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Subject: Re: [PATCH] mmap.2: MAP_FIXED is okay if the address range has been
 reserved
To:     Michal Hocko <mhocko@kernel.org>, Jann Horn <jannh@google.com>
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From:   "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com>
Message-ID: <43db1d63-fa64-4b4a-f3c3-edc73892bd23@gmail.com>
Date:   Wed, 2 May 2018 15:06:34 +0200
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Jann, Micha,

On 04/16/2018 11:11 PM, Michal Hocko wrote:
> On Mon 16-04-18 22:17:40, Jann Horn wrote:
>> On Mon, Apr 16, 2018 at 9:57 PM, Michal Hocko <mhocko@kernel.org> wrote:
>>> On Mon 16-04-18 21:30:09, Jann Horn wrote:
>>>> On Mon, Apr 16, 2018 at 9:18 PM, Michal Hocko <mhocko@kernel.org> wrote:
>>> [...]
>>>>> Yes, reasonably well written application will not have this problem.
>>>>> That, however, requires an external synchronization and that's why
>>>>> called it error prone and racy. I guess that was the main motivation for
>>>>> that part of the man page.
>>>>
>>>> What requires external synchronization? I still don't understand at
>>>> all what you're talking about.
>>>>
>>>> The following code:
>>>>
>>>> void *try_to_alloc_addr(void *hint, size_t len) {
>>>>   char *x = mmap(hint, len, ...);
>>>>   if (x == MAP_FAILED) return NULL;
>>>>   if (x == hint) return x;
>>>
>>> Any other thread can modify the address space at this moment.
>>
>> But not parts of the address space that were returned by this mmap() call.
> ?
>>> Just
>>> consider that another thread would does mmap(x, MAP_FIXED) (or any other
>>> address overlapping [x, x+len] range)
>>
>> If the other thread does that without previously having created a
>> mapping covering the area in question, that would be a bug in the
>> other thread.
> 
> MAP_FIXED is sometimes used without preallocated address ranges.
> 
>> MAP_FIXED on an unmapped address is almost always a bug
>> (excluding single-threaded cases with no library code, and even then
>> it's quite weird) - for example, any malloc() call could also cause
>> libc to start using the memory range you're trying to map with
>> MAP_FIXED.
> 
> Yeah and that's why we there is such a large paragraph in the man page
> ;)
> 
>>> becaus it is seemingly safe as x
>>> != hint.
>>
>> I don't understand this part. Are you talking about a hypothetical
>> scenario in which a programmer attempts to segment the virtual memory
>> space into areas that are exclusively used by threads without creating
>> memory mappings for those areas?
> 
> Yeah, that doesn't sound all that over-exaggerated, right? And yes,
> such a code would be subtle and most probably buggy. I am not trying to
> argue for those hypothetical cases. All I am saying is that MAP_FIXED is
> subtle.
> 
> I _do_ agree that using it solely on the preallocated and _properly_
> managed address ranges is safe. I still maintain my position on error
> prone though. And besides that there are usecases which do not operate
> on preallocated address ranges so people really have to be careful.
> 
> I do not really care what is the form. I find the current wording quite
> informative and showing examples of how things might be broken. I do
> agree with your remark that "MAP_FIXED on preallocated ranges is safe"
> should be added. But MAP_FIXED is dangerous API and should have few big
> fat warnings.

So, at this stage, is any further patch needed to the text describing
MAP_FIXED/MAP_FIXED_REPLACE?

Cheers,

Michael


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