Re: [BUGFIX PATCH bpf-next] error-injection: Fix to prohibit jump optimization

On Mon, 12 Mar 2018 11:44:21 +0100
Daniel Borkmann [off-list ref] wrote:

Hi Masami,

On 03/12/2018 11:27 AM, Masami Hiramatsu wrote:

quoted

On Mon, 12 Mar 2018 19:00:49 +0900
Masami Hiramatsu [off-list ref] wrote:

quoted

Since the kprobe which was optimized by jump can not change
the execution path, the kprobe for error-injection must not
be optimized. To prohibit it, set a dummy post-handler as
officially stated in Documentation/kprobes.txt.

Note that trace-probe based BPF is not affected, because it
ensures the trace-probe is based on ftrace, which is not
jump optimized.

Thanks for the fix! I presume this should go via bpf instead of bpf-next
tree since 4b1a29a7f542 ("error-injection: Support fault injection framework")
is in Linus' tree as well. Unless there are objection I would rather route
it that way so it would be for 4.16.

Ah, right! It should go into 4.16. It should be applicable cleanly either tree
since there is only the above commit on kernel/fail_function.c :)

Thanks,

Thanks,
Daniel

quoted

Thanks,

quoted

Fixes: 4b1a29a7f542 ("error-injection: Support fault injection framework")
Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
---
 kernel/fail_function.c |   10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/kernel/fail_function.c b/kernel/fail_function.c
index 21b0122cb39c..1d5632d8bbcc 100644
--- a/kernel/fail_function.c
+++ b/kernel/fail_function.c

@@ -14,6 +14,15 @@
 
 static int fei_kprobe_handler(struct kprobe *kp, struct pt_regs *regs);
 
+static void fei_post_handler(struct kprobe *kp, struct pt_regs *regs,
+			     unsigned long flags)
+{
+	/*
+	 * A dummy post handler is required to prohibit optimizing, because
+	 * jump optimization does not support execution path overriding.
+	 */
+}
+
 struct fei_attr {
 	struct list_head list;
 	struct kprobe kp;

@@ -56,6 +65,7 @@ static struct fei_attr *fei_attr_new(const char *sym, unsigned long addr)
 			return NULL;
 		}
 		attr->kp.pre_handler = fei_kprobe_handler;
+		attr->kp.post_handler = fei_post_handler;
 		attr->retval = adjust_error_retval(addr, 0);
 		INIT_LIST_HEAD(&attr->list);
 	}

-- 
Masami Hiramatsu [off-list ref]