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

[Michael Kerrisk (man-pages)]

On 04/12/2018 09:24 PM, John Hubbard wrote:
> 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.

So, you mean remove this entire paragraph:

              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 MAP_FIXED_NOREPLACE that
              should be used instead; older kernels require the caller to
              use addr as a hint (without MAP_FIXED) and take appropriate
              action if the kernel places the new mapping at a  different
              address.

It seems like some version of the first half of the paragraph is worth 
keeping, though, so as to point the reader in the direction of a remedy.
How about replacing that text with the following:

              Since  Linux 4.17, the MAP_FIXED_NOREPLACE flag can be used
              in a multithreaded program to avoid  the  hazard  described
              above.

?

Thanks,

Michael

-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/