livepatch/ftrace: Add recursion protection to the ftrace callback

If a ftrace callback does not supply its own recursion protection and
does not set the RECURSION_SAFE flag in its ftrace_ops, then ftrace will
make a helper trampoline to do so before calling the callback instead of
just calling the callback directly.

The default for ftrace_ops is going to change. It will expect that handlers
provide their own recursion protection, unless its ftrace_ops states
otherwise.

Link: https://lkml.kernel.org/r/20201028115613.291169246@goodmis.org
Link: https://lkml.kernel.org/r/20201106023547.122802424@goodmis.org
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Jiri Kosina <jikos@kernel.org>
Cc: Joe Lawrence <joe.lawrence@redhat.com>
Cc: live-patching@vger.kernel.org
Reviewed-by: Petr Mladek <pmladek@suse.com>
Acked-by: Miroslav Benes <mbenes@suse.cz>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

diff --git a/kernel/livepatch/patch.c b/kernel/livepatch/patch.c
index b552cf2..6c0164d 100644 (file)
--- a/kernel/livepatch/patch.c
+++ b/kernel/livepatch/patch.c
@@ -45,9 +45,13 @@ static void notrace klp_ftrace_handler(unsigned long ip,
        struct klp_ops *ops;
        struct klp_func *func;
        int patch_state;
+       int bit;
 
        ops = container_of(fops, struct klp_ops, fops);
 
+       bit = ftrace_test_recursion_trylock();
+       if (bit < 0)
+               return;
        /*
         * A variant of synchronize_rcu() is used to allow patching functions
         * where RCU is not watching, see klp_synchronize_transition().
@@ -117,6 +121,7 @@ static void notrace klp_ftrace_handler(unsigned long ip,
 
 unlock:
        preempt_enable_notrace();
+       ftrace_test_recursion_unlock(bit);
 }
 
 /*