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SubjectRe: PATCH] debug: add notifier chain debugging
On Fri, 15 Aug 2008 15:29:38 -0700 Arjan van de Ven <arjan@infradead.org> wrote:

> From: Arjan van de Ven <arjan@linux.intel.com>
> Subject: [PATCH] debug: add notifier chain debugging
>
> during some development we suspected a case where we left something
> in a notifier chain that was from a module that was unloaded already...
> and that sort of thing is rather hard to track down.
>
> This patch adds a very simple sanity check (which isn't all that
> expensive) to make sure the notifier we're about to call is
> actually from either the kernel itself of from a still-loaded
> module, avoiding a hard-to-chase-down crash.
>
> Signed-off-by: Arjan van de Ven <arjan@linux.intel.com>
> ---
> kernel/notifier.c | 16 ++++++++++++++++
> lib/Kconfig.debug | 10 ++++++++++
> 2 files changed, 26 insertions(+), 0 deletions(-)
>
> diff --git a/kernel/notifier.c b/kernel/notifier.c
> index 823be11..143fdd7 100644
> --- a/kernel/notifier.c
> +++ b/kernel/notifier.c
> @@ -21,6 +21,10 @@ BLOCKING_NOTIFIER_HEAD(reboot_notifier_list);
> static int notifier_chain_register(struct notifier_block **nl,
> struct notifier_block *n)
> {
> + if (!kernel_text_address((unsigned long)n->notifier_call)) {
> + WARN(1, "Invalid notifier registered!");
> + return 0;
> + }
> while ((*nl) != NULL) {
> if (n->priority > (*nl)->priority)
> break;
> @@ -34,6 +38,10 @@ static int notifier_chain_register(struct notifier_block **nl,
> static int notifier_chain_cond_register(struct notifier_block **nl,
> struct notifier_block *n)
> {
> + if (!kernel_text_address((unsigned long)n->notifier_call)) {
> + WARN(1, "Invalid notifier registered!");
> + return 0;
> + }

Seems strange to add checks to the registration functions. What could
be that broken?

> while ((*nl) != NULL) {
> if ((*nl) == n)
> return 0;
> @@ -82,6 +90,14 @@ static int __kprobes notifier_call_chain(struct notifier_block **nl,
>
> while (nb && nr_to_call) {
> next_nb = rcu_dereference(nb->next);
> +
> +#ifdef CONFIG_DEBUG_NOTIFIERS
> + if (!kernel_text_address((unsigned long)nb->notifier_call)) {
> + WARN(1, "Invalid notifier called!");
> + nb = next_nb;
> + continue;
> + }
> +#endif
> ret = nb->notifier_call(nb, val, v);
>
> if (nr_calls)
> diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
> index 800ac84..f4bb36e 100644
> --- a/lib/Kconfig.debug
> +++ b/lib/Kconfig.debug
> @@ -536,6 +536,16 @@ config DEBUG_SG
>
> If unsure, say N.
>
> +config DEBUG_NOTIFIERS
> + bool "Debug notifier call chains"
> + depends on DEBUG_KERNEL
> + help
> + Enable this to turn on sanity checking for notifier call chains.
> + This is most useful for kernel developers to make sure that
> + modules properly unregister themselves from notifier chains.
> + This is a relatively cheap check but if you care about maximum
> + performance, say N.
> +

If we remove the first two checks then surely we can afford to add the
remaining check unconditionally and lose the new config option and its
ifdef.



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