Re: [PATCH] netlink: add policy attribute range validation

[Johannes Berg <johannes@sipsolutions.net>]

On Wed, 2018-09-26 at 22:06 +0200, Johannes Berg wrote:
> From: Johannes Berg <johannes.berg@intel.com>
> 
> Without further bloating the policy structs, we can overload
> the `validation_data' pointer with a struct of s16 min, max
> and use those to validate ranges in NLA_{U,S}{8,16,32,64}
> attributes.
> 
> It may sound strange to validate NLA_U32 with a s16 max, but
> in many cases NLA_U32 is used for enums etc. since there's no
> size benefit in using a smaller attribute width anyway, due
> to netlink attribute alignment; in cases like that it's still
> useful, particularly when the attribute really transports an
> enum value.

That said, I did find a few places where we could benefit from a larger
type here - e.g. having a NLA_U16 that must be non-zero cannot be
represented in the policy as is, since you can't set max to 65535.

However, I don't think we want to push the policy struct to 12 bytes on
32-bit platforms? It's currently 16 bytes on 64-bit due to the pointer
(and alignment), but only 8 bytes on 32-bit.

Keeping the few places that needed this validation is unlikely to be a
larger win than the policy size increase due to the larger type.

johannes