Cqual++ Documentation

The goal of Cqual++ is to find flow-insensitive dataflow bugs.

Introductory Examples

Dataflow and dataflow bugs are best introduced by example.

Jeff Foster's Canonical Cqual Example

Here is an example of one such bug, the "printf() format-string bug", from Jeff Foster's Cqual documentation. You can find it in oink/Test/taint1.c.

char $tainted *getenv(const char *name);
int printf(const $untainted char *fmt, ...);
int main(void)
  char *s, *t;
  s = getenv("LD_LIBRARY_PATH");
  t = s;

The idea of taint analysis is as follows.

In this program $tainted flows into $untainted, which is a bug. Let's watch Cqual++ find it.

make qual-check-demo-taint1
./qual -q-config ../libqual/config/lattice Test/taint1.c
Test/taint1.c:6 WARNING:  ( (getenv) ( ("LD_LIBRARY_PATH")))' treated as $tainted and $untainted

( (getenv) ( ("LD_LIBRARY_PATH")))': $tainted $untainted
Test/taint1.c:6      $tainted <=  ( (getenv) ( ("LD_LIBRARY_PATH")))'
Test/taint1.c:6               <=  (s)''
Test/taint1.c:7               <=  (t)''
Test/taint1.c:8               <= fmt''
Test/taint1.c:2               <= $untainted

Cqual++ deduces that this program is not safe because the $tainted data eventually flows into a place that must be $untainted.

A Horrible C++ Example

One big improvement of Cqual++ over Cqual is that we handle C++. Here is the example oink/Test/horrible1.cc.

 1: // class template, reference member, pointer to virtual function
 2: // member

 4: template<class T> struct A {
 5:   virtual T f(int x) {}
 6: };

 8: struct B : A<int> {
 9:   int &y;
10:   B(int &y0) : y(y0) {}
11:   int f(int x) {
12:     y = x;
13:   }
14: };

16: int main() {
17:   int $untainted y;
18:   A<int> *a0 = new B(y);
19:   int (A<int>::*q)(int) = &A<int>::f;
20:   int $tainted x = 0;
21:   (a0->*q)(x);
22: }

What happens is as follows.

Let's see Cqual++ follow all of that.

make qual-check-demo-horrible1
./qual -q-config ../libqual/config/lattice Test/horrible1.cc
Test/horrible1.cc:11 WARNING: x' treated as $tainted and $untainted

x': $tainted $untainted
Test/horrible1.cc:21     $tainted <=  (x)'
Test/horrible1.cc:21              <= anon_var  t62350261-int'
Test/horrible1.cc:21              <= anon_var  t62350261-int'
Test/horrible1.cc:19              <= x'
Test/horrible1.cc:19              <= x'
Test/horrible1.cc:19              <= x'
Test/horrible1.cc:11              <= x'
Test/horrible1.cc:11              <= x'
Test/horrible1.cc:11              <=  (x)'
Test/horrible1.cc:12              <=  ( (* (this)).y)'
Test/horrible1.cc:12              <= y'
Test/horrible1.cc:10              <=  (y0)'
Test/horrible1.cc:10              <= y0'
Test/horrible1.cc:10              <= y0'
Test/horrible1.cc:18              <=  (y)'
Test/horrible1.cc:18              <= $untainted

Some Notes on Usage

If you run this in emacs compile mode, these lines will be parsed just as compile errors and repeated use of C-x back-tick will show you the successive points along the dataflow path. Note that there are various nodes for things in the path that you can't see; for example, when a variable is used, there is a node for the variable and then another for the expression containing the variable.

The L-value (reference) identity of a variable (what you can assign to) is represented by its name, "y", whereas its R-value identity (its value in an expression) is the name with an apostrophe following, "y'". If it is a pointer, the R-value pointed to will have two apostrophes "y"; note that the L-value identity of a thing pointed to is the R-value identity of the pointer pointing to it.

Be sure to run Cqual++ on code that has been through the preprocessor, commonly refereed to as the ".i" files. You may find the tool Build Interceptor handy for obtaining these from a software package that you would like to analyze without having to modify its build process. If you would like Cqual++ (or any other Oink tool) to print line numbers from the .i file instead of using the #line directives in the .i file to map back to the original source file, pass the flag "-tr nohashline".

What Cqual++ Is and Is Not

David Wagner recently wrote a summary of Cqual++ in an email to a user. I thought it was great documentation so I just re-use it here; there are other indented quotes throughout this document that are also due to David and are marked as such and there is one definition from Rob Johnson. I have occasionally edited these.

[David] CQual++ seems best at reasoning about the flow of data: I read in something from the network over here in the code, and it flowed through my program and eventually affected that variable over there. That kind of thing.

To get concrete, we're starting with format string bugs. This is research in progress, but the goal is to build a highly precise analysis to find format string bugs -- hopefully, with low enough false alarm rates and good enough engineering that you could use it as a "filter" before release to check for code cleanliness.

