Re: [PATCH v2 7/8] net: Rename TCA*BPF_DIGEST to ..._SHA256
From: Andy Lutomirski <luto@amacapital.net>
Date: 2017-01-11 18:20:13
Also in:
linux-crypto, lkml
On Wed, Jan 11, 2017 at 1:09 AM, Daniel Borkmann [off-list ref] wrote:
Hi Andy, On 01/11/2017 04:11 AM, Andy Lutomirski wrote:quoted
On Tue, Jan 10, 2017 at 4:50 PM, Daniel Borkmann [off-list ref] wrote:quoted
On 01/11/2017 12:24 AM, Andy Lutomirski wrote:quoted
This makes it easier to add another digest algorithm down the road if needed. It also serves to force any programs that might have been written against a kernel that had the old field name to notice the change and make any necessary changes. This shouldn't violate any stable API policies, as no released kernel has ever had TCA*BPF_DIGEST.Imho, this and patch 6/8 is not really needed. Should there ever another digest alg be used (doubt it), then you'd need a new nl attribute and fdinfo line anyway to keep existing stuff intact. Nobody made the claim that you can just change this underneath and not respecting abi for existing applications when I read from above that such apps now will get "forced" to notice a change.Fair enough. I was more concerned about prerelease iproute2 versions, but maybe that's a nonissue. I'll drop these two patches.Ok. Sleeping over this a bit, how about a general rename into "prog_tag" for fdinfo and TCA_BPF_TAG resp. TCA_ACT_BPF_TAG for the netlink attributes, fwiw, it might reduce any assumptions on this being made? If this would be preferable, I could cook that patch against -net for renaming it?
That would be fine with me. I think there are two reasonable approaches to computing the actual tag. 1. Use a standard, modern cryptographic hash. SHA-256, SHA-512, Blake2b, whatever. SHA-1 is a bad choice in part because it's partly broken and in part because the implementation in lib/ is a real mess to use (as you noticed while writing the code). 2. Use whatever algorithm you like but make the tag so short that it's obviously not collision-free. 48 or 64 bits is probably reasonable. The intermediate versions are just asking for trouble. Alexei wants to make the tag shorter, but I admit I still don't understand why he prefers that over using a better crypto hash and letting user code truncate the tag if it wants.