On Thu, Dec 4, 2014 at 4:48 PM, Yury Gribov <y.gri...@samsung.com> wrote: > On 12/04/2014 03:47 PM, Dmitry Vyukov wrote: >> >> size_in_bytes = -1 instrumentation is too slow to be the default in >> kernel. >> >> If we want to pursue this, I propose a different scheme. >> Handle 8+ byte accesses as 1/2/4 accesses. No changes to 1/2/4 access >> handling. >> Currently when we allocate, say, 17-byte object we store 0 0 1 into >> shadow. An 8-byte access starting at offset 15 won't be detected, >> because the corresponding shadow value is 0. Instead we start storing >> 0 9 1 into shadow. Then the first shadow != 0 check will fail, and the >> precise size check will catch the OOB access. >> Make this scheme the default for kernel (no additional flags). >> >> This scheme has the following advantages: >> - load shadow only once (as opposed to the current size_in_bytes = -1 >> check that loads shadow twice) >> - less code in instrumentation >> - accesses to beginning and middle of the object are not slowed down >> (shadow still contains 0, so fast-path works); only accesses to the >> very last bytes of the object are penalized. > > > Makes sense. The scheme actually looks bullet-proof, why Asan team > preferred current (fast but imprecise) algorithm? > > BTW I think we'll want this option in userspace so well so we'll probably > need to update libasan.
We've discussed this scheme, but nobody has shown that it's important enough. It bloats binary (we do have issues with binary sizes) and slows down execution a bit. And if it is non-default mode, then it adds more flags (which is bad) and adds more configurations to test. For this to happen somebody needs to do research on (1) binary size increase, (2) slowdown, (3) number of additional bugs it finds (we can run it over extensive code base that is currently asan-clean). Here is the issue with some notes: https://code.google.com/p/address-sanitizer/issues/detail?id=100 -- You received this message because you are subscribed to the Google Groups "address-sanitizer" group. To unsubscribe from this group and stop receiving emails from it, send an email to address-sanitizer+unsubscr...@googlegroups.com. For more options, visit https://groups.google.com/d/optout.
