On Fri, Dec 15, 2017 at 12:06 AM, John Hubbard <jhubbard@xxxxxxxxxx> wrote: > On 12/13/2017 06:52 PM, Jann Horn wrote: >> On Wed, Dec 13, 2017 at 10:31 AM, Michal Hocko <mhocko@xxxxxxxxxx> wrote: >>> From: John Hubbard <jhubbard@xxxxxxxxxx> > [...] >>> +.IP >>> +Furthermore, this option is extremely hazardous (when used on its own), because >>> +it forcibly removes pre-existing mappings, making it easy for a multi-threaded >>> +process to corrupt its own address space. >> >> I think this is worded unfortunately. It is dangerous if used >> incorrectly, and it's a good tool when used correctly. >> >> [...] >>> +Thread B need not create a mapping directly; simply making a library call >>> +that, internally, uses >>> +.I dlopen(3) >>> +to load some other shared library, will >>> +suffice. The dlopen(3) call will map the library into the process's address >>> +space. Furthermore, almost any library call may be implemented using this >>> +technique. >>> +Examples include brk(2), malloc(3), pthread_create(3), and the PAM libraries >>> +(http://www.linux-pam.org). >> >> This is arkward. This first mentions dlopen(), which is a very niche >> case, and then just very casually mentions the much bigger >> problem that tons of library functions can allocate memory through >> malloc(), causing mmap() calls, sometimes without that even being >> a documented property of the function. >> > > Hi Jann, > > Here is some proposed new wording, to address your two comments above. What do > you think of this: > > NOTE: this option can be hazardous (when used on its own), because it > forcibly removes pre-existing mappings, making it easy for a multi- > threaded process to corrupt its own address space. For example, thread A > looks through /proc/<pid>/maps and locates an available address range, > while thread B simultaneously acquires part or all of that same address > range. Thread A then calls mmap(MAP_FIXED), effectively overwriting the > mapping that thread B created. > > Thread B need not create a mapping directly; simply making a library call > whose implementation calls malloc(3), mmap(), or dlopen(3) will suffice, > because those calls all create new mappings. Thanks! That sounds better to me. >>> +.IP >>> +Newer kernels >>> +(Linux 4.16 and later) have a >>> +.B MAP_FIXED_SAFE >>> +option that avoids the corruption problem; if available, MAP_FIXED_SAFE >>> +should be preferred over MAP_FIXED. >> >> This is bad advice. MAP_FIXED is completely safe if you use it on an address >> range you've allocated, and it is used in this way by core system libraries to >> place multiple VMAs in virtually contiguous memory, for example: > [...] >> MAP_FIXED is a better solution for these usecases than MAP_FIXED_SAFE, >> or whatever it ends up being called. Please remove this advice or, better, >> clarify what MAP_FIXED should be used for (creation of virtually contiguous >> VMAs) and what MAP_FIXED_SAFE should be used for (attempting to >> allocate memory at a fixed address for some reason, with a failure instead of >> the normal fallback to using a different address). >> > > Rather than risk another back-and-forth with Michal (who doesn't want any advice > on how to use this safely, in the man page), I've simply removed this advice > entirely. Makes sense.
