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From:   Marco Elver <elver@google.com>
Date:   Mon, 14 Nov 2022 10:48:38 +0100
Message-ID: <CANpmjNO_ujNwaFxpsAWWXhBajhV8LJMXQjCHiSLHKG2Dc+od4A@mail.gmail.com>
Subject: Re: [PATCH v2 5/6] panic: Introduce warn_limit
To:     Kees Cook <keescook@chromium.org>
Cc:     Jann Horn <jannh@google.com>, Jonathan Corbet <corbet@lwn.net>,
        Andrew Morton <akpm@linux-foundation.org>,
        Baolin Wang <baolin.wang@linux.alibaba.com>,
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        tangmeng <tangmeng@uniontech.com>,
        "Guilherme G. Piccoli" <gpiccoli@igalia.com>,
        Tiezhu Yang <yangtiezhu@loongson.cn>,
        Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
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On Wed, 9 Nov 2022 at 21:00, Kees Cook <keescook@chromium.org> wrote:
>
> Like oops_limit, add warn_limit for limiting the number of warnings when
> panic_on_warn is not set.
>
> Cc: Jonathan Corbet <corbet@lwn.net>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
> Cc: "Jason A. Donenfeld" <Jason@zx2c4.com>
> Cc: Eric Biggers <ebiggers@google.com>
> Cc: Huang Ying <ying.huang@intel.com>
> Cc: Petr Mladek <pmladek@suse.com>
> Cc: tangmeng <tangmeng@uniontech.com>
> Cc: "Guilherme G. Piccoli" <gpiccoli@igalia.com>
> Cc: Tiezhu Yang <yangtiezhu@loongson.cn>
> Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> Cc: linux-doc@vger.kernel.org
> Signed-off-by: Kees Cook <keescook@chromium.org>
> ---
>  Documentation/admin-guide/sysctl/kernel.rst |  9 +++++++++
>  kernel/panic.c                              | 13 +++++++++++++
>  2 files changed, 22 insertions(+)
>
> diff --git a/Documentation/admin-guide/sysctl/kernel.rst b/Documentation/admin-guide/sysctl/kernel.rst
> index 09f3fb2f8585..c385d5319cdf 100644
> --- a/Documentation/admin-guide/sysctl/kernel.rst
> +++ b/Documentation/admin-guide/sysctl/kernel.rst
> @@ -1508,6 +1508,15 @@ entry will default to 2 instead of 0.
>  2 Unprivileged calls to ``bpf()`` are disabled
>  = =============================================================
>
> +
> +warn_limit
> +==========
> +
> +Number of kernel warnings after which the kernel should panic when
> +``panic_on_warn`` is not set. Setting this to 0 or 1 has the same effect
> +as setting ``panic_on_warn=1``.
> +
> +
>  watchdog
>  ========
>
> diff --git a/kernel/panic.c b/kernel/panic.c
> index 3afd234767bc..b235fa4a6fc8 100644
> --- a/kernel/panic.c
> +++ b/kernel/panic.c
> @@ -58,6 +58,7 @@ bool crash_kexec_post_notifiers;
>  int panic_on_warn __read_mostly;
>  unsigned long panic_on_taint;
>  bool panic_on_taint_nousertaint = false;
> +static unsigned int warn_limit __read_mostly = 10000;
>
>  int panic_timeout = CONFIG_PANIC_TIMEOUT;
>  EXPORT_SYMBOL_GPL(panic_timeout);
> @@ -88,6 +89,13 @@ static struct ctl_table kern_panic_table[] = {
>                 .extra2         = SYSCTL_ONE,
>         },
>  #endif
> +       {
> +               .procname       = "warn_limit",
> +               .data           = &warn_limit,
> +               .maxlen         = sizeof(warn_limit),
> +               .mode           = 0644,
> +               .proc_handler   = proc_douintvec,
> +       },
>         { }
>  };
>
> @@ -203,8 +211,13 @@ static void panic_print_sys_info(bool console_flush)
>
>  void check_panic_on_warn(const char *reason)
>  {
> +       static atomic_t warn_count = ATOMIC_INIT(0);
> +
>         if (panic_on_warn)
>                 panic("%s: panic_on_warn set ...\n", reason);
> +
> +       if (atomic_inc_return(&warn_count) >= READ_ONCE(warn_limit))
> +               panic("Warned too often (warn_limit is %d)", warn_limit);

Shouldn't this also include the "reason", like above? (Presumably a
warning had just been generated to console so the reason is easy
enough to infer from the log, although in that case "reason" also
seems redundant above.)