Re: [PATCH v4 1/4] mm/mlock: return EINVAL if len overflows for mlock/munlock

[mawupeng]

On 2023/3/22 17:01, David Hildenbrand wrote:
> On 22.03.23 09:54, David Hildenbrand wrote:
>> On 22.03.23 03:14, mawupeng wrote:
>>>
>>>
>>> On 2023/3/21 22:19, David Hildenbrand wrote:
>>>> On 21.03.23 08:44, mawupeng wrote:
>>>>>
>>>>>
>>>>> On 2023/3/20 18:54, David Hildenbrand wrote:
>>>>>> On 20.03.23 03:47, Wupeng Ma wrote:
>>>>>>> From: Ma Wupeng <mawupeng1@xxxxxxxxxx>
>>>>>>>
>>>>>>> While testing mlock, we have a problem if the len of mlock is ULONG_MAX.
>>>>>>> The return value of mlock is zero. But nothing will be locked since the
>>>>>>> len in do_mlock overflows to zero due to the following code in mlock:
>>>>>>>
>>>>>>>       len = PAGE_ALIGN(len + (offset_in_page(start)));
>>>>>>>
>>>>>>> The same problem happens in munlock.
>>>>>>>
>>>>>>> Add new check and return -EINVAL to fix this overflowing scenarios since
>>>>>>> they are absolutely wrong.
>>>>>>
>>>>>> Thinking again, wouldn't we reject mlock(0, ULONG_MAX) now as well?
>>>>>
>>>>> mlock will return 0 if len is zero which is the same w/o this patchset.
>>>>> Here is the calltrace if len is zero.
>>>>>
>>>>> mlock(len == 0)
>>>>>       do_mlock(len == 0)
>>>>>           if (!len)
>>>>>               return 0
>>>>>
>>>>
>>>> I was asking about addr=0, len=ULONG_MAX.
>>>>
>>>> IIUC, that used to work but could now fail? I haven't played with it, though.
>>>
>>> Thanks for reviewing.
>>>
>>> Previous for add = 0 and len == ULONG_MAX, mlock will return ok(0) since len overflows to zero.
>>> IFAICT, this is not right since mlock do noting(lock nothing) and return ok(0).
>>>
>>> With this patch, for the same situation, mlock can return EINVAL as expected.
>>
>> Quoting the man page:
>>
>> "EINVAL (mlock(),  mlock2(),  and  munlock()) The result of the addition
>> addr+len was less than addr (e.g., the addition may have resulted in an
>> overflow)."
>>
>> ULONG_MAX+0 = ULONG_MAX
>>
>> There is no overflow expected. The proper way to implement it would be
>> to handle that case and not fail with EINVAL.
>>
>> At least that would be expected when reading the man page.
>>
> 
> As a side note, I agree that failing with EINVAL is better than doing noting (mlocking nothing). But I am not sure what we are expected to do in that case ... the man page is a bit vague on that.

Thanks for you reviewing.

Can we try to expand the man page since overflow is considered in man page?

>