On Tue, Jul 26, 2016 at 11:27:11AM -0700, william.c.roberts@xxxxxxxxx wrote: > From: William Roberts <william.c.roberts@xxxxxxxxx> > > This patch introduces the ability randomize mmap locations where the > address is not requested, for instance when ld is allocating pages for > shared libraries. It chooses to randomize based on the current > personality for ASLR. > > Currently, allocations are done sequentially within unmapped address > space gaps. This may happen top down or bottom up depending on scheme. > > For instance these mmap calls produce contiguous mappings: > int size = getpagesize(); > mmap(NULL, size, flags, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x40026000 > mmap(NULL, size, flags, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x40027000 > > Note no gap between. > > After patches: > int size = getpagesize(); > mmap(NULL, size, flags, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x400b4000 > mmap(NULL, size, flags, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x40055000 > > Note gap between. And why is it good? > Using the test program mentioned here, that allocates fixed sized blocks > till exhaustion: https://www.linux-mips.org/archives/linux-mips/2011-05/msg00252.html, > no difference was noticed in the number of allocations. Most varied from > run to run, but were always within a few allocations of one another > between patched and un-patched runs. > > Performance Measurements: > Using strace with -T option and filtering for mmap on the program > ls shows a slowdown of approximate 3.7% NAK. It's just too costly. And no obvious benefits. -- Kirill A. Shutemov -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>