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

[Jann Horn]

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.

> 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.