x86/kprobe: Add comments to arch_{,un}optimize_kprobes()

Add a few words describing how it is safe to overwrite the 4 bytes
after a kprobe. In specific it is possible the JMP.d32 required for
the optimized kprobe overwrites multiple instructions.

Tested-by: Alexei Starovoitov <[email protected]>
Tested-by: Steven Rostedt (VMware) <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Acked-by: Alexei Starovoitov <[email protected]>
Cc: Andy Lutomirski <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: Brian Gerst <[email protected]>
Cc: Denys Vlasenko <[email protected]>
Cc: H. Peter Anvin <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Ingo Molnar <[email protected]>

Author: Peter Zijlstra

arch/x86/kernel/kprobes/opt.c

@@ -414,8 +414,12 @@ int arch_prepare_optimized_kprobe(struct optimized_kprobe *op,
 }
 
 /*
- * Replace breakpoints (int3) with relative jumps.
+ * Replace breakpoints (INT3) with relative jumps (JMP.d32).
  * Caller must call with locking kprobe_mutex and text_mutex.
+ *
+ * The caller will have installed a regular kprobe and after that issued
+ * syncrhonize_rcu_tasks(), this ensures that the instruction(s) that live in
+ * the 4 bytes after the INT3 are unused and can now be overwritten.
  */
 void arch_optimize_kprobes(struct list_head *oplist)
 {
@@ -441,7 +445,13 @@ void arch_optimize_kprobes(struct list_head *oplist)
 	}
 }
 
-/* Replace a relative jump with a breakpoint (int3).  */
+/*
+ * Replace a relative jump (JMP.d32) with a breakpoint (INT3).
+ *
+ * After that, we can restore the 4 bytes after the INT3 to undo what
+ * arch_optimize_kprobes() scribbled. This is safe since those bytes will be
+ * unused once the INT3 lands.
+ */
 void arch_unoptimize_kprobe(struct optimized_kprobe *op)
 {
 	arch_arm_kprobe(&op->kp);