That's admittedly just one category of bugs, but we think we can do a better job than anyone else has done of finding format string vulnerabilities -- and our goal is to find all such vulnerabilities, so that if the tool says it couldn't find any format string bugs, then there really aren't any in the code (modulo some reasonable assumptions). Also, we hope it will give us experience that we can build on to look at various kinds of input validation flaws (e.g., maybe Unicode bugs, cross-site scripting, SQL injection; who knows?) -- but we haven't started on any of the latter categories, and don't expect to until we have more experience with format string bugs.

By the way, I hope you don't have the impression that all static analysis tools are interchangeable. CQual++ has a pretty tight focus (at least right now), and there are a lot of things you might like to check about code that CQual++ just isn't the right tool for. If you come up with the top 10 kinds of bugs you wish a static analysis tool could do, I would be pretty happy if only 1 or 2 of those could be handled by CQual++ (to give you some idea that we have a pretty narrow focus). There are many different static analysis techniques out there. The right way to go would be to figure out what properties you want to check, and then identify the most suitable static analysis techniques for each, and then go build some tools. Type inference, as used in CQual++, is just one among many techniques, so you should expect that it is the best tool for the job for only a certain fraction of all properties you might want to check.

Anyway, that's what we're working on. As for what you want to do with Cqual++, well, you don't need to be constrained by the specific security bugs that we're currently working on. If you want to brainstorm ideas to work on, I would suggest thinking about the category of input validation bugs, particularly where data flows from a tainted source to trusting sinks, and trying to identify one or two vulnerability idioms in that universe that seem important to you. A good place to start would be to ask: what are the types of input validation bugs that you care most about, or that are biting your butt the hardest? That's probably the most likely source of applications where CQual++ might be a good tool for the job.

Qualifier Syntax

Cqual++ will parse a type qualifier, such as $tainted, wherever "const" would legally go in C or C++; if there really are any built-in qualifiers such as "const" Cqual++ qualifiers should go after them or it might not parse. There are examples throughout this document.

char $tainted *getenv(const char *name);
int const $untainted x;

Qualifiers are attached to variables or expressions; things that at runtime hold data. The How Qualifier Inference Works section says exactly how Cqual++ computes what they mean.

'Funky' Polymorphic Qualifiers for Adding to the Dataflow Graph

If we have a function declaration but no body, such as for a libc library function, Cqual++ will behave as if data that flows into it just disappears. However, sometimes in reality the data doesn't dissapear but emerges somewhere else, such as in the return value or in a place pointed to by one of the other arguments. Unless this is modeled properly the dataflow graph to be broken and a bug missed.

We want to give the function a "dataflow body" that captures how data flows in and out of it, even if the dataflow body is not an accurate model of what is really going in inside. Consider the example of strcat(). We wouldn't want to analyize the real body of a libc function such as strcat() anyway as the optimizations it does are probably not typesafe. Let's see what the semantics are from the manpage.

The strcat() function appends the src string to the dest string overwriting the `\0' character at the end of dest, and then adds a terminating `\0' character. The strings may not overlap, and the dest string must have enough space for the result. . . . The strcat() and strncat() functions return a pointer to the resulting string dest.

We could just write a fake body for for strcat() to capture these semantics; there is no need for the fake body to actually work, it just has to yeild the dataflow graph that you want. Here is one that captures my understanding.

char *strcat(char *dest, const char *src) {
  *dest = *src;
  return dest;

I find this fake body a bit disconcerting: I am inventing code that I can't test and I hope it gives me the dataflow graph I want. I would rather just write down the graph. Additionally, I would have to actually remove the previous body or the compiler or linker would complain of a double-definition.

"Funky" polymorphic qualifiers are a syntacitc convenience for just writing down the dataflow graph for Cqual++ on a function declaration; this does just what we want and doesn't interfere with compilation. [Note that there are several meanings of the word "polymorphic", so I distinguish this one by calling it "funky".]

Now we write funky qualifiers on the strcat() function header that captures that semantics. Note that data flows from *src to *dest but not the other way, whereas data flows in both directions between *dest and *RET where RET is the return value.

char $_1_2 *strcat(char $_1_2 *dest, const char $_1 *src);

int main() {
  char $tainted *x;
  char $untainted *y;
  strcat(y, x);

And it works.

make qual-check-demo-funky
./qual -q-config ../libqual/config/lattice Test/funky_strcat1.c
Test/funky_strcat1.c:6 WARNING:  (y)'' treated as $tainted and $untainted

 (y)'': $tainted $untainted
Test/funky_strcat1.c:6      $tainted <=  (x)''
Test/funky_strcat1.c:6               <= src''
Test/funky_strcat1.c:6               <= src''
Test/funky_strcat1.c:1               <= src''
Test/funky_strcat1.c:1               <= dest''
Test/funky_strcat1.c:1               <= dest''
Test/funky_strcat1.c:6               <= dest''
Test/funky_strcat1.c:6               <=  (y)''
Test/funky_strcat1.c:6               <= $untainted

The 'ignore-body-qual' Control for Subtracting from the Dataflow graph

Sometimes we do have the body of a function but we don't like it. We would like to replace it with another body. Again, the implementation of a function may not be typesafe if it is optimized with assembly or just horrible unsafe C. We can give it a new "dataflow body" that means what we want by using the techniques in the previous section, but since Cqual++ is non-deterministic, both both of them will be called. We would like to subtract the old body.

