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

[Michal Hocko <mhocko@kernel.org>]

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. Just
consider that another thread would does mmap(x, MAP_FIXED) (or any other
address overlapping [x, x+len] range) becaus it is seemingly safe as x
!= hint. This will succeed and ...
>   munmap(x, len);
... now you are munmaping somebody's else memory range

>   return NULL;

Do code _is_ buggy but it is not obvious at all.

> }
> 
> has no need for any form of external synchronization.

If the above mmap/munmap section was protected by a lock and _all_ other
mmaps (direct or indirect) would use the same lock then you are safe
against that.
-- 
Michal Hocko
SUSE Labs