
> > directly and IDK what we'd have to name such a accumulation helper...
> >
> > So maybe just make cost and size a u64 and we should be in the clear.
> >  
> 
> Hmm, but both:
>   int bpf_map_charge_init(struct bpf_map_memory *mem, size_t size);
>   void *bpf_map_area_alloc(size_t size, int numa_node);
> pass size as size_t, so casting to u64 doesn't really help on 32-bit
> systems, right?

Yup :( IOW looks like the overflows will not be caught on 32bit
machines in all existing code that does the (u64) cast. I hope 
I'm wrong there.

> Wdyt about simply adding:
>   if (cost < size)
>     return ERR_PTR(-EINVAL)
> after the cost calculation for explicit overflow checking?

We'd need that for all users of these helpers. Could it perhaps makes
sense to pass "alloc_size" and "extra_cost" as separate size_t to
bpf_map_charge_init() and then we can do the overflow checking there,
centrally?

We can probably get rid of all the u64 casting too at that point,
and use standard overflow helpers, yuppie :)

> So, if size's struct_size overflows, the allocation will fail.
> And we'll catch the cost overflow with the if-statement, no?
> 
> Another option is changing the size_t in bpf_map_... to u64. Maybe
> that's better, since arraymap and devmap uses u64 for cost/size.