This is done by putting an ignore-body-qual for this function into a kind of configuration file for Oink/Cqual++ called a "control file". See section Control Files for more.

How Qualifier Inference Works

Cqual++ does not know however what qualifiers mean: which ones may legally flow into one another, etc.. Cqual++ takes a configuration file that explains these to it. [Note that it is actually Jeff Foster's and Rob Johnson's backend in oink-stack/libqual that is doing this level of the analysis, but having said that I will return to saying Cqual++ to avoid confusion.]

-q-config FILE         : set the configuration file to FILE

A popular configuration file is oink-stack/libqual/config/lattice. Here is the section describing the semantics of the $tainted/$untainted qualifiers ("taint analysis") to Cqual++. We describe what it means as we describe inference through the section.

partial order {
  $untainted [level = value, color = "pam-color-untainted", sign = neg]
  $tainted [level = value, color = "pam-color-tainted", sign = pos]
  $untainted < $tainted

Dataflow is Modeled as a Partial Order

Dataflow is modeled as a partial order:

That is, when we conclude from examination of the Abstract Syntax Tree that data flows from point A to point B, we model this as <u>A <= B</u> in our partial order.

Consider this program and ignore the qualifiers for a moment; just think about the dataflow.

int $tainted a;
int b = a;
int $untainted c = b;

In our model we now introduce two facts:

The Sign of a Qualifier Shows How to Attach It

Now consider the above program again and notice the qualifiers on the variable declarations. The configuration file tells us that $tainted is a positive qualifier, so we add the fact to our model that it bounds below the variable it annotates (why "positive" means below is beyond me, you just need some label); similarly the fact that $untainted is a negative means it bounds above the variable it annotates. We now have these additional facts:

The Config File Gives Axioms for the Qualifiers

The config file tells us one more sentence that we must add to our model; this sentence tells us the true, deep meaning of $tainted and $untainted and their relationship to one another. Note the inequality is a strict less-than this time.

The Model Must Be Consistent

We now reach a horrific conclusion about the true nature of our program which we have known for so long and trusted so much, possibly with our very life even.

A node cannot be less than itself and therefore our model is inconsistent.

Ref- and Value-Level Qualifiers

In the previous example, you will note that the configuration file specified that $tainted and $untainted were both "value"-level qualifiers; R-value level if you work on compilers. This means that you should think of them as qualifying the content of the data at runtime: the actual data itself is tainted. It is what you think of when you think "dataflow".

Here is another partial order for doing const analysis. Note that "const" is a qualifier that already exists in C and C++, so there is no need to precede it with a "$".

partial order {
    const [level = ref, sign = pos]
    $nonconst [level = ref, sign = neg]
    $nonconst < const

Notice that the qualifiers here are "ref"-level; L-value level if you work on compilers. This means you should think of them as qualifying the context of the data at runtime: the "box" that the data lives in, not the data in the box. That is, data is not const, the container of the data is const. Note that it is not a bug to assign from a const variable into a nonconst variable, because the data is copied but the variables themselves do not mix. However it is a problem to assign a pointer to a const to a pointer to a nonconst because you can now write through the nonconst pointer to the const context. Note that going the other way is ok.

In practice, this means that ref-level qualifiers are attached to the reference to the data rather than the data; otherwise inference is done just as above. In C++ this is exactly the notion of a C++ reference. In C for variables it means its L-value identity and for data it means the pointer to the data.


Oh, and the color field? That's a suggested color for things qualified with that qualifier. We thought of everything. :-)

There is No 'Time': Cqual++ is Non-Deterministic

There is no 'time' to the Cqual++ analysis. We are oblivious to any actual decisions made by the program. We see the syntax as a jumble of nodes and edges but not happening in any particular order.

Both our control and data flow models are non-deterministic: if a program attempts to filter tainted data using controlflow, we don't notice. Note however that filtering the dataflow in other ways does work if the dataflow explicitly goes through some actual syntax; see below.

[David and correspondent]
> I'm not sure, actually. For example, we see this kind of thing a lot in
> OpenSSL...
> char localbuf[10];
> if(tainted_stuff > 10)
>     goto err;
> memcpy(localbuf,other_tainted_stuff,tainted_stuff);
> Its not clear to me whether this kind of analysis is completely out of
> scope for you, but its what interests me most.
I take it that you want to (a) check that tainted stuff is never used as the third (size) argument to memcpy, but (b) mark anything whose size has been checked as untainted after the check. Right? Yup, that's hard for us, because we can't express the notion that 'tainted_stuff' is tainted before the if-statement and untainted after (at least, not with CQual++).

