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DSA sets up a switch tree little by little. Every switch of the N
members of the tree calls dsa_register_switch, and (N - 1) will just
touch the dst->ports list with their ports and quickly exit. Only the
last switch that calls dsa_register_switch will find all DSA links
complete in dsa_tree_setup_routing_table, and not return zero as a
result but instead go ahead and set up the entire DSA switch tree
(practically on behalf of the other switches too).
The trouble is that the (N - 1) switches don't clean up after themselves
after they get an error such as EPROBE_DEFER. Their footprint left in
dst->ports by dsa_switch_touch_ports is still there. And switch N, the
one responsible with actually setting up the tree, is going to work with
those stale dp, dp->ds and dp->ds->dev pointers. In particular ds and
ds->dev might get freed by the device driver.
Be there a 2-switch tree and the following calling order:
- Switch 1 calls dsa_register_switch
- Calls dsa_switch_touch_ports, populates dst->ports
- Calls dsa_port_parse_cpu, gets -EPROBE_DEFER, exits.
- Switch 2 calls dsa_register_switch
- Calls dsa_switch_touch_ports, populates dst->ports
- Probe doesn't get deferred, so it goes ahead.
- Calls dsa_tree_setup_routing_table, which returns "complete == true"
due to Switch 1 having called dsa_switch_touch_ports before.
- Because the DSA links are complete, it calls dsa_tree_setup_switches
now.
- dsa_tree_setup_switches iterates through dst->ports, initializing
the Switch 1 ds structure (invalid) and the Switch 2 ds structure
(valid).
- Undefined behavior (use after free, sometimes NULL pointers, etc).
Real example below (debugging prints added by me, as well as guards
against NULL pointers):
[ 5.477947] dsa_tree_setup_switches: Setting up port 0 of switch ffffff803df0b980 (dev ffffff803f775c00)
[ 6.313002] dsa_tree_setup_switches: Setting up port 1 of switch ffffff803df0b980 (dev ffffff803f775c00)
[ 6.319932] dsa_tree_setup_switches: Setting up port 2 of switch ffffff803df0b980 (dev ffffff803f775c00)
[ 6.329693] dsa_tree_setup_switches: Setting up port 3 of switch ffffff803df0b980 (dev ffffff803f775c00)
[ 6.339458] dsa_tree_setup_switches: Setting up port 4 of switch ffffff803df0b980 (dev ffffff803f775c00)
[ 6.349226] dsa_tree_setup_switches: Setting up port 5 of switch ffffff803df0b980 (dev ffffff803f775c00)
[ 6.358991] dsa_tree_setup_switches: Setting up port 6 of switch ffffff803df0b980 (dev ffffff803f775c00)
[ 6.368758] dsa_tree_setup_switches: Setting up port 7 of switch ffffff803df0b980 (dev ffffff803f775c00)
[ 6.378524] dsa_tree_setup_switches: Setting up port 8 of switch ffffff803df0b980 (dev ffffff803f775c00)
[ 6.388291] dsa_tree_setup_switches: Setting up port 9 of switch ffffff803df0b980 (dev ffffff803f775c00)
[ 6.398057] dsa_tree_setup_switches: Setting up port 10 of switch ffffff803df0b980 (dev ffffff803f775c00)
[ 6.407912] dsa_tree_setup_switches: Setting up port 0 of switch ffffff803da02f80 (dev 0000000000000000)
[ 6.417682] dsa_tree_setup_switches: Setting up port 1 of switch ffffff803da02f80 (dev 0000000000000000)
[ 6.427446] dsa_tree_setup_switches: Setting up port 2 of switch ffffff803da02f80 (dev 0000000000000000)
[ 6.437212] dsa_tree_setup_switches: Setting up port 3 of switch ffffff803da02f80 (dev 0000000000000000)
[ 6.446979] dsa_tree_setup_switches: Setting up port 4 of switch ffffff803da02f80 (dev 0000000000000000)
[ 6.456744] dsa_tree_setup_switches: Setting up port 5 of switch ffffff803da02f80 (dev 0000000000000000)
[ 6.466512] dsa_tree_setup_switches: Setting up port 6 of switch ffffff803da02f80 (dev 0000000000000000)
[ 6.476277] dsa_tree_setup_switches: Setting up port 7 of switch ffffff803da02f80 (dev 0000000000000000)
[ 6.486043] dsa_tree_setup_switches: Setting up port 8 of switch ffffff803da02f80 (dev 0000000000000000)
[ 6.495810] dsa_tree_setup_switches: Setting up port 9 of switch ffffff803da02f80 (dev 0000000000000000)
[ 6.505577] dsa_tree_setup_switches: Setting up port 10 of switch ffffff803da02f80 (dev 0000000000000000)
[ 6.515433] dsa_tree_setup_switches: Setting up port 0 of switch ffffff803db15b80 (dev ffffff803d8e4800)
[ 7.354120] dsa_tree_setup_switches: Setting up port 1 of switch ffffff803db15b80 (dev ffffff803d8e4800)
[ 7.361045] dsa_tree_setup_switches: Setting up port 2 of switch ffffff803db15b80 (dev ffffff803d8e4800)
[ 7.370805] dsa_tree_setup_switches: Setting up port 3 of switch ffffff803db15b80 (dev ffffff803d8e4800)
[ 7.380571] dsa_tree_setup_switches: Setting up port 4 of switch ffffff803db15b80 (dev ffffff803d8e4800)
[ 7.390337] dsa_tree_setup_switches: Setting up port 5 of switch ffffff803db15b80 (dev ffffff803d8e4800)
[ 7.400104] dsa_tree_setup_switches: Setting up port 6 of switch ffffff803db15b80 (dev ffffff803d8e4800)
[ 7.409872] dsa_tree_setup_switches: Setting up port 7 of switch ffffff803db15b80 (dev ffffff803d8e4800)
[ 7.419637] dsa_tree_setup_switches: Setting up port 8 of switch ffffff803db15b80 (dev ffffff803d8e4800)
[ 7.429403] dsa_tree_setup_switches: Setting up port 9 of switch ffffff803db15b80 (dev ffffff803d8e4800)
[ 7.439169] dsa_tree_setup_switches: Setting up port 10 of switch ffffff803db15b80 (dev ffffff803d8e4800)
The solution is to recognize that the functions that call
dsa_switch_touch_ports (dsa_switch_parse_of, dsa_switch_parse) have side
effects, and therefore one should clean up their side effects on error
path. The cleanup of dst->ports was taken from dsa_switch_remove and
moved into a dedicated dsa_switch_release_ports function, which should
really be per-switch (free only the members of dst->ports that are also
members of ds, instead of all switch ports).
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
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Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.