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

Thu Mar 9 05:58:31 PST 2017

Hi Ivan,

Thanks for posting this; I'm excited by this proposal - if we can get 
this kind of support in without making the implementation 
non-trivially-harder to maintain, that would be a positive development. 
As Sean mentioned, I did something along these lines to adapt ASan to 
the IBM BG/Q - an HPC system that uses a lightweight operating system. 
On the BG/Q, the lightweight operating system does support virtual 
memory for some special-purpose mappings, but it does not support 
mapping unreserved pages (i.e. MAP_NORESERVE is not supported, and this 
functionality is not supported any other way). As a result, the 
mechanism that the sanitizers use to cover the complete address space 
using shadow memory - by mapping a large region of unreserved pages - 
won't work in this environment. Systems without virtual memory at all 
will obviously have the same problem: All shadow memory must be 
physically backed. I'll also mention that many normal Linux HPC 
environments are configured with overcommit turned off, and I believe 
that using the sanitizers in such environments would also currently not 
work.

Because all shadow memory must be physically backed, it must be 
allocated judicially, and the mapping process might need to be more 
complicated than a simple shift/offset. On the BG/Q, there were a few 
distinct regions of virtual memory that needed to be mapped into a 
single shadow region in the part of the address space where heap 
allocations could be made - as a result, I used a more-complicated 
mapping function.

In this light, I'm trying to understand your proposal. I see that you're 
proposing to add support for some kind of additional translation scheme 
between virtual addresses and physical addresses, but I'm not exactly 
sure how you propose to use them. It might help if you were to provide 
some hypothetical implementation of these translations for a simple 
system so that we can understand the usage model better. I'd also like 
to better understand how the instrumentation works; if the mapping 
always replaced by these __asan_mem_to_vshadow/__asan_mem_to_pshadow calls?

Finally, I recommend that we layer this support so that we have:

[regular system] -> [system without (sufficient) unreserved pages] -> 
[system without any mmu]

I'd like a clear explanation of how these last two differ. It looks like 
you have support for manually zeroing pages for the last category. 
Please explain exactly how this scheme works.

Thanks,

Hal

On 02/23/2017 12:16 PM, Ivan A. Kosarev via llvm-dev 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_PSHADOW(mem) VSHADOW_TO_PSHADOW(MEM_TO_VSHADOW(mem))
>    #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,
>
>
>
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-- 
Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory

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