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SubjectRe: Oops caused by race between livepatch and ftrace
On Mon, 20 May 2019 16:19:31 -0500
Josh Poimboeuf <jpoimboe@redhat.com> wrote:

> diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
> index a12aff849c04..8259d4ba8b00 100644
> --- a/kernel/trace/ftrace.c
> +++ b/kernel/trace/ftrace.c
> @@ -34,6 +34,7 @@
> #include <linux/hash.h>
> #include <linux/rcupdate.h>
> #include <linux/kprobes.h>
> +#include <linux/memory.h>
>
> #include <trace/events/sched.h>
>
> @@ -2610,10 +2611,12 @@ static void ftrace_run_update_code(int command)
> {
> int ret;
>
> + mutex_lock(&text_mutex);
> +

Hmm, this may blow up with lockdep, as I believe we already have a
locking dependency of:

text_mutex -> ftrace_lock

And this will reverses it. (kprobes appears to take the locks in this
order).

Perhaps have live kernel patching grab ftrace_lock?

-- Steve


> ret = ftrace_arch_code_modify_prepare();
> FTRACE_WARN_ON(ret);
> if (ret)
> - return;
> + goto out_unlock;
>
> /*
> * By default we use stop_machine() to modify the code.
> @@ -2625,6 +2628,9 @@ static void ftrace_run_update_code(int command)
>
> ret = ftrace_arch_code_modify_post_process();
> FTRACE_WARN_ON(ret);
> +
> +out_unlock:
> + mutex_unlock(&text_mutex);
> }
>
> static void ftrace_run_modify_code(struct ftrace_ops *ops, int command,
> @@ -5776,6 +5782,7 @@ void ftrace_module_enable(struct module *mod)
> struct ftrace_page *pg;
>
> mutex_lock(&ftrace_lock);
> + mutex_lock(&text_mutex);
>
> if (ftrace_disabled)
> goto out_unlock;
> @@ -5837,6 +5844,7 @@ void ftrace_module_enable(struct module *mod)
> ftrace_arch_code_modify_post_process();
>
> out_unlock:
> + mutex_unlock(&text_mutex);
> mutex_unlock(&ftrace_lock);
>
> process_cached_mods(mod->name);

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