[llvm-dev] [RFC] Design of a TBAA sanitizer

Dean Michael Berris via llvm-dev llvm-dev at lists.llvm.org
Thu Apr 6 20:15:53 PDT 2017

Hi Hal,

I'm not too knowledgable about these areas so pardon the potentially ignorant questions.

First, does this mean the instrumentation/rewriting happens at the front-end so we can identify places where the aliasing might happen and annotate those when generating the IR? Say, in clang, does it only annotate potentially egregious cases or does it have to do it for all pointer operations?

Second, how do you report the errors in the sanitiser? Is the intent to run like ASAN where it will fail on cases where it trips? Or does it only collect the information?

Third, what would the results look like? Can it tell where the aliasing violations happened?

Lastly, how do features like c++11 unions get tracked, or the upcoming std::variant<...> implementations that might do some trickery? I suspect this is also dependent on things like alignment and padding, and even with packed representations of structs that get union'ed with character arrays, etc.

/me quickly Googles for TBAA's definition.


> On 5 Apr 2017, at 06:13, Hal Finkel via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> Hi everyone,
> At EuroLLVM, Chandler and I chatted about the design for a potential TBAA sanitizer. Here's my attempt to summarize:
> C/C++ have type-based aliasing rules, and LLVM's optimizer can exploit these given TBAA metadata added by Clang. Roughly, a pointer of given type cannot be used to access an object of a different type (with, of course, certain exceptions). Unfortunately, there's a lot of code in the wild that violates these rules (e.g. for type punning), and such code often must be built with -fno-strict-aliasing. Performance is often sacrificed as a result. Part of the problem is the difficulty of finding TBAA violations. A sanitizer would help.
> A design goal of a TBAA sanitizer is to limit the shadow-memory overhead of the implementation. ASan, for example, uses 1 bit per byte. Here we're hoping to keep the overhead down to 2 bits per byte for the TBAA sanitizing. We might be able to do this, while handling all common types on the fast path, if we use both alignment and type information. When accessing data of B bytes, 2*B bits of shadow memory can be used. Thus, we'll get 2 bits for a one-byte type, 4 bits for a two-byte type, etc. Moreover, we need appropriate holes in the encoding space so that no type has a shadow encoding that overlaps with an aligned part of a larger type's encoding.
> For example, we need to detect:
>  double f = ...; return *(int*) &f; // We should catch this.
> We might use the following encoding. The idea is that the common case, for which we need a reasonable fast path, is that type types are exactly equal. For this case, we want a simple comparison of the shadow type encodings to be sufficient to validate the access. For cases where the encodings don't match (and isn't zero to indicate an unknown type), or for which there is no direct encoding for the access type, a slow path must be used. All of this assumes that we're validating the the pointer alignment first, and then checking the type encodings.
> 1 Byte:
> 00 = 0  = unknown type
> 01 = 1 = hole
> 10 = 2 = hole
> 11 = 3  = all one-byte types (slow path, see note later on this)
> 2 Bytes:
> 0000 = 0 = unknown type
> 0101 = 5 = short
> 0110 = 6 = hole (A)
> 0111 = 7 = wchar_t (under some ABIs)
> 1001 = 9 = hole (B)
> 1010 = 10 = hole (C)
> 1011 = 11 = char16_t
> 1111 = 15 = all other types (slow path)
> It is important here to have wchar_t have a direct encoding here because wchar_t is two bytes on Windows, and moreover, wchar_t is very commonly used on Windows. The partial encoding overlap of wchar_t (i.e. 0111) with the 11 one-byte-type encoding works because 11 always indicates a slow-path check.
> 4 Bytes:
> 0000 0000 = 0 = unknown type
> A A = int
> A B = float
> B A  = pointer (under some ABIs)
> B B = long (under some ABIs)
> A 1111 = wchar_t (under some ABIs)
> B 1111 = char32_t
> A C = hole (D)
> C A = hole (E)
> B C = hole (F)
> C B = hole (G)
> C C = hole (H)
> 1111 1111 = 255 = all other types (slow path)
> 8 Bytes:
> 0000 0000 0000 0000 = 0 = unknown type
> D D = double
> D E = long (under some ABIs)
> E D = long long (under some ABIs)
> E E = long double (under some ABIs)
> D F = pointer (under some ABIs)
> F D = hole (I)
> E F = hole (J)
> F E = hole
> F F = hole
> ...
> 1111 1111 1111 1111 = 65535 = all other types
> 16 Bytes:
> 0 = unknown type
> | | = __int128_t
> I J = long long (under some ABIs)
> J I = long double (under some ABIs)
> J J = hole
> ...
> -1 = all other types
> For pointers, this scheme would consider all pointers to be the same (regardless of pointee type). Doing otherwise would mostly requiring putting pointer-type checking on the slow path (i.e. access via a pointer pointer), and that could add considerable overhead. We might, however, split out function pointers from other pointers. We could provide a compile-time option to control the granularity of pointer-type checks.
> Builtin vector types for which the vector element type has a direct encoding also naturally have a direct encoding (the concatenation of the encoding for the element type).
> Obviously the fact that we have no fast-path encodings for one-byte types could be problematic. Note however that:
> 1. If a larger type is being used to access a smaller type (plus more), the encodings won't match, so we always end up on the slow path.
> 2. If the access type is a one-byte type, we would want to validate quickly. However, most common one-byte types are universally aliasing (i.e. not subject to TBAA violations). Specifically, for C/C++, pointers to char, unsigned char, signed char (C only), and std::byte, can be used to access any part of any type. That leaves signed char (C++ only), bool/_Bool, and enums with a [signed/unsigned] char base type (C++ only, std::byte exempted) as pointee types we might wish to validate. We'd always need to fall back to the slow path to validate these. We could provide a compile-time option to disable such one-byte access checking if necessary.
> How would the slow path work? First, the code needs to find the beginning of the allocation. It can do this by scanning backwards in the ASan shadow memory. Once located, we'll read a pointer to a type-description structure from that "red zone" location. For dynamic allocations, ASan's allocator will ensure such a space for the pointer exists. For static allocations and globals, the compiler will ensure it exists. The compiler will make sure that all constructors locate this field and fill it in. Destructors can clear it. If two of these type-description-structure pointers are equal, then we can conclude that the types are equal. If not, then we need to interpret the structure. The pointer itself might be to an interval map (to deal with arrays, placement new, etc. - we can use the low bit of the pointer to differentiate between an actual type-description structure and an interval map), and the leaves of the interval map point to actual type-description structures. The type-description structure is an array of (offset, type) pairs, where the type field is also a type-description-structure pointer. The type-description structures themselves are comdat globals emitted in each relevant translation unit, where the comdat key is formed using the mangled type name (and size, etc.), and pointers to these symbols are then used to identify the types.
> Thoughts?
> Thanks again,
> Hal
> -- 
> Hal Finkel
> Lead, Compiler Technology and Programming Languages
> Leadership Computing Facility
> Argonne National Laboratory
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-- Dean

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