[DPDK/core Bug 1970] fib: trie leaks a tbl8 group and can corrupt the dataplane when a tbl8 allocation fails during insert

[bugzilla@dpdk.org]

http://bugs.dpdk.org/show_bug.cgi?id=1970

            Bug ID: 1970
           Summary: fib: trie leaks a tbl8 group and can corrupt the
                    dataplane  when a tbl8 allocation fails during insert
           Product: DPDK
           Version: unspecified
          Hardware: All
                OS: All
            Status: UNCONFIRMED
          Severity: normal
          Priority: Normal
         Component: core
          Assignee: dev@dpdk.org
          Reporter: maxime@leroys.fr
  Target Milestone: ---

Created attachment 356
  --> http://bugs.dpdk.org/attachment.cgi?id=356&action=edit
Patch to reproduce the bug

On a full or tight tbl8 pool, an add whose install runs out of tbl8 groups
mid-way
is not rolled back: the group(s) already taken are leaked and the tbl24 entry
already written is left dangling.
trie_modify() undoes only the RIB node (rte_rib6_remove), never the dataplane
or the tbl8 pool. A later, valid add is then wrongly refused with -ENOSPC, and
a multi-interval update can leave the dataplane inconsistent.


Steps to reproduce (fixed pool, no RCU)
---------------------------------------

IPv6 trie, 2-byte next hop, config.trie.num_tbl8 = 1:

    rte_fib6_add(fib, 2001:db8::, 32, nh);   ->  -ENOSPC
    rte_fib6_add(fib, fc00::,     25, nh);   ->  -ENOSPC   (WRONG)

Expected: the /25 needs a single tbl8 group and the pool has capacity
1, so it must succeed. It is refused because the failed /32 leaked the
only group.

Instrumenting tbl8_pool_pos at the modify_dp() call confirms the leak:

    add /32: ret=-ENOSPC, tbl8_pool_pos 0 -> 1  (group taken, not returned)
    add /25: ret=-ENOSPC, tbl8_pool_pos 1 -> 1  (no free group left)

QSBR defer mode (pool of 2, a reader kept non-quiescent):

    add /25 ; delete /25    (group deferred; rsvd_tbl8s back to 0)
    add /32                 ->  passes the reservation check, then leaks
    add /25 (other prefix)  ->  -ENOSPC (WRONG: a group was leaked)

A reproducer is attached (test/fib6). Both new cases fail on current
main:
    0001-test-fib6-reproduce-tbl8-leak-on-failed-trie-insert.patch


Root cause
----------

install_to_dp() (build_common_root()/write_edge()) allocates and
publishes tbl8 groups incrementally and has no failure path: on
tbl8_alloc() failure it returns an error, leaving the partial writes
and the taken groups in place. trie_modify() only removes the RIB node.

Two reasons the pre-check does not prevent it:

1. Transient peak. trie_modify() checks the steady-state footprint:

       if (dp->rsvd_tbl8s + count_empty_levels(node) > dp->number_tbl8s)

   but install_to_dp() transiently needs footprint + 1 for a
   byte-aligned prefix: build_common_root() descends one byte level
   past the prefix and allocates a group there; recycle_root_path()
   frees it only at the end. So the check passes with a pool of one,
   the install takes the first group, then tbl8_alloc() for the
   transient group fails.

2. Logical vs physical, under QSBR defer. rsvd_tbl8s tracks the
   logical reservation, not physical occupancy. In
   RTE_FIB6_QSBR_MODE_DQ a recycled group stays out of the pool until
   a grace period elapses, so rsvd_tbl8s drops while the group is
   still allocated, and the check accepts an add that then fails.

modify_dp() also calls install_to_dp() once per gap around covering
more-specifics; under defer mode each interval's transient group
accumulates, so the peak can exceed footprint + 1, and a failure on a
late interval leaves earlier intervals already written (a lookup in
one range returns the new next hop while another returns the old one).

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