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

[Michael Kerrisk (man-pages)]

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.
> 
>>>> +.TP
>>>> +.BR MAP_FIXED_NOREPLACE " (since Linux 4.17)"
>>>> +Similar to MAP_FIXED with respect to the
>>>> +.I
>>>> +addr
>>>> +enforcement, but different in that MAP_FIXED_NOREPLACE never clobbers a pre-existing
>>>> +mapped range. If the requested range would collide with an existing
>>>> +mapping, then this call fails with
>>>> +.B EEXIST.
>>>> +This flag can therefore be used as a way to atomically (with respect to other
>>>> +threads) attempt to map an address range: one thread will succeed; all others
>>>> +will report failure. Please note that older kernels which do not recognize this
>>>> +flag will typically (upon detecting a collision with a pre-existing mapping)
>>>> +fall back to a "non-MAP_FIXED" type of behavior: they will return an address that
>>>> +is different than the requested one. Therefore, backward-compatible software
>>>> +should check the returned address against the requested address.
>>>>  .TP
>>>>  .B MAP_GROWSDOWN
>>>>  This flag is used for stacks.
>>>> @@ -487,6 +508,12 @@ is not a valid file descriptor (and
>>>>  .B MAP_ANONYMOUS
>>>>  was not set).
>>>>  .TP
>>>> +.B EEXIST
>>>> +range covered by
>>>> +.IR addr ,
>>>> +.IR length
>>>> +is clashing with an existing mapping.
>>>
>>> Maybe add something like ", and MAP_FIXED_NOREPLACE was specified"? I
>>> think most manpages explicitly document which error conditions can be
>>> triggered by which flags.
>>
>> sure, no objection from me.

I've added the suggested piece from Jann to the EEXIST error description.

Cheers,

Michael



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