Managing the Dataflow by Interposing with Explicit Syntax

[David] Is there any way to rewrite the above code to introduce two different variables, one for the tainted stuff before the check, and one for the untainted stuff after? And are you willing to do rewrite the code in that kind of way? For instance, would something like this work?
char localbuf[10];

untainted_len = upperbound(tainted_len, 10);

// for the implementation; hide this from the analysis
size_t upperbound(size_t i, size_t bound) {
    if (i > bound)
    return i;
If it's ok to rewrite the code in that style, then I think we should be able to do a good job of this. We'd annotate memcpy like this:
    void *memcpy(void *dest, const void *src, size_t $untainted n);
and (give this to the analysis instead of the above implementation):
    size_t $untainted upperbound(size_t i, size_t $untainted bound);
Then because we have two different variables, with two different names (e.g., tainted_len and untainted_len), rather than just one variable (tainted_stuff), we can correctly track that 'tainted_len' is $tainted and 'untainted_len' is $untainted.

Breaking dataflow deliberately

Sometimes one wants to simply break the dataflow for a syntatic exception to a rule. Just pass the dataflow through a cast that contains a user-qualifier. For example, you can cast a flow to $untainted, which will break the flow.

int main() {
    $tainted int x;
    $untainted float y;
    y = (float $untainted) x;

This breaking of the flow will happen even if -fcasts-preserve is enabled.

Features of Cqual++

I attempt a short overview of the Cqual++ features; some of them are technically Oink features but they are only used in Cqual++ so I document them here. As with oink you can get a list of the command line flags and a hint at their function by typing ./qual --help, you can get a list of the defaults with ./qual --verbose, and you can negate boolean flags with -f-no. Rather than just repeat the list of flags here, I will introduce aspects of the tool and the relevant command-line flags at appropriate points.

Modifying the kind of analysis done

Cqual++ will allows you to modify what analysis it does.

-fq-inference            : do inference (on by default, obviously)
-fq-strict-const         : enforce const, rather than allow const inference
-fq-casts-preserve       : inference goes through casts, rather than blocking
-fq-casts-preserve-below-functions : -fcasts-preserve works below functions
-fq-use-const-subtyping  : no back edge below pointers if pointing to const
-fq-poly                 : do polymorphic analysis
-fo-instance-sensitive   : C mode only.
                           1) fields get unique values per struct instance
                           2) 'void's get unique values by type
-q-catch-qual QNAME    : attach QNAME to singleton exception variable

Modifying the Output

You can modify what kind of output Cqual++ will give you.

-fq-print-trans-qual      : in pretty printing, annotate inferred qualifiers
-fq-explain-errors        : print bad dataflow path when one is found
-fq-exit-at-inconsistency : exit when an inference inconsistency is detected

Printing the Cqual++ Inference Graph

Cqual++ translates the syntax of C++ into a dataflow graph which the backend then thinks about to tell you if your program has a dataflow bug or not. You can print out the dataflow graph and examine it manually. This is a handy feature if you are debugging Cqual++ (oink/qual) and the input parses and typechecks but the inference conclusion is wrong. Here are the command-line arguments.

-fq-print-quals-graph    : print out quals.dot of inference graph
-fq-ugly                 : put address of qvar into quals.dot node names
-fq-name-with-loc        : put location info on qualifier variable names

There is a make target to help with all the typing: Use the 'qual-graph/%' target in the qual_result_test.incl.mk makefile to print the graph as a Graph Viz 'dot' file and then convert it to a postscript file for easy viewing. You can also do it manually like this.

./qual -q-config ../libqual/config/lattice -fprint-quals-graph -fq-no-exit-at-inconsistency FILE.cc
dot -Tps -o FILE-cc.ps quals.dot

Even for small files the output can be a huge graph that can overwhelm your postscript viewer. If the problem is that you expect, say, $tainted *to* flow to $untainted and it does *not*, you can use the included utility 'qual_graph_component'; run it twice to get just the connected components containing the two nodes of interest respectively (a useful idea of Scott Mc Peak's).

make qual-graph
rm -f quals.dot Test/*.dot Test/*.ps
./qual -fq-no-explain-errors -fq-name-with-loc -q-config ../libqual/config/lattice -fq-print-quals-graph -fq-ugly -fq-no-ret-inf Test/t1000.cc
Test/t1000.cc:5 WARNING: Test/t1000.cc:5:3  (x)' treated as $tainted and $untainted

dot -Tps -o Test/t1000.cc-cc.ps quals.dot
./qual_graph_component -root '$tainted'   < quals.dot > Test/t1000.cc-tainted.dot
dot -Tps -o Test/t1000.cc-tainted.cc.ps   Test/t1000.cc-tainted.dot
./qual_graph_component -root '$untainted' < quals.dot > Test/t1000.cc-untainted.dot
dot -Tps -o Test/t1000.cc-untainted.cc.ps Test/t1000.cc-untainted.dot

If the problem is that you expect $tainted to *not* flow to $untainted and it *does*, then just use either one as the -root argument and just examine the one connected component containing both of them.

