TypeSanitizer — Clang 22.1.8 documentation

# Clang 22.1.8 documentation

## TypeSanitizer

## Introduction

The TypeSanitizer is a detector for strict type aliasing violations. It consists of a compiler instrumentation module and a run-time library. C/C++ has type-based aliasing rules, and LLVM can exploit these for optimizations given the TBAA metadata Clang emits. In general, a pointer of a given type cannot access an object of a different type, with only a few exceptions.

These rules aren’t always apparent to users, which leads to code that violates these rules (e.g. for type punning). This can lead to optimization passes introducing bugs unless the code is build with `-fno-strict-aliasing`, sacrificing performance.

TypeSanitizer is built to catch when these strict aliasing rules have been violated, helping users find where such bugs originate in their code despite the code looking valid at first glance.

Typical memory overhead introduced by TypeSanitizer is about 8x. Runtime slowdown varies greatly depending on how often the instrumented code relies on type aliasing. In the best case slowdown is 2x-3x.

## The TypeSanitizer Algorithm

For each TBAA type-access descriptor, encoded in LLVM IR using TBAA Metadata, the instrumentation pass generates descriptor tables. Thus there is a unique pointer to each type (and access descriptor). These tables are comdat (except for anonymous-namespace types), so the pointer values are unique across the program.

The descriptors refer to other descriptors to form a type aliasing tree, like how LLVM’s TBAA data does.

The runtime uses 8 bytes of shadow memory, the size of the pointer to the type descriptor, for every byte of accessed data in the program. The first byte of a type will have its shadow memory be set to the pointer to its type descriptor. Aside from that, there are some other values it may be.

- 0 is used to represent an unknown type

- Negative numbers represent an interior byte: A byte inside a type that is not the first one. As an example, a value of -2 means you are in the third byte of a type.

The Instrumentation first checks for an exact match between the type of the current access and the type for that address in the shadow memory. This can quickly be done by checking pointer values. If it matches, it checks the remaining shadow memory of the type to ensure they are the correct negative numbers. If this fails, it calls the “slow path” check. If the exact match fails, we check to see if the value, and the remainder of the shadow bytes, is 0. If they are, we can set the shadow memory to the correct type descriptor pointer for the first byte, and the correct negative numbers for the rest of the type’s shadow.

If the type in shadow memory is neither an exact match nor 0, we call the slower runtime check. It uses the full TBAA algorithm, just as the compiler does, to determine when two types are permitted to alias.

The instrumentation pass inserts calls to the memset intrinsic to set the memory updated by memset, memcpy, and memmove, as well as allocas/byval (and for lifetime.start/end) to reset the shadow memory to reflect that the type is now unknown. The runtime intercepts memset, memcpy, etc. to perform the same function for the library calls.

## How to build

Build LLVM/Clang with CMake and enable the `compiler-rt` runtime. An example CMake configuration that will allow for the use/testing of TypeSanitizer:

    $ cmake -DCMAKE_BUILD_TYPE=Release -DLLVM_ENABLE_PROJECTS="clang" -DLLVM_ENABLE_RUNTIMES="compiler-rt" <path to source>/llvm

## Usage

Compile and link your program with `-fsanitize=type` flag. The TypeSanitizer run-time library should be linked to the final executable, so make sure to use `clang` (not `ld`) for the final link step. To get a reasonable performance add `-O1` or higher. TypeSanitizer by default doesn’t print the full stack trace in error messages. Use `TYSAN_OPTIONS=print_stacktrace=1` to print the full trace. To get nicer stack traces in error messages add `-fno-omit-frame-pointer` and `-g`. To get perfect stack traces you may need to disable inlining (just use `-O1`) and tail call elimination (`-fno-optimize-sibling-calls`).

    % cat example_AliasViolation.c
    int main(int argc, char **argv) {
      int x = 100;
      float *y = (float*)&x;
      *y += 2.0f;          // Strict aliasing violation
      return 0;
    }

    # Compile and link
    % clang++ -g -fsanitize=type example_AliasViolation.cc

The program will print an error message to `stderr` each time a strict aliasing violation is detected. The program won’t terminate, which will allow you to detect many strict aliasing violations in one run.

