[PATCH -v5 16/17] x86/kprobe: Add comments to arch_{,un}optimize_kprobes()

Peter Zijlstra
2019-11-11T14:47:58Z

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@infradead.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);