netdev - Re: [PATCH net-next 1/4] net: mpls: Convert number of nexthops to u8

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:   Mon, 27 Mar 2017 08:43:05 -0600
From:   David Ahern <dsa@...ulusnetworks.com>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     netdev@...r.kernel.org, roopa@...ulusnetworks.com,
        rshearma@...cade.com
Subject: Re: [PATCH net-next 1/4] net: mpls: Convert number of nexthops to u8

On 3/26/17 9:11 PM, Eric W. Biederman wrote:
> I don't like this.  Byte writes don't exist on all architectures.
> 
> So while I think always writing to rtn_nhn_alive under the
> rtn_lock ensures that we don't have wrong values written
> it is quite subtle.  And I don't know how this will interact with other
> fields that you are introducing.
> 
> AKA this might be ok, but I expect this formulation of the code
> will easily bit-rot and break.

net/ has other use cases -- e.g., ipv6 tunneling has proto as a u8.

It unrealistic for a route to have 255 or more nexthops so the point of
this patch is to not waste 8 bytes tracking it - especially when
removing it gets routes with ipv4 and ipv6 via's into a cache line.

I can make the alive counter a u16 without increasing the size of the
struct. I'd prefer to leave it as an u8 to have a u8 hole for flags
should something be needed later.