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Re: [RFC PATCH 7/7] sound: core: Avoid using timespec for struct snd_timer_tread

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On 22 September 2017 at 15:57, Arnd Bergmann <arnd@arndb.de> wrote:
> On Fri, Sep 22, 2017 at 5:00 AM, Baolin Wang <baolin.wang@linaro.org> wrote:
>> On 21 September 2017 at 21:09, Arnd Bergmann <arnd@arndb.de> wrote:
>>> On Thu, Sep 21, 2017 at 8:18 AM, Baolin Wang <baolin.wang@linaro.org> wrote:
>>>
>>>> +static int snd_timer_user_tread(void __user *argp, struct snd_timer_user *tu,
>>>> +                               unsigned int cmd)
>>>> +{
>>>> +       int __user *p = argp;
>>>> +       int xarg, old_tread;
>>>> +
>>>> +       if (tu->timeri) /* too late */
>>>> +               return -EBUSY;
>>>> +       if (get_user(xarg, p))
>>>> +               return -EFAULT;
>>>> +
>>>> +       old_tread = tu->tread;
>>>> +#if __BITS_PER_LONG == 64
>>>> +       tu->tread = xarg ? 2 : 0;
>>>> +#ifdef IA32_EMULATION
>>>> +       tu->tread = xarg ? 3 : 0;
>>>> +#endif
>>>> +#else
>>>> +       if (cmd == SNDRV_TIMER_IOCTL_TREAD64)
>>>> +               tu->tread = xarg ? 2 : 0;
>>>> +       else
>>>> +               tu->tread = xarg ? 1 : 0;
>>>> +#endif
>>>
>>> The 64-bit case looks broken here:
>>>
>>> - The tread flag is different for compat and native mode, so you
>>>    must pass a flag to identify whether you are called from
>>>    __snd_timer_user_ioctl or from snd_timer_user_ioctl_compat().
>>
>> I have some confusion here. For 64-bit, we will set tu->tread = 2 no
>> matter it is native mode or compat mode, only we will set tu->tread =
>> 3 for x86_32 in compat mode, right?
>> So I think we do not need to identify whether called from native mode
>> or compat mode.
>
> When we have a user space program with 32-bit time_t in compat mode
> (i.e. cmd==SNDRV_TIMER_IOCTL_TREAD) on a 64-bit kernel, we want
> to set tread=1, and that is different from the native mode that wants to
> set tread=2.

I understand your meaning now, thanks for explanation.

>
> For determining whether to use tread=2 or tread=3, we have to check
> both compat mode and x32 mode. This could be done by checking for
> "if (IS_ENABLED(CONFIG_IA32_EMULATION) && in_compat_syscall() &&
> is_x32_task())", but the in_compat_syscall() check can be skipped when
> you know that you were called from .compat_ioct().

OK.

-- 
Baolin.wang
Best Regards

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[RFC PATCH 7/7] sound: core: Avoid using timespec for struct snd_timer_tread Baolin Wang <baolin.wang@linaro.org> - 2017-09-21 08:20 +0200
  Re: [RFC PATCH 7/7] sound: core: Avoid using timespec for struct snd_timer_tread Arnd Bergmann <arnd@arndb.de> - 2017-09-21 15:10 +0200
    Re: [RFC PATCH 7/7] sound: core: Avoid using timespec for struct snd_timer_tread Baolin Wang <baolin.wang@linaro.org> - 2017-09-22 05:10 +0200
      Re: [RFC PATCH 7/7] sound: core: Avoid using timespec for struct snd_timer_tread Arnd Bergmann <arnd@arndb.de> - 2017-09-22 10:00 +0200
        Re: [RFC PATCH 7/7] sound: core: Avoid using timespec for struct snd_timer_tread Baolin Wang <baolin.wang@linaro.org> - 2017-09-22 10:40 +0200

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