[StaticAnalyzer] Question about IPA function call inlining

Clang Frontend
1

Hi,

I noticed a strange behavior of function call inlining when I was debugging my checker.
For a very simple code example:

void func_with_loop(int loop_times) {
for (int i = 0; i < loop_times; i++) {
//Do nothing
}
}

void call_loop(int tag) {
func_with_loop(tag);
}

When I run clang static analyzer on this piece of code with scan-build, the function “func_with_loop()” will not be inlined by ExprEngine.

To demo this behavior, here is my patch to the clang to print if the function call is inlined: https://gist.github.com/zeroomega/bbe2cfa298a912a1b5b37fa4b9cd76b5 . The output of clang after this patch is:

testmxchannel.c:204:3func_with_loop Should inline: Direct inlined
Block Count exceeded maxBlockVisitOnPath 4Dump SRC

[B1]
1: i
2: [B1.1]++
Preds (1): B2
Succs (1): B2

Dump Dst:

[B2]
1: i
2: [B2.1] (ImplicitCastExpr, LValueToRValue, int)
3: loop_times
4: [B2.3] (ImplicitCastExpr, LValueToRValue, int)
5: [B2.2] < [B2.4]
T: for (…; [B2.5]; …)
Preds (2): B1 B3
Succs (2): B1 B0

testmxchannel.c:204:3func_with_loop InlinedFailedState is not null Should not inline

It seems that the ExprEngine did try to inline the function “func_with_loop”, but the total block count of this function exceeded “maxBlockVisitOnPath”, which by default is 4. The inline process was rolled back and this function is evaluated by “conservativeEvalCall”. I manually increased the value of “maxBlockVisitOnPath” but “func_with_loop” was still not inlined.

Is it an intended behavior of clang static analyzer or is it a bug? In my understanding, by default, when clang static analyzer evaluate a loop that the total times of the loop cannot determined, the loop will be evaluated for 4 times and an additional ExplodedNode will be created with the loop condition evaluated to false. But why in this code example it causes the function inline to be rolled back instead of creating an additional node that evaluate the loop conditions to false?

Another question I have is that is it possible in checkPreCall() callback of a checker to determine if the function call will not be inlined by ExprEngine?

Thanks for any help,

Haowei

2

Hello Haowei,

You described most of the behaviors right but not exactly.

3

Hi Peter,

Thanks for you clarification, it helps a lot. If I understood correctly, for following code:

void func_with_loop(int tag) {
for (int i = 0; i < 12; i++) {
// Do nothing
}
// Some complex body
}

If the func_with_loop() is a top level function, the code after the for loop will not be evaluated at all because maxBlockVisitOnPath will be exceeded. Is it correct?

I am currently working on a checker but have some false positive cases due to the behaviors of the loops you mentioned earlier. A simple example (simplified from a real FP) would be like:

typedef int mx_status_t;
typedef typeof(sizeof(int)) mx_handle_t;

mx_status_t mx_channel_create(
uint32_t options,
mx_handle_t* out0,
mx_handle_t* out1);

mx_status_t mx_handle_close(mx_handle_t handle);

void func_with_loop(mx_handle_t h, int loop_times) {
mx_handle_close(h);
for (int i = 0; i < loop_times; ++i) {
// Some unrelated body
}
}

void func_call_loop(int loop_times) {
mx_handle_t sa, sb;
mx_channel_create(0, &sa, &sb);
// Some unrelated body
func_with_loop(sa, loop_times);
mx_handle_close(sb);
}

The function mx_channel_create() will allocate two handles (you can think it is like a file descriptor) and save them to the pointers pointed by out1 and out2. Function mx_handle_close() will release the handle passed in. My checker is basically looking for paths that a handle is allocated but not released, which is very similar to the SimpleStreamChecker in the clang.

In the code example above, the handle is properly released in every path. However, as the loop condition of the for loop in function func_with_loop cannot be determined, the maxBlockVisitOnPath will be exceeded and the “replaying-without-inlining” feature will kick in. So there exists paths that func_with_loop is not inlined, which in this case, the handle in sa is not considered as released at the end of analysis and the checker will report a leak.

Is it possible to detect this type of false positives and not to report them within a checker? Is so, what callbacks and APIs should I use? Or for this special case, there is nothing I can do in a checker to fix this issue unless I modified the way ExprEngine evaluates a loop?

The source code of the checker I am working on can be found here: https://gist.github.com/zeroomega/0a2abb371d85ff0444a8a9562d119b80 . It is still under development so contains a lot of debug code.

Thanks,
Haowei

4

Generally, your checker needs to be able to handle calls like "foo(sa)", where "foo()" isn't inlined but closes "sa". This has nothing to do with loops - when "foo()" isn't inlined for whatever reason, on a certain path, and "sa" *escapes* into "foo()" (which means that "foo()" may potentially access the value of "sa" within its unavailable body), the checker should stop tracking "sa", avoiding the false positive.

In SimpleStreamChecker this is accomplished in checkPointerEscape() callback. Unfortunately, your handles are not pointers, so it may not work as expected. It's a known issue that checkPointerEscape() doesn't work for non-pointers; i believe it should be extended to work with non-pointers; it shouldn't be hard; but currently no checkers use it for non-pointers, so nobody has fixed this problem so far.

P.S. In case you have any plans for contributing the checker back to the mainline analyzer, it's a great idea to start early and submit it in small chunks to phabricator, which we can discuss and help you with potential issues - it should be significantly easier than contributing a finished checker.