On Mon 16-04-18 15:55:36, Jann Horn wrote: > On Mon, Apr 16, 2018 at 12:07 PM, Michal Hocko <mho...@kernel.org> wrote: > > On Fri 13-04-18 18:17:36, Jann Horn wrote: > >> On Fri, Apr 13, 2018 at 6:05 PM, Jann Horn <ja...@google.com> wrote: > >> > On Fri, Apr 13, 2018 at 6:04 PM, Michal Hocko <mho...@kernel.org> wrote: > >> >> 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? > >> > > >> > No, it isn't. > > > > I could have been more specific, sorry. > > > >> mmap() with a hint (without MAP_FIXED) will always non-racily allocate > >> a memory region for you or return an error code. If it does allocate a > >> memory region, it belongs to you until you deallocate it. It might be > >> at a different address than you requested - > > > > Yes, this all is true. Except the atomicity is guaranteed only for the > > syscall. Once you return to the userspace any error handling is error > > prone and racy because your mapping might change under you feet. So... > > Can you please elaborate on why you think anything could change the > mapping returned by mmap() under the caller's feet?
Because as soon as the mmap_sem is dropped then any other thread can modify the shared address space. > When mmap() returns a memory area to the caller, that memory area > belongs to the caller. No unrelated code will touch it, unless that > code is buggy. 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. -- Michal Hocko SUSE Labs
