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

[Jann Horn]

On Fri, Apr 13, 2018 at 6:04 PM, Michal Hocko <mhocko@xxxxxxxxxx> wrote:
> On Fri 13-04-18 17:04:09, Jann Horn wrote:
>> 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.
>
> This is still racy in multithreaded application which is the main point
> of the whole section, no?

No, it isn't.