[llvm-dev] "compiler-rt" - DataFlowSanitizer

[Sam Kerner via llvm-dev]

On Tue, Apr 16, 2019 at 3:44 PM dareen khalid via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
>
> Hi all,
>
> I have some questions about "DataFlowSanitizer" from "compiler-rt".
> I want to know how I can test the "DataFlowSanitizer"?

This document is a good reference for DataFlowSanitizer:
  https://clang.llvm.org/docs/DataFlowSanitizer.html

> Can I configure it to label only some values,

The section named "Example" in the document above shows a simple
program that sets and tests for labels.

dfsan_create_label() creates a label.

dfsan_set_label() applies a label to the memory holding a variable.

> i.e, the return values from specific functions?

To label the return value of a function, add a call to
dfsan_set_label() on the return value of the function:

  // Outside the function:
  dfsan_label return_label = dfsan_create_label("return_label", 0);

  // An example function:
  int MyFunction(int a, int b) {
    ...
    int result = ...;

    // Set a label on the returned value:
    dfsan_set_label(return_label, &result, sizeof(result));

    return result;
  }

> Also, how can I print these labels?

To discover the label on a variable, you can test for it and print the result:

  int var = ...;

  // Does 'var' have label 'return_label'?
  dfsan_label var_label = dfsan_get_label(var);
  if (dfsan_has_label(var_label, return_label)) {
    printf("'var' has the label ''return_label");
  }

To see the state of all labels at the time the program exits set, set
the shell variable DFSAN_OPTIONS to "dump_labels_at_exit=<file path>".
For example, suppose the example program in the document is in a file
named "dfsan.c".  Here are the commands I ran to see the state of all
labels when it exits:

  # Compile dfsan.c into a binary named "dfsan":
  $ clang -g -fsanitize=dataflow dfsan.c -o dfsan

  # Run it.  There is no output because all assertions pass:
  $ ./dfsan

  # Run it again with shell variable DFSAN_OPTIONS set to export label
state to standard out on exit:
  $ env DFSAN_OPTIONS=dump_labels_at_exit=/dev/stdout ./dfsan
  ==21994==INFO: DataFlowSanitizer: dumping labels to /dev/stdout
  1 0 0 i
  2 0 0 j
  3 0 0 k
  4 1 2
  5 3 4

If you tell us more about what you are trying to accomplish with
DataFlowSanitizer, we may be able to give more specific advice.

>
> Thanks,
> Dareen
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