netdev - Re: [PATCH bpf v4] bpf: zero-fill re-used per-cpu map element

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Message-ID: <CAEf4BzaK4Lbac=A8dsRyCLCbdwTbXEJFXFU113kYv1UVCV9TyQ@mail.gmail.com>
Date:   Wed, 4 Nov 2020 12:45:50 -0800
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     David Verbeiren <david.verbeiren@...sares.net>
Cc:     bpf <bpf@...r.kernel.org>, Andrii Nakryiko <andrii@...nel.org>,
        Networking <netdev@...r.kernel.org>,
        Matthieu Baerts <matthieu.baerts@...sares.net>
Subject: Re: [PATCH bpf v4] bpf: zero-fill re-used per-cpu map element

On Wed, Nov 4, 2020 at 3:26 AM David Verbeiren
<david.verbeiren@...sares.net> wrote:
>
> 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")
> Acked-by: Matthieu Baerts <matthieu.baerts@...sares.net>
> Signed-off-by: David Verbeiren <david.verbeiren@...sares.net>
> ---
>
> Notes:
>     v4:
>       - Replaced racy "once" test by getpgid syscall with
>         check on pid. (Andrii)
>       - Copyright lines use /* */ (Andrii)

Acked-by: Andrii Nakryiko <andrii@...nel.org>

>
>     v3:
>       - Added selftest that was initially provided as separate
>         patch, and reworked to
>         * use skeleton (Andrii, Song Liu)
>         * skip test if <=1 CPU (Song Liu)
>
>     v2:
>       - Moved memset() to separate pcpu_init_value() function,
>         which replaces pcpu_copy_value() but delegates to it
>         for the cases where no memset() is needed (Andrii).
>       - This function now also avoids doing the memset() for
>         the current cpu for which the value must be set
>         anyhow (Andrii).
>       - Same pcpu_init_value() used for per-cpu LRU map
>         (Andrii).
>
>  kernel/bpf/hashtab.c                          |  30 ++-
>  .../selftests/bpf/prog_tests/map_init.c       | 214 ++++++++++++++++++
>  .../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
>

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