[llvm-dev] [RFC] ASan: patches to support 32-byte shadow granularity
Walter Lee via llvm-dev
llvm-dev at lists.llvm.org
Tue Oct 31 14:29:26 PDT 2017
I've prepared a preliminary set of patches that makes ASan work with
32-byte shadow granularity, and I would like to get some feedback on
those patches as well as my general plan.
Some background: I am porting ASan to the Myriad platform. I'm
looking to break up that port into components that may be
useful/relevant to other platforms -- the first of those pieces is the
ability to use a coarser shadow granularity. This is important for us
because Myriad has a limited amount of physical memory and no virtual
memory, so it is important to limit the amount of shadow memory
My end-goal for this part is to be able to configure a build that
overrides the default shadow granularity, that can cleanly run the
clang/llvm test suite for 32-byte shadow granularity on i386/x86_64,
so we can set up buildbot for that configuration.
My basic plan:
1. Add build support to override of default shadow scale.
2. Fix various issues with 32-byte shadow granularity.
3. Propose improvements to 32-byte shadow granularity support.
4. Make test suite run cleanly for 32-byte shadow granularity.
5. Set up build bot for 32-byte shadow granularity on i386/x86_64.
My initial set of patches adds the build support and makes some
essential fixes to the compiler and run-time. They are:
https://reviews.llvm.org/D39469 [asan] Add cmake hook to override
default shadow scale
https://reviews.llvm.org/D39470 [asan] Fix size/alignment issues with
non-default shadow scale
https://reviews.llvm.org/D39471 [asan] Fix small X86_64 ShadowOffset
for non-default shadow scale
https://reviews.llvm.org/D39472 [asan] Ensure that the minimum redzone
is at least SHADOW_GRANULARITY
https://reviews.llvm.org/D39473 [sanitizers] Increase alignment of low
https://reviews.llvm.org/D39474 [asan] Avoid assert failure for
non-default shadow scale
https://reviews.llvm.org/D39475 [asan] Improve stack error reports for
large shadow granularity
The following features don't work yet, but can be fixed:
- i386/x86_64 assembly instrumentation.
- Prelink support. The memory map with MidMem for this configuration
is more complicated that that expected by compiler_rt. The current
HighShadow would overlap with MidMem so would need to be adjusted.
It's not clear to me that whether is an important feature?
- Intra object overflow. This appears to be an experimental feature,
and I have not ported the padding insertion for different
Some features will not work as well with the larger shadow
- Stack errors: it seems sensible not to insert 32-byte sentinels
between every object, but the result is that some stack overflow
gets reported as unknown or use-after-scope. I have a patch that
improves on the default behavior, but there remains cases where the
error reports will not be as good.
- __asan_poison_memory_region is now more limited. A typical case
that doesn't work is the poisoning of 8-byte or 16-byte that maps to
the middle of a shadow byte.
For testing, I have a few questions:
- Would it make sense to provide an internal compiler flag to set the
shadow granularity, so that there we can at least run the
instrumentation tests for 32-byte granularity in normal builds?
- Is there a reasonable subset of tests I can port to 32-byte
granularity to provide reasonable coverage, or should I aim to port
For now, I've tested my changes against check-all (for default shadow
granularity), and also set up a 32-byte shadow granularity build and
manually inspected the failures to ensure that they are not
More information about the llvm-dev