Re: Alignment in BPF verifier

[David Miller <davem@davemloft.net>]

From: Edward Cree <ecree@solarflare.com>
Date: Fri, 19 May 2017 21:00:13 +0100

> Well, I've managed to get somewhat confused by reg->id.
> In particular, I'm unsure which bpf_reg_types can have an id, and what
>  exactly it means.  There seems to be some code that checks around map value
>  pointers, which seems strange as maps have fixed sizes (and the comments in
>  enum bpf_reg_type make it seem like id is a PTR_TO_PACKET thing) - is this
>  maybe because of map-of-maps support, can the contained maps have differing
>  element sizes?  Or do we allow *(map_value + var + imm), if map_value + var
>  was appropriately bounds-checked?
> 
> Does the 'id' identify the variable that was added to an object pointer, or
>  the object itself?  Or does it blur these and identify (what the comment in
>  enum bpf_reg_type calls) "skb->data + (u16) var"?

The reg->id value changes any time a variable gets added to a packet
pointer.

You will also notice right now that only packet pointers have their
alignment tracked.

I have changes pending that will do that for MAP pointers too, but
it needs more work.