Guillaume Nault gnault@redhat.com writes:
The networking stack currently doesn't clearly distinguish between DSCP and ECN bits. The entire DSCP+ECN bits are stored in u8 variables (or structure fields), and each part of the stack handles them in their own way, using different macros. This has created several bugs in the past and some uncommon code paths are still unfixed.
Such bugs generally manifest by selecting invalid routes because of ECN bits interfering with FIB routes and rules lookups (more details in the LPC 2021 talk[1] and in the RFC of this series[2]).
This patch series aims at preventing the introduction of such bugs (and detecting existing ones), by introducing a dscp_t type, representing "sanitised" DSCP values (that is, with no ECN information), as opposed to plain u8 values that contain both DSCP and ECN information. dscp_t makes it clear for the reader what we're working on, and Sparse can flag invalid interactions between dscp_t and plain u8.
This series converts only a few variables and structures:
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 any more. This contrasts with the previous behaviour where all 8 bits of the Traffic Class field were used. It is believed that this change is acceptable as matching ECN bits wasn't usable for IPv4, so only IPv6-only deployments could be depending on it. Also the previous behaviour made DSCP-based ip6-rules fail for packets with both a DSCP and an ECN mark, which is another reason why any such deploy is unlikely.
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 would never match, as the packets would have their ECN bits cleared before doing the rule lookup.
Patch 3 converts the fc_tos field of struct fib_config. This is equivalent to patch 2, but for IPv4 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 packet, since their ECN bits are cleared before the lookup.
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 had user facing consequences, this patch shouldn't have any side effect and is there to give an overview of what future conversion patches will look like. Conversions are quite mechanical, but imply some code churn, which is the price for the extra clarity a possibility of type checking.
To summarise, all the behaviour changes required for the dscp_t type approach to work should be contained in patches 1-3. These changes are edge cases of ip-route and ip-rule that don't currently work properly. So they should be safe. Also, a kernel selftest is added for each of them.
Finally, this work also paves the way for allowing the usage of the 3 high order DSCP bits in IPv4 (a few call paths already handle them, but in general the stack clears them before IPv4 rule and route lookups).
LGTM; thanks again for doing this!
Reviewed-by: Toke Høiland-Jørgensen toke@redhat.com