From mboxrd@z Thu Jan 1 00:00:00 1970 From: Jann Horn Subject: Re: [PATCH] mmap.2: MAP_FIXED is okay if the address range has been reserved Date: Mon, 16 Apr 2018 23:12:48 +0200 Message-ID: References: <20180413160435.GA17484@dhcp22.suse.cz> <20180416100736.GG17484@dhcp22.suse.cz> <20180416191805.GS17484@dhcp22.suse.cz> <20180416195726.GT17484@dhcp22.suse.cz> <20180416211115.GU17484@dhcp22.suse.cz> Mime-Version: 1.0 Content-Type: text/plain; charset="UTF-8" Return-path: In-Reply-To: <20180416211115.GU17484@dhcp22.suse.cz> Sender: linux-kernel-owner@vger.kernel.org To: Michal Hocko Cc: "Michael Kerrisk (man-pages)" , John Hubbard , linux-man , Andrew Morton , Linux-MM , lkml , Linux API List-Id: linux-man@vger.kernel.org On Mon, Apr 16, 2018 at 11:11 PM, Michal Hocko wrote: > On Mon 16-04-18 22:17:40, Jann Horn wrote: >> On Mon, Apr 16, 2018 at 9:57 PM, Michal Hocko wrote: >> > On Mon 16-04-18 21:30:09, Jann Horn wrote: >> >> On Mon, Apr 16, 2018 at 9:18 PM, Michal Hocko 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. >> >> But not parts of the address space that were returned by this mmap() call. > ? >> > Just >> > consider that another thread would does mmap(x, MAP_FIXED) (or any other >> > address overlapping [x, x+len] range) >> >> If the other thread does that without previously having created a >> mapping covering the area in question, that would be a bug in the >> other thread. > > MAP_FIXED is sometimes used without preallocated address ranges. Wow, really? Can you point to an example? >> MAP_FIXED on an unmapped address is almost always a bug >> (excluding single-threaded cases with no library code, and even then >> it's quite weird) - for example, any malloc() call could also cause >> libc to start using the memory range you're trying to map with >> MAP_FIXED. > > Yeah and that's why we there is such a large paragraph in the man page > ;) > >> > becaus it is seemingly safe as x >> > != hint. >> >> I don't understand this part. Are you talking about a hypothetical >> scenario in which a programmer attempts to segment the virtual memory >> space into areas that are exclusively used by threads without creating >> memory mappings for those areas? > > Yeah, that doesn't sound all that over-exaggerated, right? And yes, > such a code would be subtle and most probably buggy. I am not trying to > argue for those hypothetical cases. All I am saying is that MAP_FIXED is > subtle. > > I _do_ agree that using it solely on the preallocated and _properly_ > managed address ranges is safe. I still maintain my position on error > prone though. And besides that there are usecases which do not operate > on preallocated address ranges so people really have to be careful. > > I do not really care what is the form. I find the current wording quite > informative and showing examples of how things might be broken. I do > agree with your remark that "MAP_FIXED on preallocated ranges is safe" > should be added. But MAP_FIXED is dangerous API and should have few big > fat warnings. > -- > Michal Hocko > SUSE Labs