Linking and Multiple Translation Units

Cqual++ is whole-program: you can give it multiple translation units and it will reason about them as a whole by linking together the global symbols very much as a real linker would. Note that our linker is a bit naive compared to a real linker; for example it doesn't implement weak or common symbols, etc. This is work in progress, but it works for basic linking. See the make target qual-check-unsat-symbol in oink/qual_serialization_test.incl.mk for tests that exercise this feature.

-fo-report-unsatisfied-symbols : print unsatisfied function symbols in linker

Control Files

Oink has a general "control file" mechanism that is intended to include instructions on detailed modifications to how to analyze a particular file; the suffix for a control file is .ctl.

-o-control *file*                      : a file for controlling the behavior of oink
-fo-print-controls-and-exit    : print the controls and stop
-fo-report-unused-controls    : print controls that go unused

Right now the only control implemented is "ignore-body-qual" which tells the dataflow not to process the body of a function. Here are some from Test/simple1.ctl and Test/unsat_symbol3c3.ctl.

    file =    bar.i              # some comments mixed with data
name = ::foo::f2 SIG (int x, char y)
visibility = extern

    file = *
    name = foo
    visibility = static

The semantics of each field are as follows.

See the make target qual-check-control-files in oink/qual_serialization_test.incl.mk for tests that exercise this feature. For the limitations of the control file syntax it is easiest to see an example; see the control file parsing test input Test/simple1.ctl.


Much like a real compiler, Cqual++ can "compile" an input C or C++ file to a dataflow graph and serialize it out for you to a .qdir directory. This can be later supplied on the command line to another instance of Cqual++ and the graph will be "linked" in to the analysis.

Note that the resulting dataflow graph does not contain the Abstract Syntax Tree. Further, all we care about this graph is the connections it induces between the global symbols, not all of the internal details; when compactification is on, the backend will compact this graph down by about an order of magnitude before rendering it out. Thus the result of serializing and then de-serializing is much smaller than the in-core image of a file just parsed from source.

Using serialization and compactification to "compile" each translation unit separately and then "linking" together the resulting graphs. you should be able to analyze large programs that you could not otherwise analyze all at once in core.

-q-srz FILE;           : serialize qual inference graph to FILE
-fq-compactify-graph     : compactify graph before serialization


Serialization example

Here are two files where the tainted-ness flows between translation units when they run.


int return_tainted();
void want_untainted(int $untainted fmt, ...);
int main() {
  int a = return_tainted();


int return_tainted() {
  int $tainted x;
  return x;

Here we can see serialization in action. It is a bit disconcerting, but we get separate warnings in each file.

make qual-srz-demo
./qual -fq-no-explain-errors -q-config ../libqual/config/lattice Test/taint2_a.c -fq-no-ret-inf -q-srz Test/taint2_a.c.qdir
./qual -fq-no-explain-errors -q-config ../libqual/config/lattice Test/taint2_b.c -fq-no-ret-inf -q-srz Test/taint2_b.c.qdir
./qual -q-config ../libqual/config/lattice Test/taint2_a.c.qdir Test/taint2_b.c.qdir
Test/taint2_b.c:1 WARNING: return_tainted_ret treated as $tainted and $untainted

return_tainted_ret: $tainted $untainted
Test/taint2_b.c:1      $tainted <= return_tainted_ret
Test/taint2_b.c:1               <= return_tainted_ret
Test/taint2_a.c:4               <= $untainted

Test/taint2_a.c:4 WARNING:  ( (return_tainted) ()) treated as $tainted and $untainted

 ( (return_tainted) ()): $tainted $untainted
Test/taint2_b.c:1      $tainted <= return_tainted_ret
Test/taint2_b.c:1               <= return_tainted_ret
Test/taint2_a.c:4               <=  (return_tainted)_ret
Test/taint2_a.c:4               <=  ( (return_tainted) ())
Test/taint2_a.c:4               <= $untainted

Kinds of Dataflow

David Wagner again on the various kinds of dataflow.

[David] Oh, dear. I wonder whether I led you astray with the term "flow-sensitive". I don't know how much you know about the program analysis literature, but the term "flow-sensitive" is a piece of jargon with some specific (and not very intuitive) meaning. Also, by the way, flow analysis is a pretty broad category: there's data flow, and control flow, and intra-procedural analysis, and inter-procedural analysis, and all kinds of (whatever)-sensitivity.


char *s = read_from_network();
char *d = malloc(strlen(s)+1);
strcpy(d, s);

That's the kind of data flow that Cqual++ is very good at tracking. Basically, we annotate read_from_network() to say that it returns a "char $tainted *", like this:

char $tainted *read_from_network();

We annotate ``use_in_trusting_way()`` to say that it requires its input to be a "char $untainted *", like this:

void use_in_trusting_way(char $untainted *);

Finally, we have some library of annotations on things like ``strcpy()`` expressing how information flows. (Most or all of these annotations can be application-specific, and written separately from the rest of the code.) With these annotations, CQual++ should be able to handle this kind of flow easily, even if the data is passed through functions, stored on the heap, copied all over the place, and so on.

