Re: [PATCH] mmap.2: document new MAP_FIXED_NOREPLACE flag

[Michael Kerrisk (man-pages)]

Hi Jann,

On 04/11/2018 06:40 PM, Jann Horn wrote:
> On Wed, Apr 11, 2018 at 6:36 PM, Michal Hocko <mhocko@xxxxxxxxxx> wrote:
>> On Wed 11-04-18 17:37:46, Jann Horn wrote:
>>> On Wed, Apr 11, 2018 at 2:04 PM,  <mhocko@xxxxxxxxxx> wrote:
>>>> From: Michal Hocko <mhocko@xxxxxxxx>
>>>>
>>>> 4.17+ kernels offer a new MAP_FIXED_NOREPLACE flag which allows the caller to
>>>> atomicaly probe for a given address range.
>>>>
>>>> [wording heavily updated by John Hubbard <jhubbard@xxxxxxxxxx>]
>>>> Signed-off-by: Michal Hocko <mhocko@xxxxxxxx>
>>>> ---
>>>> Hi,
>>>> Andrew's sent the MAP_FIXED_NOREPLACE to Linus for the upcoming merge
>>>> window. So here we go with the man page update.
>>>>
>>>>  man2/mmap.2 | 27 +++++++++++++++++++++++++++
>>>>  1 file changed, 27 insertions(+)
>>>>
>>>> diff --git a/man2/mmap.2 b/man2/mmap.2
>>>> index ea64eb8f0dcc..f702f3e4eba2 100644
>>>> --- a/man2/mmap.2
>>>> +++ b/man2/mmap.2
>>>> @@ -261,6 +261,27 @@ Examples include
>>>>  and the PAM libraries
>>>>  .UR http://www.linux-pam.org
>>>>  .UE .
>>>> +Newer kernels
>>>> +(Linux 4.17 and later) have a
>>>> +.B MAP_FIXED_NOREPLACE
>>>> +option that avoids the corruption problem; if available, MAP_FIXED_NOREPLACE
>>>> +should be preferred over MAP_FIXED.
>>>
>>> This still looks wrong to me. There are legitimate uses for MAP_FIXED,
>>> and for most users of MAP_FIXED that I'm aware of, MAP_FIXED_NOREPLACE
>>> wouldn't work while MAP_FIXED works perfectly well.
>>>
>>> MAP_FIXED is for when you have already reserved the targeted memory
>>> area using another VMA; MAP_FIXED_NOREPLACE is for when you haven't.
>>> Please don't make it sound as if MAP_FIXED is always wrong.
>>
>> Well, this was suggested by John. I think, nobody is objecting that
>> MAP_FIXED has legitimate usecases. The above text just follows up on
>> the previous section which emphasises the potential memory corruption
>> problems and it suggests that a new flag is safe with that regards.
>>
>> If you have specific wording that would be better I am open for changes.
> 
> I guess I'd probably also want to change the previous text; so I
> should probably send a followup patch once this one has landed.
Okay -- I'm ready to take that piece now. Please send me a patch!

Cheers,

Michael

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