DataFlowSanitizer — Clang 11 documentation

# Clang 11 documentation

## DataFlowSanitizer

- Introduction

- How to build libc++ with DFSan

- Usage

  - ABI List

- Example

- Current status

- Design

## Introduction

DataFlowSanitizer is a generalised dynamic data flow analysis.

Unlike other Sanitizer tools, this tool is not designed to detect a specific class of bugs on its own. Instead, it provides a generic dynamic data flow analysis framework to be used by clients to help detect application-specific issues within their own code.

## How to build libc++ with DFSan

DFSan requires either all of your code to be instrumented or for uninstrumented functions to be listed as `uninstrumented` in the ABI list.

If you’d like to have instrumented libc++ functions, then you need to build it with DFSan instrumentation from source. Here is an example of how to build libc++ and the libc++ ABI with data flow sanitizer instrumentation.

    cd libcxx-build

    # An example using ninja
    cmake -GNinja path/to/llvm-project/llvm \
      -DCMAKE_C_COMPILER=clang \
      -DCMAKE_CXX_COMPILER=clang++ \
      -DLLVM_USE_SANITIZER="DataFlow" \
      -DLLVM_ENABLE_LIBCXX=ON \
      -DLLVM_ENABLE_PROJECTS="libcxx;libcxxabi"

    ninja cxx cxxabi

Note: Ensure you are building with a sufficiently new version of Clang.

## Usage

With no program changes, applying DataFlowSanitizer to a program will not alter its behavior. To use DataFlowSanitizer, the program uses API functions to apply tags to data to cause it to be tracked, and to check the tag of a specific data item. DataFlowSanitizer manages the propagation of tags through the program according to its data flow.

The APIs are defined in the header file `sanitizer/dfsan_interface.h`. For further information about each function, please refer to the header file.

### ABI List

DataFlowSanitizer uses a list of functions known as an ABI list to decide whether a call to a specific function should use the operating system’s native ABI or whether it should use a variant of this ABI that also propagates labels through function parameters and return values. The ABI list file also controls how labels are propagated in the former case. DataFlowSanitizer comes with a default ABI list which is intended to eventually cover the glibc library on Linux but it may become necessary for users to extend the ABI list in cases where a particular library or function cannot be instrumented (e.g. because it is implemented in assembly or another language which DataFlowSanitizer does not support) or a function is called from a library or function which cannot be instrumented.

DataFlowSanitizer’s ABI list file is a Sanitizer special case list. The pass treats every function in the `uninstrumented` category in the ABI list file as conforming to the native ABI. Unless the ABI list contains additional categories for those functions, a call to one of those functions will produce a warning message, as the labelling behavior of the function is unknown. The other supported categories are `discard`, `functional` and `custom`.

- `discard` – To the extent that this function writes to (user-accessible) memory, it also updates labels in shadow memory (this condition is trivially satisfied for functions which do not write to user-accessible memory). Its return value is unlabelled.

- `functional` – Like `discard`, except that the label of its return value is the union of the label of its arguments.

- `custom` – Instead of calling the function, a custom wrapper `__dfsw_F` is called, where `F` is the name of the function. This function may wrap the original function or provide its own implementation. This category is generally used for uninstrumentable functions which write to user-accessible memory or which have more complex label propagation behavior. The signature of `__dfsw_F` is based on that of `F` with each argument having a label of type `dfsan_label` appended to the argument list. If `F` is of non-void return type a final argument of type `dfsan_label *` is appended to which the custom function can store the label for the return value. For example:

    void f(int x);
    void __dfsw_f(int x, dfsan_label x_label);

    void *memcpy(void *dest, const void *src, size_t n);
    void *__dfsw_memcpy(void *dest, const void *src, size_t n,
                        dfsan_label dest_label, dfsan_label src_label,
                        dfsan_label n_label, dfsan_label *ret_label);

If a function defined in the translation unit being compiled belongs to the `uninstrumented` category, it will be compiled so as to conform to the native ABI. Its arguments will be assumed to be unlabelled, but it will propagate labels in shadow memory.

For example:

    # main is called by the C runtime using the native ABI.
    fun:main=uninstrumented
    fun:main=discard

    # malloc only writes to its internal data structures, not user-accessible memory.
    fun:malloc=uninstrumented
    fun:malloc=discard

    # tolower is a pure function.
    fun:tolower=uninstrumented
    fun:tolower=functional

    # memcpy needs to copy the shadow from the source to the destination region.
    # This is done in a custom function.
    fun:memcpy=uninstrumented
    fun:memcpy=custom

## Example

The following program demonstrates label propagation by checking that the correct labels are propagated.

    #include <sanitizer/dfsan_interface.h>
    #include <assert.h>

    int main(void) {
      int i = 1;
      dfsan_label i_label = dfsan_create_label("i", 0);
      dfsan_set_label(i_label, &i, sizeof(i));

      int j = 2;
      dfsan_label j_label = dfsan_create_label("j", 0);
      dfsan_set_label(j_label, &j, sizeof(j));

      int k = 3;
      dfsan_label k_label = dfsan_create_label("k", 0);
      dfsan_set_label(k_label, &k, sizeof(k));

      dfsan_label ij_label = dfsan_get_label(i + j);
      assert(dfsan_has_label(ij_label, i_label));
      assert(dfsan_has_label(ij_label, j_label));
      assert(!dfsan_has_label(ij_label, k_label));

      dfsan_label ijk_label = dfsan_get_label(i + j + k);
      assert(dfsan_has_label(ijk_label, i_label));
      assert(dfsan_has_label(ijk_label, j_label));
      assert(dfsan_has_label(ijk_label, k_label));

      return 0;
    }

## Current status

DataFlowSanitizer is a work in progress, currently under development for x86_64 Linux.

## Design

Please refer to the design document.