Re: [PATCH v3 2/4] fprobe: make fprobe_kprobe_handler recursion free

[Google]

On Wed, 17 May 2023 11:45:07 +0800
Ze Gao <zegao2021@xxxxxxxxx> wrote:

> Current implementation calls kprobe related functions before doing
> ftrace recursion check in fprobe_kprobe_handler, which opens door
> to kernel crash due to stack recursion if preempt_count_{add, sub}
> is traceable in kprobe_busy_{begin, end}.
> 
> Things goes like this without this patch quoted from Steven:
> "
> fprobe_kprobe_handler() {
>    kprobe_busy_begin() {
>       preempt_disable() {
>          preempt_count_add() {  <-- trace
>             fprobe_kprobe_handler() {
> 		[ wash, rinse, repeat, CRASH!!! ]
> "
> 
> By refactoring the common part out of fprobe_kprobe_handler and
> fprobe_handler and call ftrace recursion detection at the very beginning,
> the whole fprobe_kprobe_handler is free from recursion.
> 
> Signed-off-by: Ze Gao <zegao@xxxxxxxxxxx>
> Acked-by: Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>
> Link: https://lore.kernel.org/linux-trace-kernel/20230516071830.8190-3-zegao@xxxxxxxxxxx
> ---
>  kernel/trace/fprobe.c | 59 ++++++++++++++++++++++++++++++++-----------
>  1 file changed, 44 insertions(+), 15 deletions(-)
> 
> diff --git a/kernel/trace/fprobe.c b/kernel/trace/fprobe.c
> index 9abb3905bc8e..097c740799ba 100644
> --- a/kernel/trace/fprobe.c
> +++ b/kernel/trace/fprobe.c
> @@ -20,30 +20,22 @@ struct fprobe_rethook_node {
>  	char data[];
>  };
>  
> -static void fprobe_handler(unsigned long ip, unsigned long parent_ip,
> -			   struct ftrace_ops *ops, struct ftrace_regs *fregs)
> +static inline void __fprobe_handler(unsigned long ip, unsigned long
> +		parent_ip, struct ftrace_ops *ops, struct ftrace_regs *fregs)

OK, I picked up this series to probes/fixes. Note that I fixed this line 
because the "unsigned long parent_ip" was split into 2 lines.

Thank you,


>  {
>  	struct fprobe_rethook_node *fpr;
>  	struct rethook_node *rh = NULL;
>  	struct fprobe *fp;
>  	void *entry_data = NULL;
> -	int bit, ret;
> +	int ret;
>  
>  	fp = container_of(ops, struct fprobe, ops);
> -	if (fprobe_disabled(fp))
> -		return;
> -
> -	bit = ftrace_test_recursion_trylock(ip, parent_ip);
> -	if (bit < 0) {
> -		fp->nmissed++;
> -		return;
> -	}
>  
>  	if (fp->exit_handler) {
>  		rh = rethook_try_get(fp->rethook);
>  		if (!rh) {
>  			fp->nmissed++;
> -			goto out;
> +			return;
>  		}
>  		fpr = container_of(rh, struct fprobe_rethook_node, node);
>  		fpr->entry_ip = ip;
> @@ -61,23 +53,60 @@ static void fprobe_handler(unsigned long ip, unsigned long parent_ip,
>  		else
>  			rethook_hook(rh, ftrace_get_regs(fregs), true);
>  	}
> -out:
> +}
> +
> +static void fprobe_handler(unsigned long ip, unsigned long parent_ip,
> +		struct ftrace_ops *ops, struct ftrace_regs *fregs)
> +{
> +	struct fprobe *fp;
> +	int bit;
> +
> +	fp = container_of(ops, struct fprobe, ops);
> +	if (fprobe_disabled(fp))
> +		return;
> +
> +	/* recursion detection has to go before any traceable function and
> +	 * all functions before this point should be marked as notrace
> +	 */
> +	bit = ftrace_test_recursion_trylock(ip, parent_ip);
> +	if (bit < 0) {
> +		fp->nmissed++;
> +		return;
> +	}
> +	__fprobe_handler(ip, parent_ip, ops, fregs);
>  	ftrace_test_recursion_unlock(bit);
> +
>  }
>  NOKPROBE_SYMBOL(fprobe_handler);
>  
>  static void fprobe_kprobe_handler(unsigned long ip, unsigned long parent_ip,
>  				  struct ftrace_ops *ops, struct ftrace_regs *fregs)
>  {
> -	struct fprobe *fp = container_of(ops, struct fprobe, ops);
> +	struct fprobe *fp;
> +	int bit;
> +
> +	fp = container_of(ops, struct fprobe, ops);
> +	if (fprobe_disabled(fp))
> +		return;
> +
> +	/* recursion detection has to go before any traceable function and
> +	 * all functions called before this point should be marked as notrace
> +	 */
> +	bit = ftrace_test_recursion_trylock(ip, parent_ip);
> +	if (bit < 0) {
> +		fp->nmissed++;
> +		return;
> +	}
>  
>  	if (unlikely(kprobe_running())) {
>  		fp->nmissed++;
>  		return;
>  	}
> +
>  	kprobe_busy_begin();
> -	fprobe_handler(ip, parent_ip, ops, fregs);
> +	__fprobe_handler(ip, parent_ip, ops, fregs);
>  	kprobe_busy_end();
> +	ftrace_test_recursion_unlock(bit);
>  }
>  
>  static void fprobe_exit_handler(struct rethook_node *rh, void *data,
> -- 
> 2.40.1
> 


-- 
Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>