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? -- Michal Hocko SUSE Labs