Note that this analysis doesn't require us to understand anything special about the order in which statements can execute -- it only requires tracking the flow of data from one variable to another -- which is where CQual++ really shines.

Cqual++ is Polymorphic

The word "polymorphic" is very overloaded: we have two meanings of polymorphic in this document (the one I am about to give and the Funky Qualifiers meaning). Neither one of these meanings are what Object Oriented people mean by polymorphic, which we simply call "inheritance" (Programming Languages people will just call it "subtyping"). In the context of static dataflow analysis

Here is example oink/Test/poly1.c. Notice that there is really no problem; $tainted never flows to $untainted at runtime.

int f(int x) {
    return x;

int main() {
    int $tainted x0;
    int y0;
    y0 = f(x0);

    int x1;
    int $untainted y1;
    y1 = f(x1);

However a non-polymorphic analysis will simply flow the $tainted in at the first call to f() and flow it out at every call to f().

./qual -fq-no-poly -q-config ../libqual/config/lattice Test/poly1.c; test $? -eq\
Test/poly1.c:8 WARNING:  (x0)' treated as $tainted and $untainted

 (x0)': $tainted $untainted
Test/poly1.c:8      $tainted <=  (x0)'
Test/poly1.c:8               <= x'
Test/poly1.c:8               <= x'
Test/poly1.c:12              <= x'
Test/poly1.c:12              <= x'
Test/poly1.c:1               <= x'
Test/poly1.c:1               <=  (x)'
Test/poly1.c:2               <=  t54412836-int
Test/poly1.c:1               <=  t54412836-int
Test/poly1.c:1               <= f_ret
Test/poly1.c:12              <=  (f)_ret
Test/poly1.c:12              <= f_ret
Test/poly1.c:12              <=  (f)_ret
Test/poly1.c:12              <=  ( (f) ( (x1)))
Test/poly1.c:12              <=  (y1)'
Test/poly1.c:12              <= $untainted

A polymorphic analysis does not make this mistake.

./qual -fq-poly -q-config ../libqual/config/lattice Test/poly1.c

Cqual++ is Instance-Sensitive

There is alway the question of how far a static analysis should go to model the heap. A simple way to handle structs is to just treat all structs of the same type as if they were the same struct; that is, to just treat a struct as a namespace and its members as global variables. This will not give you false negatives since if two structs might be the same, we just assume they are the same. However the false positive rate gets bad.

Cqual refers to this as 'field-sensitivity'. I consider 'instance-sensitivity' to be the accurate term for it and respectfully disagree with the Cqual designers.

Cqual++ is instance-sensitive in C mode. Instance-sensitivity is off by default. This restriction to C mode is due to inheritance making notions of "same type" complicated. We could fix this but we have not yet.

Here two structs are independent: they don't ever flow together. A line that would make them flow together is commented out.

struct A {
    int q;

int main() {
    struct A *a1;
    struct A *a2;
    int $tainted x;
    int $untainted y;
    a1->q = x;
    // NOTE that we do not do this: a2 = a1;
    y = a2->q;

With instance-sensitivity turned on, Cqual++ correctly allows the file to pass.

make qual-check-demo-instance-sens
./qual -q-config ../libqual/config/lattice -fo-instance-sensitive Test/instance_sensitive_demo.c

With instance-sensitivity turned off, Cqual++ uses one model for both structs and the two instances of the field q are collapsed into one, allowing $tainted to apparently flow to $untainted.

./qual -q-config ../libqual/config/lattice Test/instance_sensitive_demo.c
Test/instance_sensitive_demo.c:10 WARNING:  ( (* (a1)).q)' treated as $tainted and $untainted

 ( (* (a1)).q)': $tainted $untainted
Test/instance_sensitive_demo.c:10     $tainted <=  (x)'
Test/instance_sensitive_demo.c:10              <=  ( (* (a1)).q)'
Test/instance_sensitive_demo.c:10              <= q'
Test/instance_sensitive_demo.c:12              <=  ( (* (a2)).q)'
Test/instance_sensitive_demo.c:12              <=  (y)'
Test/instance_sensitive_demo.c:12              <= $untainted

Cqual++ is Void-Polymorphic

Sometimes programmers will put an int* into a void* at one place and get it out later, and then put a float* into the same void* someplace else and get it out later. If we assume that the programmer is not breaking the typesystem, then the int and float should not get tangled together when they are inside the void*.

