Re: [PATCH v8 3/6] Uprobes: Support SDT markers having reference count (semaphore)

Ravi Bangoria
2018-08-14T04:37:46Z

Hi Song,

On 08/13/2018 10:42 PM, Song Liu wrote:
> On Mon, Aug 13, 2018 at 6:17 AM, Oleg Nesterov <oleg@redhat.com> wrote:
>> On 08/13, Ravi Bangoria wrote:
>>>
>>>> But damn, process creation (exec) is trivial. We could add a new uprobe_exec()
>>>> hook and avoid delayed_uprobe_install() in uprobe_mmap().
>>>
>>> I'm sorry. I didn't get this.
>>
>> Sorry for confusion...
>>
>> I meant, if only exec*( could race with _register(), we could add another uprobe
>> hook which updates all (delayed) counters before return to user-mode.
>>
>>>> Afaics, the really problematic case is dlopen() which can race with _register()
>>>> too, right?
>>>
>>> dlopen() should internally use mmap() right? So what is the problem here? Can
>>> you please elaborate.
>>
>> What I tried to say is that we can't avoid uprobe_mmap()->delayed_uprobe_install()
>> because dlopen() can race with _register() too, just like exec.
>>
>> Oleg.
>>
>
> How about we do delayed_uprobe_install() per file? Say we keep a list
> of delayed_uprobe
> in load_elf_binary(). Then we can install delayed_uprobe after loading
> all sections of the
> file.

I'm not sure if I totally understood the idea. But how this approach can
solve dlopen() race with _register()?

Rather, making delayed_uprobe_list an mm field seems simple and effective
idea to me. The only overhead will be list_empty(mm->delayed_list) check.

Please let me know if I misunderstood you.

Thanks,
Ravi