inet: Separate DSCP from ECN bits using new dscp_t type

Following my talk at LPC 2021 [1], here's a patch series whose objective is to start fixing the problems with how DSCP and ECN bits are handled in the kernel. This approach seemed to make consensus among the participants, although it implies a few behaviour changes for some corner cases of ip rule and ip route. Let's see if this consensus can survive a wider review :).

The problem to solve
====================

Currently, the networking stack generally doesn't clearly distinguish between DSCP and ECN bits. The entire DSCP+ECN bits are stored in a single u8 variable (or structure field), and each part of the stack handles them in their own way, using different macros.

This has been, and still is, the source of several bugs:

* Invalid use of RT_TOS(), leading to regression in VXLAN:
- a0dced17ad9d ("Revert "vxlan: fix tos value before xmit"")

* Other invalid uses of RT_TOS() in IPv6 code, where it doesn't make sense (RT_TOS() follows the old RFC 1349, which was never meant for IPv6):
- grep for 'ip6_make_flowinfo(RT_TOS(tos)' for several examples that need to be fixed.

* Failure to properly mask the ECN bits before doing IPv4 route
lookups, leading to returning a wrong route:
- 2e5a6266fbb1 ("netfilter: rpfilter: mask ecn bits before fib lookup"),
- 8d2b51b008c2 ("udp: mask TOS bits in udp_v4_early_demux()"),
- 21fdca22eb7d ("ipv4: Ignore ECN bits for fib lookups in fib_compute_spec_dst()"),
- 1ebf179037cb ("ipv4: Fix tos mask in inet_rtm_getroute()"),
- some more, uncommon, code paths still need to be fixed.

Also, this creates inconsistencies between subsystems:

* IPv4 routes accept tos/dsfield options that have ECN bits set, but no packet can actually match them, as their ECN bits are cleared before lookup.

* IPv4 rules accept tos/dsfield options that have the high order ECN bit set (but rejects those using the low order ECN bit). Like IPv4 routes, such rules can't match actual packets as the rule lookup is done after clearing the packets ECN bits.

* IPv6 routes accept the tos/dsfield options without any restriction, but silently ignores them.

* IPv6 rules also accept any value of tos/dsfield, but compares all 8 bits, including ECN. Therefore, a rule matching packets with a particular DSCP value stops working as soon as ECN is used (a work around is to define 4 rules for each DSCP value to match, one for each possible ECN code point).

* Modules that want to distinguish between the DSCP and ECN bits create their own local macros (Netfilter, SCTP).

What this RFC does
==================

This patch series creates a dscp_t type, meant to represent pure DSCP values, excluding ECN bits. Doing so allows to clearly separate the interpretation of DSCP and ECN bits and enables Sparse to detect invalid combinations of dscp_t with the plain u8 variables generally used to store the full TOS/Traffic Class/DS field.

Note, this patch series differs slightly from that of the original talk (slide 14 [2]). For the talk, I just cleared the ECN bits, while in this series, I do a bit-shift. This way dscp_t really represents DSCP values, as defined in RFCs. Also I've renamed the helper functions to replace "u8" by "dsfield", as I felt "u8" was ambiguous. Using "dsfield" makes it clear that dscp_t to u8 conversion isn't just a plain cast, but that a bit-shift happens and the result has the two ECN bits.

The new dscp_t type is then used to convert several field members:

* Patch 1 converts the tclass field of struct fib6_rule. It effectively forbids the use of ECN bits in the tos/dsfield option of ip -6 rule. Rules now match packets solely based on their DSCP bits, so ECN doesn't influence the result anymore. This contrasts with previous behaviour where all 8 bits of the Traffic Class field was used. It is believed this change is acceptable as matching ECN bits wasn't usable for IPv4, so only IPv6-only deployments could be depending on it (that is, it's unlikely enough that a someone uses ip6 rules matching ECN bits in production).

* Patch 2 converts the tos field of struct fib4_rule. This one too effectively forbids defining ECN bits, this time in ip -4 rule. Before that, setting ECN bit 1 was accepted, while ECN bit 0 was rejected. But even when accepted, the rule wouldn't match as the packets would normally have their ECN bits cleared while doing the rule lookup.

* Patch 3 converts the fc_tos field of struct fib_config. This is like patch 2, but for ip4 routes. Routes using a tos/dsfield option with any ECN bit set is now rejected. Before this patch, they were accepted but, as with ip4 rules, these routes couldn't match any real packet, since callers were supposed to clear their ECN bits beforehand.

* Patch 4 converts the fa_tos field of struct fib_alias. This one is pure internal u8 to dscp_t conversion. While patches 1-3 dealed with user facing consequences, this patch shouldn't have any side effect and is just there to give an overview of what such conversion patches will look like. These are quite mechanical, but imply some code churn.

Note that there's no equivalent of patch 3 for IPv6 (ip route), since the tos/dsfield option is silently ignored for IPv6 routes.

Future work
===========

This is a minimal series, the objective of this RFC is just to validate the dscp_t approach. More work will be needed to fully iron out the problem:

* Convert more internal structures to dscp_t (in particular the flowi4_tos field of struct flowi4).

* Slowly remove the uses of IPTOS_TOS_MASK, and hence RT_TOS(), in the kernel, as it clears only one of the ECN bits and the TOS is going to be masked again anyway by IPTOS_RT_MASK, which is stricter.

* Finally, start allowing high DSCP values in IPv4 routes and rules (the IPTOS_RT_MASK used in IPv4 clears the 3 high order bits of the DS field).

* Maybe find a way to warn users that use the ignored tos/dsfield option with ip -6 route.

Feedbacks welcome!

Thanks,

William

[1] LPC talk: https://linuxplumbersconf.org/event/11/contributions/943/
[2] LPC slides: https://linuxplumbersconf.org/event/11/contributions/943/attachments/901/1780/inet_tos_lpc2021.pdf

Guillaume Nault (4):
ipv6: Define dscp_t and stop taking ECN bits into account in ip6-rules
ipv4: Stop taking ECN bits into account in ip4-rules
ipv4: Reject routes specifying ECN bits in rtm_tos
ipv4: Use dscp_t in struct fib_alias

 include/net/inet_dscp.h  | 54 +++++++++++++++++++++++++++++++++++++
 include/net/ip_fib.h     |  3 ++-
 include/net/ipv6.h       |  6 +++++
 net/ipv4/fib_frontend.c  | 10 ++++++-
 net/ipv4/fib_lookup.h    |  3 ++-
 net/ipv4/fib_rules.c     | 17 ++++++------
 net/ipv4/fib_semantics.c | 14 +++++-----
 net/ipv4/fib_trie.c      | 58 +++++++++++++++++++++++-----------------
 net/ipv4/route.c         |  3 ++-
 net/ipv6/fib6_rules.c    | 18 ++++++++-----
 10 files changed, 138 insertions(+), 48 deletions(-)
 create mode 100644 include/net/inet_dscp.h

2.21.3