Cqual++ is void-polymorphic in C mode. Void-polymorphism is off by default and is actually controlled by the same flag as instance-sensitivity, -fo-instance-sensitive, since the underlying mechanism and difficulties with C++ are very similar. Again, This restriction to C mode is due to inheritance making notions of "same type" complicated. We could fix this but we have not yet.

int main() {
    int $tainted *i;
    int *i2;
    float *f;
    float $untainted *f2;
    void *v = i;
    f2 = v;

With void-polymorphism turned on, Cqual++ correctly allows the file to pass.

make qual-check-demo-void-poly
./qual -q-config ../libqual/config/lattice -fo-instance-sensitive Test/void_poly_demo.c

With void-polymorphism turned off, Cqual++ uses one model for everything that goes into the void the two pointed-to things are collapsed into one, allowing $tainted to apparently flow to $untainted.

./qual -q-config ../libqual/config/lattice Test/void_poly_demo.c
Test/void_poly_demo.c:6 WARNING:  (i)'' treated as $tainted and $untainted

 (i)'': $tainted $untainted
Test/void_poly_demo.c:6      $tainted <=  (i)''
Test/void_poly_demo.c:6               <=  (v)''
Test/void_poly_demo.c:7               <=  (f2)''
Test/void_poly_demo.c:7               <= $untainted

Cqual++ is Not Flow-Sensitive

[David] I wonder if words like "flow-sensitive" and "[data] flow analysis" might be causing some miscommunication here (note: they're not the same concept!). Note that


char *s = read_from_network();
s = "innocuous";

That's an example of code that CQual++ doesn't handle as well. The above is safe, but CQual++ doesn't recognize it as safe. The reason is that CQual++ doesn't try to reason about the order in which various statements occur; it just conservatively assumes that they can be executed in any order whatsoever. (In the jargon, we say that CQual++ is "flow-insensitive".) Consequently, CQual++ cannot recognize that the second assignment to 's' overwrites the previous value, and thus mistakenly thinks that use_in_trusting_way() can receive tainted data.

Recognizing this kind of code, and all its generalizations, requires an analysis that is "flow-sensitive": i.e., an analysis that tries to determine what order statements can be executed in, and refines the results accordingly. A flow-sensitive analysis would allow the variable s to have different types at different places in the code (e.g., type "char $tainted *" after the first line, and type "char $untainted *" after the third line). CQual++ doesn't do that.

I suspect it's not so easy to do well. [http://www.cs.umd.edu/~jfoster/ Jeff] built a flow-sensitive version of CQual, and my impression is that it was a little messy and required some pointer analysis (e.g., restrict annotations). He got a paper out of it, so that gives a lower bound on the ease of implementing it: it was at least hard enough to be worth a publication. :-)

Static Single Assignment Considerations

It was pointed out to me that the example above can be fixed by using a trick called Static Single Assignment (SSA): whenever a stack variable is modified in the code, instead make a new variable with a fresh name (that's a technical term actually) and make all the following references to it refer to the new name. That is, stack variables become immutable. In C++ terminology, we say that they can be *initialized* but not *assigned*.

At first SSA sounds really promising and it will fix flow-sensitive problems for variables on the *stack*, such as in the example above. However, the exact same problem for a variable on the *heap* (that is, a memory-location) cannot be fixed in the same way: the analogue of SSA on the heap is a restriction that you cannot mutate heap data, only copy it. Thus, while the above example can be fixed by adding SSA to the analysis, the same example but with an extra layer of pointer indirection will demonstrate again 1) the lack of flow sensitivity and 2) the real difficulty in adding it to the analysis.

Right now the dataflow analysis of Oink (and therefore Cqual++) is completely purely not flow-sensitive. Adding SSA (for stack data) would make the dataflow more flow sensitive, we would now be in a more complex point in the design space: flow sensitivity for the stack but not the heap. I think that it is better to have a simpler point in the design space that is simpler to think about: a qualifier for a variable or memory location is a property that does not change over time. If you want to keep $tainted and $untainted data separate from one another, use different variables; your design will be fundamentally simpler if these qualifiers do not depend on the order of operations.

Cqual++ is Not Path-Sensitive

[Rob Johnson]

[David Wagner] CQual++ doesn't do that. Example:

char *s;
if (flag)
    s = read_from_network():
    s = "hello, world";
if (flag)

That's another kind of code that we can't handle very well. CQual++ will ignore the conditional in the if-statement and conservatively assume that both branches can be taken. Consequently, CQual++ will think that it might be possible for data to flow from read_from_network() into the argument to use_in_trusting_way(), and will issue a warning -- but the above code isn't a real bug, as any self-respecting programmer can see (assuming 'flag' cannot change between the two if-statements).

What Exactly is 'Dataflow'?

The most useful tool would be one that reported a bug exactly when there was one: no false positives and no false negatives. One expression of the Halting Problem is that this is not possible do to at static (compile) time. Therefore in any tool that finds non-trivial bugs, approximations must be made. As a general strategy, Cqual++ considers data to flow at static (compile) time if data could flow at dynamic (run) time; generally this strategy is called a conservative analysis.

