linux-kernel - [PATCH -v5 16/17] x86/kprobe: Add comments to arch_{,un}optimize

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Date:   Mon, 11 Nov 2019 14:13:08 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     x86@...nel.org
Cc:     peterz@...radead.org, linux-kernel@...r.kernel.org,
        rostedt@...dmis.org, mhiramat@...nel.org, bristot@...hat.com,
        jbaron@...mai.com, torvalds@...ux-foundation.org,
        tglx@...utronix.de, mingo@...nel.org, namit@...are.com,
        hpa@...or.com, luto@...nel.org, ard.biesheuvel@...aro.org,
        jpoimboe@...hat.com, jeyu@...nel.org, alexei.starovoitov@...il.com
Subject: [PATCH -v5 16/17] 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.

Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
---
 arch/x86/kernel/kprobes/opt.c |   14 ++++++++++++--
 1 file changed, 12 insertions(+), 2 deletions(-)

--- a/arch/x86/kernel/kprobes/opt.c
+++ b/arch/x86/kernel/kprobes/opt.c
@@ -414,8 +414,12 @@ int arch_prepare_optimized_kprobe(struct
 }
 
 /*
- * 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_h
 	}
 }
 
-/* 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);