linux-kernel - Re: [PATCH v2 1/2] kprobes: propagate error from arm_kprobe

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Date:   Fri, 3 Nov 2017 10:03:17 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Jessica Yu <jeyu@...nel.org>
Cc:     Masami Hiramatsu <mhiramat@...nel.org>,
        Ananth N Mavinakayanahalli <ananth@...ux.vnet.ibm.com>,
        Anil S Keshavamurthy <anil.s.keshavamurthy@...el.com>,
        "David S . Miller" <davem@...emloft.net>,
        Ingo Molnar <mingo@...nel.org>, Petr Mladek <pmladek@...e.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Joe Lawrence <joe.lawrence@...hat.com>,
        Jiri Kosina <jikos@...nel.org>,
        Miroslav Benes <mbenes@...e.cz>, live-patching@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/2] kprobes: propagate error from
 arm_kprobe_ftrace()

On Thu,  2 Nov 2017 17:33:33 +0100
Jessica Yu <jeyu@...nel.org> wrote:

> Improve error handling when arming ftrace-based kprobes. Specifically, if
> we fail to arm a ftrace-based kprobe, register_kprobe()/enable_kprobe()
> should report an error instead of success. Previously, this has lead to
> confusing situations where register_kprobe() would return 0 indicating
> success, but the kprobe would not be functional if ftrace registration
> during the kprobe arming process had failed. We should therefore take any
> errors returned by ftrace into account and propagate this error so that we
> do not register/enable kprobes that cannot be armed. This can happen if,
> for example, register_ftrace_function() finds an IPMODIFY conflict (since
> kprobe_ftrace_ops has this flag set) and returns an error. Such a conflict
> is possible since livepatches also set the IPMODIFY flag for their ftrace_ops.
> 
> arm_all_kprobes() keeps its current behavior and attempts to arm all
> kprobes. It returns the last encountered error and gives a warning if
> not all kprobes could be armed.
> 
> This patch is based on Petr Mladek's original patchset (patches 2 and 3)
> back in 2015, which improved kprobes error handling, found here:
> 
>    https://lkml.org/lkml/2015/2/26/452
> 
> However, further work on this had been paused since then and the patches
> were not upstreamed.
> 
> Based-on-patches-by: Petr Mladek <pmladek@...e.com>
> Signed-off-by: Jessica Yu <jeyu@...nel.org>
> ---
>  kernel/kprobes.c | 88 ++++++++++++++++++++++++++++++++++++++++----------------
>  1 file changed, 63 insertions(+), 25 deletions(-)
> 
> diff --git a/kernel/kprobes.c b/kernel/kprobes.c
> index da2ccf142358..f4a094007cb5 100644
> --- a/kernel/kprobes.c
> +++ b/kernel/kprobes.c
> @@ -978,18 +978,27 @@ static int prepare_kprobe(struct kprobe *p)
>  }
>  
>  /* Caller must lock kprobe_mutex */
> -static void arm_kprobe_ftrace(struct kprobe *p)
> +static int arm_kprobe_ftrace(struct kprobe *p)
>  {
> -	int ret;
> +	int ret = 0;
>  
>  	ret = ftrace_set_filter_ip(&kprobe_ftrace_ops,
>  				   (unsigned long)p->addr, 0, 0);
> -	WARN(ret < 0, "Failed to arm kprobe-ftrace at %p (%d)\n", p->addr, ret);
> -	kprobe_ftrace_enabled++;
> -	if (kprobe_ftrace_enabled == 1) {
> +	if (WARN(ret < 0, "Failed to arm kprobe-ftrace at %p (%d)\n", p->addr, ret))
> +		return ret;
> +
> +	if (kprobe_ftrace_enabled == 0) {
>  		ret = register_ftrace_function(&kprobe_ftrace_ops);
> -		WARN(ret < 0, "Failed to init kprobe-ftrace (%d)\n", ret);
> +		if (WARN(ret < 0, "Failed to init kprobe-ftrace (%d)\n", ret))
> +			goto err_ftrace;
>  	}
> +
> +	kprobe_ftrace_enabled++;
> +	return ret;
> +
> +err_ftrace:
> +	ftrace_set_filter_ip(&kprobe_ftrace_ops, (unsigned long)p->addr, 1, 0);

Hmm, this could have a very nasty side effect. If you remove a function
from the ops, and it was the last function, an empty ops means to trace
*all* functions.

Perhaps you want to add it to the "notrace" list. Which would require
implementing a ftrace_set_notrace_ip() function. Which I believe is
what you want. Any function in the notrace hash will have the same
functions in the filter hash be ignored.

I'll let Masami review the rest.

-- Steve


> +	return ret;
>  }
>