Comment 4 for bug 1917609

A quick bisect against groovy shows:
dd773889e74605c1a0a97d701f589733800c9dc8 is the first bad commit
commit dd773889e74605c1a0a97d701f589733800c9dc8
Author: David Verbeiren <email address hidden>
Date: Wed Nov 4 12:23:32 2020 +0100

    bpf: Zero-fill re-used per-cpu map element

    BugLink: https://bugs.launchpad.net/bugs/1908555

    [ Upstream commit d3bec0138bfbe58606fc1d6f57a4cdc1a20218db ]

    Zero-fill element values for all other cpus than current, just as
    when not using prealloc. This is the only way the bpf program can
    ensure known initial values for all cpus ('onallcpus' cannot be
    set when coming from the bpf program).

    The scenario is: bpf program inserts some elements in a per-cpu
    map, then deletes some (or userspace does). When later adding
    new elements using bpf_map_update_elem(), the bpf program can
    only set the value of the new elements for the current cpu.
    When prealloc is enabled, previously deleted elements are re-used.
    Without the fix, values for other cpus remain whatever they were
    when the re-used entry was previously freed.

    A selftest is added to validate correct operation in above
    scenario as well as in case of LRU per-cpu map element re-use.

    Fixes: 6c9059817432 ("bpf: pre-allocate hash map elements")
    Signed-off-by: David Verbeiren <email address hidden>
    Signed-off-by: Alexei Starovoitov <email address hidden>
    Acked-by: Matthieu Baerts <email address hidden>
    Acked-by: Andrii Nakryiko <email address hidden>
    Link: https://<email address hidden>
    Signed-off-by: Sasha Levin <email address hidden>
    Signed-off-by: Kamal Mostafa <email address hidden>
    Signed-off-by: Ian May <email address hidden>

 kernel/bpf/hashtab.c | 30 ++-
 tools/testing/selftests/bpf/prog_tests/map_init.c | 214 ++++++++++++++++++++++
 tools/testing/selftests/bpf/progs/test_map_init.c | 33 ++++
 3 files changed, 275 insertions(+), 2 deletions(-)
 create mode 100644 tools/testing/selftests/bpf/prog_tests/map_init.c
 create mode 100644 tools/testing/selftests/bpf/progs/test_map_init.c

$ git tag --contains dd773889e74605c1a0a97d701f589733800c9dc8
Ubuntu-5.8.0-44.50
Ubuntu-5.8.0-45.51