Messages in this thread | | | Date | Mon, 16 Apr 2018 21:57:26 +0200 | From | Michal Hocko <> | Subject | Re: [PATCH] mmap.2: MAP_FIXED is okay if the address range has been reserved |
| |
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
| |