[tip: core/kprobes] x86/kprobe: Add comments to arch_{,un}optimize_kprobes()

[tip-bot2 for Peter Zijlstra]

The following commit has been merged into the core/kprobes branch of tip:

Commit-ID:     f2cb4f95b7571f2bebcf226cd92b448fd58950ca
Gitweb:        https://git.kernel.org/tip/f2cb4f95b7571f2bebcf226cd92b448fd58950ca
Author:        Peter Zijlstra <peterz@xxxxxxxxxxxxx>
AuthorDate:    Mon, 11 Nov 2019 14:02:10 +01:00
Committer:     Ingo Molnar <mingo@xxxxxxxxxx>
CommitterDate: Wed, 27 Nov 2019 07:44:25 +01:00

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 <ast@xxxxxxxxxx>
Tested-by: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Acked-by: Alexei Starovoitov <ast@xxxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxxxx>
Cc: Brian Gerst <brgerst@xxxxxxxxx>
Cc: Denys Vlasenko <dvlasenk@xxxxxxxxxx>
Cc: H. Peter Anvin <hpa@xxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Link: https://lkml.kernel.org/r/20191111132458.401696663@xxxxxxxxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
---
 arch/x86/kernel/kprobes/opt.c | 14 ++++++++++++--
 1 file changed, 12 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/kprobes/opt.c b/arch/x86/kernel/kprobes/opt.c
index 26e0d6c..3f45b5c 100644
--- a/arch/x86/kernel/kprobes/opt.c
+++ b/arch/x86/kernel/kprobes/opt.c
@@ -414,8 +414,12 @@ err:
 }
 
 /*
- * 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);