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.
