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

[Jann Horn]

On Fri, Apr 13, 2018 at 8:49 AM, Michal Hocko <mhocko@xxxxxxxxxx> wrote:
> On Fri 13-04-18 08:43:27, Michael Kerrisk wrote:
> [...]
>> 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.
>
> Yes, that sounds reasonable to me.

But that kind of sounds as if you can't avoid it before Linux 4.17,
when actually, you just have to call mmap() with the address as hint,
and if mmap() returns a different address, munmap() it and go on your
normal error path.