[Bug 2125448] [NEW] Burst parameter of tc police only allows 4GB while the kernel can support much more

[Launchpad Bug Tracker]

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[Impact]
The tc police burst option limits the size of the burst up to 4 GiB, i.e., UINT_MAX for a 32 bit unsigned int. However, the underlying implementation of burst is computed as "time at the specified rate", and for higher rates, a burst size exceeding 4 GiB is feasible without modification to the kernel.
Conversely, there can be bursts smaller that 4 GiB that overflow the burst time calculation and that was not detected.

[Test Plan]
All the following commands must be run as root. We are assuming the tested interface is eth0.

Add fq_codel queuing discipline

# tc qdisc add dev eth0 root fq_codel

Check the assigned discipline number

# tc -s qdisc show dev eth0
qdisc fq_codel nnnn: root refcnt 13 limit ...

Note the 4 digits nnnn number. It should be substituted for the nnnn
number in the rest of the following commands.

1) Tests before the patch is applied

Try to set a filter with a burst of exactly 4Gb at 10Gbit rate and see
that it fails:

# tc filter add dev eth0 protocol ip prio 1 parent nnnn: handle 1 fw police rate 10Gbit peakrate 10Gbit burst 4294967296 mtu 64Kb conform-exceed reclassify
Error: argument "4294967296" is wrong: buffer

Try to set a filter with a burst that is greater than the maximum time:

# tc filter add dev eth0 protocol ip prio 1 parent nnnn: handle 1 fw
police rate 1Mbit peakrate 10Gbit burst 34375000 mtu 64Kb conform-exceed
reclassify

Check if it was set correctly:

# tc -raw filter get dev eth0 protocol ip prio 1 parent nnnn: handle 1 fw
filter parent nnnn: protocol ip pref 1 fw chain 0 handle 0x1  police 0x1 rate 1Mbit burst 15261b mtu 64Kb [001d1bf8] peakrate 10Gbit action reclassify overhead 0b

We can see a result with the overflowed buffer (burst is 15261b)

2) Tests after the patch is applied:

Try to set a filter with a burst of exactly 4Gb at 10Gbit rate:

# tc filter add dev eth0 protocol ip prio 1 parent nnnn: handle 1 fw
police rate 10Gbit peakrate 10Gbit burst 4294967296 mtu 64Kb conform-
exceed reclassify

We can check that it was set correctly (burst is 4Gb):

# tc -raw filter get dev eth0 protocol ip prio 1 parent nnnn: handle 1 fw
filter parent nnnn: protocol ip pref 1 fw chain 0 handle 0x1  police 0x1 rate 10Gbit burst 4Gb mtu 64Kb [03333334] peakrate 10Gbit action reclassify overhead 0b

Remove the filter:

# tc filter del dev eth0 protocol ip prio 1 parent nnnn:

Try to set a filter with a burst that is greater than the maximum time:

# tc filter add dev eth0 protocol ip prio 1 parent nnnn: handle 1 fw police rate 1Mbit peakrate 10Gbit burst 34375000 mtu 64Kb conform-exceed reclassify
POLICE: burst out of range

We see that it is out of range

[Where problems could occur]
Externally the only difference with the installed patch is that some commands that will previously overflow without errors will now fail. That might break some scripts but it can be argued that they were already broken before.

** Affects: iproute2 (Ubuntu)
     Importance: Undecided
     Assignee: Jorge Merlino (jorge-merlino)
         Status: In Progress

** Affects: iproute2 (Ubuntu Jammy)
     Importance: Undecided
         Status: New

** Affects: iproute2 (Ubuntu Noble)
     Importance: Undecided
         Status: New

** Affects: iproute2 (Ubuntu Plucky)
     Importance: Undecided
         Status: New

** Affects: iproute2 (Ubuntu Questing)
     Importance: Undecided
     Assignee: Jorge Merlino (jorge-merlino)
         Status: In Progress


** Tags: patch
-- 
Burst parameter of tc police only allows 4GB while the kernel can support much more
https://bugs.launchpad.net/bugs/2125448
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