    % ./a.out
    ==1375532==ERROR: TypeSanitizer: type-aliasing-violation on address 0x7ffeebf1a72c (pc 0x5b3b1145ff41 bp 0x7ffeebf1a660 sp 0x7ffeebf19e08 tid 1375532)
    READ of size 4 at 0x7ffeebf1a72c with type float accesses an existing object of type int
        #0 0x5b3b1145ff40 in main example_AliasViolation.c:4:10

    ==1375532==ERROR: TypeSanitizer: type-aliasing-violation on address 0x7ffeebf1a72c (pc 0x5b3b1146008a bp 0x7ffeebf1a660 sp 0x7ffeebf19e08 tid 1375532)
    WRITE of size 4 at 0x7ffeebf1a72c with type float accesses an existing object of type int
        #0 0x5b3b11460089 in main example_AliasViolation.c:4:10

### Error terminology

There are some terms that may appear in TypeSanitizer errors that are derived from TBAA Metadata <https://llvm.org/docs/LangRef.html#tbaa-metadata>. This section hopes to provide a brief dictionary of these terms.

- `omnipotent char`: This is a special type which can alias with anything. Its name comes from the C/C++ type `char`.

- `type p[x]`: This signifies pointers to the type. `x` is the number of indirections to reach the final value. As an example, a pointer to a pointer to an integer would be `type p2 int`.

TypeSanitizer is still experimental. User-facing error messages should be improved in the future to remove references to LLVM IR specific terms.

## Sanitizer features

### Instrumentation code inlining

By default TypeSanitizer inserts instrumentation through function calls. This may lead to a reduction in runtime performance. `-fno-sanitize-type-outline-instrumentation` (default: `false`) forces all code instrumentation to be inlined. This will increase the size of the generated code and compiler overhead, but may improve the runtime performance of the resulting code.

### `__has_feature(type_sanitizer)`

In some cases one may need to execute different code depending on whether TypeSanitizer is enabled. __has_feature can be used for this purpose.

    #if defined(__has_feature)
    #  if __has_feature(type_sanitizer)
    // code that builds only under TypeSanitizer
    #  endif
    #endif

### `__attribute__((no_sanitize("type")))`

Some code you may not want to be instrumented by TypeSanitizer. One may use the function attribute `no_sanitize("type")` to disable instrumenting type aliasing. It is possible, depending on what happens in non-instrumented code, that instrumented code emits false-positives/ false-negatives. This attribute may not be supported by other compilers, so we suggest to use it together with `__has_feature(type_sanitizer)`.

### `__attribute__((disable_sanitizer_instrumentation))`

The `disable_sanitizer_instrumentation` attribute can be applied to functions to prevent all kinds of instrumentation. As a result, it may introduce false positives and incorrect stack traces. Therefore, it should be used with care, and only if absolutely required; for example for certain code that cannot tolerate any instrumentation and resulting side-effects. This attribute overrides `no_sanitize("type")`.

### Ignorelist

TypeSanitizer supports `src` and `fun` entity types in Sanitizer special case list, that can be used to suppress aliasing violation reports in the specified source files or functions. Like with other methods of ignoring instrumentation, this can result in false positives/ false-negatives.

### Limitations

- TypeSanitizer uses more real memory than a native run. It uses 8 bytes of shadow memory for each byte of user memory.

- There are transformation passes which run before TypeSanitizer. If these passes optimize out an aliasing violation, TypeSanitizer cannot catch it.

- Codebases that use unions and struct-initialized variables can see incorrect results, as TypeSanitizer doesn’t yet instrument these reliably.

- Since Clang & LLVM’s TBAA system is used to generate the checks used by the instrumentation, TypeSanitizer follows Clang & LLVM’s rules for type aliasing. There may be situations where that disagrees with the standard. However this does at least mean that TypeSanitizer will catch any aliasing violations that would cause bugs when compiling with Clang & LLVM.

- TypeSanitizer cannot currently be run alongside other sanitizers such as AddressSanitizer, ThreadSanitizer or UndefinedBehaviourSanitizer.

### Current Status

TypeSanitizer is brand new, and still in development. There are some known issues, especially in areas where Clang’s emitted TBAA data isn’t extensive enough for TypeSanitizer’s runtime.

We are actively working on enhancing the tool — stay tuned. Any help, issues, pull requests, ideas, is more than welcome. You can find the issue tracker here.