[llvm-dev] RFC: Generalize means the sanitizers work with memory

Kostya Serebryany via llvm-dev llvm-dev at lists.llvm.org
Mon Feb 27 17:32:51 PST 2017

Hi Ivan,

I've seen your message, but did not have a chance to carefully read, sorry.
Busy weeks.
I may have time next week, or maybe some one else replies earlier.
Don't hesitate to ping me ~ mid next week.

Some suggestions:
* if you use http://llvm.org/docs/Phabricator.html for patches you are more
likely to get attention from us.
* be more concrete, e.g. instead of "platforms that lack such support"
mention which exactly
  platforms are affected (and what is the rest of the LLVM support story
for them)

On Thu, Feb 23, 2017 at 10:16 AM, Ivan A. Kosarev via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> RFC: Generalize means the sanitizers work with memory
> Overview
> ========
> Currently, LLVM sanitizers, such as Asan and Tsan, are tied to a specific
> memory model that relies on presence of hardware support for virtual
> memory.
> This prevents sanitizers from being used on platforms that lack such
> support,
> but otherwise are capable of running sanitized programs. Our research
> indicates that adding support for such platforms is possible with a
> relatively
> small amount of changes to the sanitizers source code and zero performance
> and
> size penalty on currently supported systems. We also found that these
> changes
> clarify and formalize the functional and performance dependencies between
> sanitizers and system memory so they can be considered an improvement in
> terms of design and readability regardless of the added capabilities. One
> can
> think of it as a zero-cost abstraction layer.
> The Approach
> ============
> To support platforms that do not have hardware virtual memory managers,
> we need to introduce the concept of physical memory pages that work as the
> storage for data that sanitizers currently read and write by virtual
> addresses. In presence of the concept of physical memory, every time we
> access
> virtual memory we have to translate the given virtual address to a physical
> one. For example, this check:
>    *(u8 *)MEM_TO_SHADOW(allocated) == 0
> becomes:
>    *MEM_TO_PSHADOW(allocated) == 0
> where the MEM_TO_PSHADOW(mem) macro is defined as:
>    #define MEM_TO_VSHADOW(mem) /* Whatever currently MEM_TO_SHADOW() is. */
> The VSHADOW_TO_PSHADOW(vs) macro returns a pointer to a byte within a
> physical page that corresponds to the given virtual address and allocates
> this
> page if it has not been allocated before. On platforms that leverage
> hardware
> virtual memory managers this macro returns the virtual address as a
> physical
> one:
>    #define VSHADOW_TO_PSHADOW(vs) (reinterpret_cast<u8*>((vs)))
> Physical pages are required to be aligned by their size. The size of
> physical
> pages is a multiple of the shadow memory granularity (8 bytes for Asan) and
> not less than the size of the widest scalar access we have to support (16
> bytes). This makes trivial finding page offsets, which we need to implement
> RTL functions efficiently. This also simplifies handling of aligned
> accesses
> to physical memory as they are known to not cross bounds of physical pages.
> Note that RTL functions have to be fixed to not rely on specific size,
> location or order of physical pages.
> In addition to the facilities that allow handling of individual accesses to
> the virtual memory we also need a set of functions that efficiently perform
> operations on specified ranges of virtual addresses:
> // Fills a virtual memory with a given value. May release zeroed pages. For
> // DFsan we may need a version of this function that takes 16-bit values to
> // fill with.
> void vshadow_memset(uptr vs, u8 value, uptr size);
> // Similarly to vshadow_memset(), this function fills a range of virtual
> // memory with a given value and additionally claims that range as
> read-only
> // so the memory manager is not required to support modifying accesses for
> // these addresses.
> void fill_rodata_vshadow(uptr vs, u8 value, uptr size);
> // Copies potentially overlapping memory regions.
> void vshadow_memmove(uptr dest, uptr src, uptr size);
> // Returns the virtual address of the first non-zero byte in a given
> virtual
> // address range. Can also be used to test for zeroed regions.
> uptr find_non_zero_vshadow_byte(uptr vs, uptr size);
> // Explicitly releases pages that fit the specified range.
> void release_vshadow(uptr vs, uptr size);
> The Proof-of-Concept Patch
> ==========================
> To make sure the approach is feasible we have prepared a patch that
> fixes the Asan and Tsan RTL and instrumentation parts to translate virtual
> shadow memory addresses to physical ones and mmap() shadow memory as we
> access
> it. This way we simulate a software virtual memory manager that allocates
> physical storage for shadow memory on-demand.
> We used that to mock RTL for the sanitizers tests. With this mock in place
> we
> pass all Tsan tests and fail on 3 of 610 Asan tests:
> test/asan/TestCases/Linux/cuda_test.cc
> test/asan/TestCases/Linux/nohugepage_test.cc
> test/asan/TestCases/Linux/swapcontext_annotation.cc
> The first two tests rely on specific memory map after initializtion of the
> shadow memory and the latter takes too long to complete. It would probably
> be
> acceptable to XFAIL them when run with a software memory manager enabled
> and
> then consider ways to adopt them as necessary on a per-test basis.
> * * *
> With this paper we propose the changes that make it possible to use
> sanitizers
> on plaforms that have no MMUs to be part of the mainline. However, before
> moving further we would like some feedback from the community so comments
> are
> very appreciated.
> If the approach is fine, we will prepare a set of patches shortly.
> Thank you,
> --
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
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