On Fri 13-04-18 17:04:09, Jann Horn wrote:
> On Fri, Apr 13, 2018 at 8:49 AM, Michal Hocko <mho...@kernel.org> 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?