Re: [PATCH] mmap.2: MAP_FIXED is okay if the address range has been reserved

[John Hubbard]

On 04/12/2018 12:18 PM, Jann Horn wrote:
> On Thu, Apr 12, 2018 at 8:59 PM, John Hubbard <jhubbard@xxxxxxxxxx> wrote:
>> On 04/12/2018 11:49 AM, Jann Horn wrote:
>>> On Thu, Apr 12, 2018 at 8:37 PM, Michael Kerrisk (man-pages)
>>> <mtk.manpages@xxxxxxxxx> wrote:
>>>> Hi John,
>>>>
>>>> On 12 April 2018 at 20:33, John Hubbard <jhubbard@xxxxxxxxxx> wrote:
>>>>> On 04/12/2018 08:39 AM, Jann Horn wrote:
>>>>>> Clarify that MAP_FIXED is appropriate if the specified address range has
>>>>>> been reserved using an existing mapping, but shouldn't be used otherwise.
>>>>>>
>>>>>> Signed-off-by: Jann Horn <jannh@xxxxxxxxxx>
>>>>>> ---
>>>>>>  man2/mmap.2 | 19 +++++++++++--------
>>>>>>  1 file changed, 11 insertions(+), 8 deletions(-)
>>>>>>
>>>>>> diff --git a/man2/mmap.2 b/man2/mmap.2
>>> [...]
>>>>>>  .IP
>>>>>>  For example, suppose that thread A looks through
>>>>>> @@ -284,13 +285,15 @@ and the PAM libraries
>>>>>>  .UR http://www.linux-pam.org
>>>>>>  .UE .
>>>>>>  .IP
>>>>>> -Newer kernels
>>>>>> -(Linux 4.17 and later) have a
>>>>>> +For cases in which the specified memory region has not been reserved using an
>>>>>> +existing mapping, newer kernels (Linux 4.17 and later) provide an option
>>>>>>  .B MAP_FIXED_NOREPLACE
>>>>>> -option that avoids the corruption problem; if available,
>>>>>> -.B MAP_FIXED_NOREPLACE
>>>>>> -should be preferred over
>>>>>> -.BR MAP_FIXED .
>>>>>> +that should be used instead; older kernels require the caller to use
>>>>>> +.I addr
>>>>>> +as a hint (without
>>>>>> +.BR MAP_FIXED )
>>>>>
>>>>> Here, I got lost: the sentence suddenly jumps into explaining non-MAP_FIXED
>>>>> behavior, in the MAP_FIXED section. Maybe if you break up the sentence, and
>>>>> possibly omit non-MAP_FIXED discussion, it will help.
>>>>
>>>> Hmmm -- true. That piece could be a little clearer.
>>>
>>> How about something like this?
>>>
>>>               For  cases in which MAP_FIXED can not be used because
>>> the specified memory
>>>               region has not been reserved using an existing mapping,
>>> newer kernels
>>>               (Linux  4.17  and  later)  provide  an  option
>>> MAP_FIXED_NOREPLACE  that
>>>               should  be  used  instead. Older kernels require the
>>>               caller to use addr as a hint and take appropriate action if
>>>               the kernel places the new mapping at a different address.
>>>
>>> John, Michael, what do you think?
>>
>>
>> I'm still having difficulty with it, because this is in the MAP_FIXED section,
>> but I think you're documenting the behavior that you get if you do *not*
>> specify MAP_FIXED, right? Also, the hint behavior is true of both older and
>> new kernels...
> 
> The manpage patch you and mhocko wrote mentioned MAP_FIXED_NOREPLACE
> in the MAP_FIXED section - I was trying to avoid undoing a change you
> had just explicitly made.

heh. So I've succeeding in getting my own wording removed, then? Progress! :)

> 
>> So, if that's your intent (you want to sort of document by contrast to what
>> would happen if this option were not used), then how about something like this:
>>
>>
>> Without the MAP_FIXED option, the kernel would treat addr as a hint, rather
>> than a requirement, and the caller would need to take appropriate action
>> if the kernel placed the mapping at a different address. (For example,
>> munmap and try again.)
> 
> I'd be fine with removing the paragraph. As you rightly pointed out,
> it doesn't really describe MAP_FIXED.
> 

OK, that's probably the simplest fix.

thanks,
-- 
John Hubbard
NVIDIA