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

On Mon, 28 Oct 2019 23:11:50 +0100, Björn Töpel wrote:

On Mon, 28 Oct 2019 at 18:55, Jakub Kicinski
[off-list ref] wrote:

quoted

On Fri, 25 Oct 2019 09:18:40 +0200, Björn Töpel wrote:  

quoted

From: Björn Töpel <redacted>

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 <redacted>  

Thanks for the re-spin.
 

quoted

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  

quoted

@@ -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.