Re: [PATCH/RFC 17/17] tracing/uprobes: Add @+file_offset fetch method

[Oleg Nesterov <oleg@redhat.com>]

Hi Namhyung,

I'll certainly try to read (and even apply ;) this series carefully.

But let me make a couple of nits right now, even if I do not understand
this code yet.

On 11/27, Namhyung Kim wrote:
>
> +		} else if (arg[1] == '+') {
> +			struct file_offset_fetch_param *foprm;
> +
> +			/* kprobes don't support file offsets */
> +			if (is_kprobe)
> +				return -EINVAL;
> +
> +			ret = kstrtol(arg + 2, 0, &offset);
> +			if (ret)
> +				break;
> +
> +			foprm = kzalloc(sizeof(*foprm), GFP_KERNEL);
> +			if (!foprm)
> +				return -ENOMEM;
> +
> +			foprm->tu = priv;
> +			foprm->offset = offset;

Hmm. I am not sure, but can't we simplify this?

Why do we need this foprm at all? To pass tu/offset obviously. But
why we need to store this info in fetch_param?

translate_user_vaddr() needs to access utask->vaddr anyway. It seems
to me it would be more clean to do the following:

	1. Add
		struct xxx {
			struct trace_uprobe *tu;
			unsigned long bp_addr;
		};

	   in trace_uprobe.c.

	2. Add

		struct xxx info = {
			.tu = tu,
			.bp_addr = instruction_pointer(regs);
		};

		current->utask->vaddr = (long)&info;

	   into uprobe_dispatcher() and uretprobe_dispatcher() (the latter
	   should obviously use func instead of instruction_pointer).

	 3. FETCH_FUNC_NAME(file_offset, type) can do

	 	struct xxx *info = (void*)current->utask->vaddr;
		void *addr = data + info->bp_addr - info->tu->offset;

	 	return FETCH_FUNC_NAME(memory, type)(regs, aaddr, dest);

	 4. Now, the only change we need in parse_probe_arg("@") is that
	    it should use either FETCH_MTD_memory or FETCH_MTD_file_offset
	    depending on arg[0] == '+'.

	    And we do not need to pass "void *prive" to parse_probe_arg().

What do you think? One again, I can be easily wrong, I didn't read the
code yet.

>  static int uprobe_dispatcher(struct uprobe_consumer *con, struct pt_regs *regs)
>  {
>  	struct trace_uprobe *tu;
> +	struct uprobe_task *utask;
>  	int ret = 0;
>  
>  	tu = container_of(con, struct trace_uprobe, consumer);
>  	tu->nhit++;
>  
> +	utask = current->utask;
> +	if (utask == NULL)
> +		return UPROBE_HANDLER_REMOVE;

Hmm, why? The previous change ensures ->utask is not NULL? If we hit
NULL we have a bug, we should not remove this uprobe.

Oleg.