linux-kernel - Re: [PATCH] kprobes: Rollback post_handler on failed arm

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Date:   Wed, 15 Jun 2022 09:34:24 +0900
From:   Masami Hiramatsu (Google) <mhiramat@...nel.org>
To:     Chuang W <nashuiliang@...il.com>
Cc:     stable@...r.kernel.org,
        "Naveen N. Rao" <naveen.n.rao@...ux.ibm.com>,
        Anil S Keshavamurthy <anil.s.keshavamurthy@...el.com>,
        "David S. Miller" <davem@...emloft.net>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Ingo Molnar <mingo@...nel.org>, Jessica Yu <jeyu@...nel.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] kprobes: Rollback post_handler on failed arm_kprobe()

Hi Chuang,

On Tue, 14 Jun 2022 17:06:33 +0800
Chuang W <nashuiliang@...il.com> wrote:

> In a scenario where livepatch and aggrprobe coexist, if arm_kprobe()
> returns an error, ap.post_handler, while has been modified to
> p.post_handler, is not rolled back.

Would you mean 'coexist' on the same function?

> 
> When ap.post_handler is not NULL (not rolled back), the caller (e.g.
> register_kprobe/enable_kprobe) of arm_kprobe_ftrace() will always fail.

It seems this explanation and the actual code does not
match. Can you tell me what actually you observed?

Thank you,

> 
> Fixes: 12310e343755 ("kprobes: Propagate error from arm_kprobe_ftrace()")
> Signed-off-by: Chuang W <nashuiliang@...il.com>
> Cc: <stable@...r.kernel.org>
> ---
>  kernel/kprobes.c | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/kernel/kprobes.c b/kernel/kprobes.c
> index f214f8c088ed..0610b02a3a05 100644
> --- a/kernel/kprobes.c
> +++ b/kernel/kprobes.c
> @@ -1300,6 +1300,7 @@ static int register_aggr_kprobe(struct kprobe *orig_p, struct kprobe *p)
>  {
>  	int ret = 0;
>  	struct kprobe *ap = orig_p;
> +	kprobe_post_handler_t old_post_handler = NULL;
>  
>  	cpus_read_lock();
>  
> @@ -1351,6 +1352,9 @@ static int register_aggr_kprobe(struct kprobe *orig_p, struct kprobe *p)
>  
>  	/* Copy the insn slot of 'p' to 'ap'. */
>  	copy_kprobe(ap, p);
> +
> +	/* save the old post_handler */
> +	old_post_handler = ap->post_handler;
>  	ret = add_new_kprobe(ap, p);
>  
>  out:
> @@ -1365,6 +1369,7 @@ static int register_aggr_kprobe(struct kprobe *orig_p, struct kprobe *p)
>  			ret = arm_kprobe(ap);
>  			if (ret) {
>  				ap->flags |= KPROBE_FLAG_DISABLED;
> +				ap->post_handler = old_post_handler;
>  				list_del_rcu(&p->list);
>  				synchronize_rcu();
>  			}
> -- 
> 2.34.1
> 


-- 
Masami Hiramatsu (Google) <mhiramat@...nel.org>