Re: [PATCHv3 6/9] staging: vme_user: switch to returning -EFAULT on __copy_*_user errors

[Martyn Welch <martyn.welch@ge.com>]

On 06/07/15 14:10, Dmitry Kalinkin wrote:
> On Mon, Jul 6, 2015 at 3:51 PM, Martyn Welch <martyn.welch@ge.com> wrote:
>> On 26/06/15 21:39, Dmitry Kalinkin wrote:
>>>
>>> Signed-off-by: Dmitry Kalinkin <dmitry.kalinkin@gmail.com>
>>> ---
>>>    drivers/staging/vme/devices/vme_user.c | 47
>>> ++++++++--------------------------
>>>    1 file changed, 11 insertions(+), 36 deletions(-)
>>>
>>> diff --git a/drivers/staging/vme/devices/vme_user.c
>>> b/drivers/staging/vme/devices/vme_user.c
>>> index a2345db..ef876a4 100644
>>> --- a/drivers/staging/vme/devices/vme_user.c
>>> +++ b/drivers/staging/vme/devices/vme_user.c
>>> @@ -123,7 +123,6 @@ struct vme_user_vma_priv {
>>>    static ssize_t resource_to_user(int minor, char __user *buf, size_t
>>> count,
>>>                                  loff_t *ppos)
>>>    {
>>> -       ssize_t retval;
>>>          ssize_t copied = 0;
>>>
>>>          if (count > image[minor].size_buf)
>>> @@ -135,13 +134,8 @@ static ssize_t resource_to_user(int minor, char
>>> __user *buf, size_t count,
>>>          if (copied < 0)
>>>                  return (int)copied;
>>>
>>> -       retval = __copy_to_user(buf, image[minor].kern_buf,
>>> -                               (unsigned long)copied);
>>> -       if (retval != 0) {
>>> -               copied = (copied - retval);
>>> -               pr_info("User copy failed\n");
>>> -               return -EINVAL;
>>> -       }
>>> +       if (__copy_to_user(buf, image[minor].kern_buf, (unsigned
>>> long)copied))
>>> +               return -EFAULT;
>>>
>>>          return copied;
>>
>>
>> Does __copy_to_user() not return the number of bytes still to be copied? The
>> above seems to add the assumption that __copy_to_user()
>> can't return part way through a copy.
> Yes it does. And this information is useless in userspace.
> Returning -EFAULT would be more informative in this case.
> I couldn't find an example of kernel code doing this any other way:
> https://urldefense.proofpoint.com/v2/url?u=http-3A__lxr.free-2Delectrons.com_ident-3Fi-3D-5F-5Fcopy-5Fto-5Fuser&d=AwIBaQ&c=IV_clAzoPDE253xZdHuilRgztyh_RiV3wUrLrDQYWSI&r=o_jX-O9t9rDa5QmNKEzuslLxr8l2x1M2rHid0HdzmY4&m=YqOunx1QvC1jz8WjtHd_feWk5cdNWIVW0utSWesR8ag&s=BdRq0di7Bryi1LqA3LtL_Z9UeSHdpiYwghMElfWbR1A&e=

Fair enough. If that's the approach taken elsewhere, then I'm happy for 
this to become consistent.

>>
>>>    }
>>> @@ -149,21 +143,16 @@ static ssize_t resource_to_user(int minor, char
>>> __user *buf, size_t count,
>>>    static ssize_t resource_from_user(unsigned int minor, const char __user
>>> *buf,
>>>                                    size_t count, loff_t *ppos)
>>>    {
>>> -       ssize_t retval;
>>>          ssize_t copied = 0;
>>>
>>>          if (count > image[minor].size_buf)
>>>                  count = image[minor].size_buf;
>>>
>>> -       retval = __copy_from_user(image[minor].kern_buf, buf,
>>> -                                 (unsigned long)count);
>>> -       if (retval != 0)
>>> -               copied = (copied - retval);
>>> -       else
>>> -               copied = count;
>>> +       if (__copy_from_user(image[minor].kern_buf, buf, (unsigned
>>> long)count))
>>> +               return -EFAULT;
>>>
>>
>> Same here.
>>
>>>          copied = vme_master_write(image[minor].resource,
>>> image[minor].kern_buf,
>>> -                                 copied, *ppos);
>>> +                                 count, *ppos);
>>>
>>>          return copied;
>>>    }
>>> @@ -172,38 +161,24 @@ static ssize_t buffer_to_user(unsigned int minor,
>>> char __user *buf,
>>>                                size_t count, loff_t *ppos)
>>>    {
>>>          void *image_ptr;
>>> -       ssize_t retval;
>>>
>>>          image_ptr = image[minor].kern_buf + *ppos;
>>> +       if (__copy_to_user(buf, image_ptr, (unsigned long)count))
>>> +               return -EFAULT;
>>>
>>
>> Ditto.
>>
>>> -       retval = __copy_to_user(buf, image_ptr, (unsigned long)count);
>>> -       if (retval != 0) {
>>> -               retval = (count - retval);
>>> -               pr_warn("Partial copy to userspace\n");
>>> -       } else
>>> -               retval = count;
>>> -
>>> -       /* Return number of bytes successfully read */
>>> -       return retval;
>>> +       return count;
>>>    }
>>>
>>>    static ssize_t buffer_from_user(unsigned int minor, const char __user
>>> *buf,
>>>                                  size_t count, loff_t *ppos)
>>>    {
>>>          void *image_ptr;
>>> -       size_t retval;
>>>
>>>          image_ptr = image[minor].kern_buf + *ppos;
>>> +       if (__copy_from_user(image_ptr, buf, (unsigned long)count))
>>> +               return -EFAULT;
>>>
>>
>> And here.
>>
>>> -       retval = __copy_from_user(image_ptr, buf, (unsigned long)count);
>>> -       if (retval != 0) {
>>> -               retval = (count - retval);
>>> -               pr_warn("Partial copy to userspace\n");
>>> -       } else
>>> -               retval = count;
>>> -
>>> -       /* Return number of bytes successfully read */
>>> -       return retval;
>>> +       return count;
>>>    }
>>>
>>>    static ssize_t vme_user_read(struct file *file, char __user *buf, size_t
>>> count,
>>>
>>
>> --
>> Martyn Welch (Lead Software Engineer)  | Registered in England and Wales
>> GE Intelligent Platforms               | (3828642) at 100 Barbirolli Square
>> T +44(0)1327322748                     | Manchester, M2 3AB
>> E martyn.welch@ge.com                  | VAT:GB 927559189

-- 
Martyn Welch (Lead Software Engineer)  | Registered in England and Wales
GE Intelligent Platforms               | (3828642) at 100 Barbirolli Square
T +44(0)1327322748                     | Manchester, M2 3AB
E martyn.welch@ge.com                  | VAT:GB 927559189