Unfortunately, to go all the way with a conservative analysis would make the tool completely useless. Cqual++ is fundamentally based on dataflow; however "data flowing" is actually an imprecise abstraction in the first place. I wrote a little program, launder.c, to demonstrate this problem. Launder computes 'cat' but

Launder does this by "laundering" the dataflow through the controlflow. However, the things it does are not beyond what someone might write a program to actually do: the JPEG codec does do this sort of bit-granularity Huffman encoding/decoding.


#include <stdio.h>

#ifdef CQUAL
/* to analyze */
#  define TAINTED $tainted
#  define UNTAINTED $untainted
/* to compile */
#  define TAINTED
#  define UNTAINTED

/* Replace this with an internal home-grown compression algorithm if
    you don't believe this is within the realm of what people might
    write.  For example, the codec for I think JPEG uses Huffman
    encoding down to the bit granularity and therefor does things like
    this. */
char *launderString(char *in) {
    int len = strlen(in);
    char *out = malloc((len+1) * sizeof out[0]);
    int i, j;
    for (i=0; i<len; ++i) {
        out[i] = 0;
        for (j=0; j<8; ++j) {
          // NOTE: I can make this as bad as I want; Huffman decoding is
          // non-trivial
          if (in[i] & (1<<j)) out[i] |= (1<<j);
    out[i] = '\0';
    return out;

int main(int argc, char **argv) {
    int size = 1024;
    char TAINTED line[size];
    while(1) {
        char *ret = fgets(line, size, stdin);
        if (!ret) break;
        char UNTAINTED *b = launderString(line);
        /* FORMAT STRING BUG! */
    return 0;

Cqual++ Does Not Find the Bug in Launder

Run this demo with make qual-check-demo-launder.

Demonstrate that launder computes cat.

gcc -E Test/launder.c > Test/launder.i
gcc Test/launder.i -o Test/launder
echo 'Hello there!' > Test/hello.out
Test/launder < Test/hello.out
Hello there!

Demonstrate that Cqual++ does not find the dataflow.

gcc -DCQUAL -E Test/launder.c > Test/launder_qual.i
grep 'ainted' Test/launder_qual.i
char $tainted line[size];
char $untainted *b = launderString(line);
./qual -q-config ../libqual/config/lattice Test/launder_qual.i

Laundering data through an array

Here is a program that uses an array to compute the identity function.

char a[100];
int main(int argc, char **argv) {
    char i;
    for (i='a'; i<'z'; ++i) a[i] = i;
    char $tainted x = argv[0][0];
    char $untainted y = a[x];
    return 0;

Cqual++ does not find data that "flows through" the array.

make qual-check-demo-launder-array
Demonstrate that Cqual++ does not find the dataflow
when laundering the data through an array.
./qual -q-config ../libqual/config/lattice Test/launder_array.c

We could make the the dataflow flow "through" array lookups, but do you really want it to flow through all of them? The real question is: which array lookups preserve enough information that we would think of it as "data is flowing" and which of them destroy enough information that we think of it as just looking up a fact about the data. This is a subtle judgment call.

Cqual++ as a Logical Decision Procedure

The study of proofs is called 'Logic'. Call any mechanical way of deciding the truth value of a sentence 'decision procedure'. In Logic we consider the difference between 1) being able to prove a fact versus 2) the fact being true. There are two interesting properties a decision procedure can have.


We think of a static error checking tool as trying to prove that the input program does not have a error. Therefore if we were to say "An analysis is sound" it would means that it never reports a false negative; such a system would actually demonstrate the absence of bugs to the user.

As the Launder example above demonstrates, this "totally sound" strategy is basically not possible as a practical matter for Cqual++: if $tainted data were ever the argument to an 'if' or 'switch' statement then we would have to taint the program counter; then all hell brakes loose and Everything your program touches is tainted. Such an analysis might be correct, but it is not very useful. Here is a really sound analysis; if your program has a bug, it will always tell you: echo "Error".

An Attempt at an Exhaustive List of Soundness Holes

Since no completely sound analysis of C++ is going to be useful, we do our best and attempt to enumerate all of the soundness holes in Cqual++. These situations that we assume never participate in the mechanism of an actual dataflow bug; if they do, we may not find the bug.

If you find any more soundness holes, tell us and I'll give you an acknowledgment when we include it. Please illustrate with a program like the dataflow laundering program I give above. Your program qualifies if

Please note that Cqual++ seems to do the right thing in these situations since it is flow-insensitive.


Going back to logic terminology, again we think of Cqual++ as trying to prove that your program does not have dataflow bugs. Therefore if we say that Cqual++ is complete that means that it never gives a false positive. In general, there are two kinds of static analysis tools.

Cqual is if the verifier kind. That is, we think that being complete is important but not quite as important as being sound: a human can filter out some false positives, but if you give users too many, the tool will be useless to them.