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Guillaume Nault says:
====================
inet: Separate DSCP from ECN bits using new dscp_t type
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).
References:
[1] LPC 2021 talk:
- https://linuxplumbersconf.org/event/11/contributions/943/
- Direct link to slide deck:
https://linuxplumbersconf.org/event/11/contributions/943/attachments/901/1780/inet_tos_lpc2021.pdf
[2] RFC version of this series:
- https://lore.kernel.org/netdev/cover.1638814614.git.gnault@redhat.com/
====================
Link: https://lore.kernel.org/r/cover.1643981839.git.gnault@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
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