On Thu, Jan 06, 2022 at 05:00:07PM -0300, Guilherme G. Piccoli wrote:
> The kernel notifier infrastructure allows function callbacks to be
> added in multiple lists, which are then called in the proper time,
> like in a reboot or panic event. The panic_notifier_list specifically
> contains the callbacks that are executed during a panic event. As any
> other notifier list, the panic one has no filtering and all functions
> previously registered are executed.
>
> The kdump infrastructure, on the other hand, enables users to set
> a crash kernel that is kexec'ed in a panic event, and vmcore/logs
> are collected in such crash kernel. When kdump is set, by default
> the panic notifiers are ignored - the kexec jumps to the crash kernel
> before the list is checked and callbacks executed.
>
> There are some cases though in which kdump users might want to
> allow panic notifier callbacks to execute _before_ the kexec to
> the crash kernel, for a variety of reasons - for example, users
> may think kexec is very prone to fail and want to give a chance
> to kmsg dumpers to run (and save logs using pstore), or maybe
> some panic notifier is required to properly quiesce some hardware
> that must be used to the crash kernel. For these cases, we have
> the kernel parameter "crash_kexec_post_notifiers".
>
> But there's a problem: currently it's an "all-or-nothing" situation,
> the kdump user choice is either to execute all panic notifiers or
> none of them. Given that panic notifiers may increase the risk of a
> kdump failure, this is a tough decision and may affect the debug of
> hard to reproduce bugs, if for some reason the user choice is to
> enable panic notifiers, but kdump then fails.
>
> So, this patch aims to ease this decision: we hereby introduce a filter
> for the panic notifier list, in which users may select specifically
> which callbacks they wish to run, allowing a safer kdump. The allowlist
> should be provided using the parameter "panic_notifier_filter=a,b,..."
> where a, b are valid callback names. Invalid symbols are discarded.
>
> Currently up to 16 symbols may be passed in this list, we consider
> that this numbers allows enough flexibility (and no matter what
> architecture is used, at most 30 panic callbacks are registered).
> In an experiment using a qemu x86 virtual machine, by default only
> six callbacks are registered in the panic notifier list.
> Once a valid callback name is provided in the list, such function
> is allowed to be registered/unregistered in the panic_notifier_list;
> all other panic callbacks are ignored. Notice that this filter is
> only for the panic notifiers and has no effect in the other notifiers.
...
> +static int __init panic_notifier_filter_setup(char *buf)
> +{
> + char *func;
> + unsigned long addr;
> +
> + if (!buf)
> + return -EINVAL;
> +
> + while (buf) {
> + func = strsep(&buf, ",");
Don't we have a parser of this format already?
Anyway, you may reduce code by
unsigned long addr;
char *func = buf;
while ((func = strsep(&func, ",")) {
> + addr = kallsyms_lookup_name(func);
> +
> + if (!addr) {
> + pr_info("panic_notifier_filter: invalid symbol %s\n", func);
> + continue;
> + }
> +
> + if (panic_nf_count < PANIC_NF_MAX) {
> + panic_nf_functions[panic_nf_count] = addr;
> + panic_nf_count++;
> + pr_debug("panic_notifier_filter: added symbol %s\n", func);
> + } else {
> + pr_warn("panic_notifier_filter: exceeded maximum notifiers (%d), aborting\n",
> + PANIC_NF_MAX);
> + panic_nf_count = 0;
> + break;
> + }
> + }
> +
> + return 0;
> +}
> +early_param("panic_notifier_filter", panic_notifier_filter_setup);
--
With Best Regards,
Andy Shevchenko
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