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