Re: [PATCH perf/core 02/11] uprobes: Skip emulate/sstep on unique uprobe when ip is changed

[Oleg Nesterov <oleg@redhat.com>]

On 09/02, Jiri Olsa wrote:
>
> If user decided to take execution elsewhere, it makes little sense
> to execute the original instruction, so let's skip it.

Exactly.

So why do we need all these "is_unique" complications? Only a single
is_unique/exclusive consumer can change regs->ip, so I guess handle_swbp()
can just do

	handler_chain(uprobe, regs);
	if (instruction_pointer(regs) != bp_vaddr)
		goto out;


> Allowing this
> behaviour only for uprobe with unique consumer attached.

But if a non-exclusive consumer changes regs->ip, we have a problem
anyway, right?

We can probably add something like

		rc = uc->handler(uc, regs, &cookie);
	+	WARN_ON(!uc->is_unique && instruction_pointer(regs) != bp_vaddr);

into handler_chain(), although I don't think this is needed.

Oleg.

> Signed-off-by: Jiri Olsa <jolsa@kernel.org>
> ---
>  kernel/events/uprobes.c | 13 ++++++++++---
>  1 file changed, 10 insertions(+), 3 deletions(-)
> 
> diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
> index b9b088f7333a..da8291941c6b 100644
> --- a/kernel/events/uprobes.c
> +++ b/kernel/events/uprobes.c
> @@ -2568,7 +2568,7 @@ static bool ignore_ret_handler(int rc)
>  	return rc == UPROBE_HANDLER_REMOVE || rc == UPROBE_HANDLER_IGNORE;
>  }
>  
> -static void handler_chain(struct uprobe *uprobe, struct pt_regs *regs)
> +static void handler_chain(struct uprobe *uprobe, struct pt_regs *regs, bool *is_unique)
>  {
>  	struct uprobe_consumer *uc;
>  	bool has_consumers = false, remove = true;
> @@ -2582,6 +2582,9 @@ static void handler_chain(struct uprobe *uprobe, struct pt_regs *regs)
>  		__u64 cookie = 0;
>  		int rc = 0;
>  
> +		if (is_unique)
> +			*is_unique |= uc->is_unique;
> +
>  		if (uc->handler) {
>  			rc = uc->handler(uc, regs, &cookie);
>  			WARN(rc < 0 || rc > 2,
> @@ -2735,6 +2738,7 @@ static void handle_swbp(struct pt_regs *regs)
>  {
>  	struct uprobe *uprobe;
>  	unsigned long bp_vaddr;
> +	bool is_unique = false;
>  	int is_swbp;
>  
>  	bp_vaddr = uprobe_get_swbp_addr(regs);
> @@ -2789,7 +2793,10 @@ static void handle_swbp(struct pt_regs *regs)
>  	if (arch_uprobe_ignore(&uprobe->arch, regs))
>  		goto out;
>  
> -	handler_chain(uprobe, regs);
> +	handler_chain(uprobe, regs, &is_unique);
> +
> +	if (is_unique && instruction_pointer(regs) != bp_vaddr)
> +		goto out;
>  
>  	/* Try to optimize after first hit. */
>  	arch_uprobe_optimize(&uprobe->arch, bp_vaddr);
> @@ -2819,7 +2826,7 @@ void handle_syscall_uprobe(struct pt_regs *regs, unsigned long bp_vaddr)
>  		return;
>  	if (arch_uprobe_ignore(&uprobe->arch, regs))
>  		return;
> -	handler_chain(uprobe, regs);
> +	handler_chain(uprobe, regs, NULL);
>  }
>  
>  /*
> -- 
> 2.51.0
>