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

[Michal Hocko]

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?

-- 
Michal Hocko
SUSE Labs