How to find subclasses of an abstract CodeQL class

[Sixelayo]

Sort of a generic question : in order to know exactly how a CPP request worked, I needed to know what is an AllocationExpr. Online documentation doesn't tells us much nor does librairy search

When looking into Allocation.qll we find this :

/**
 * An allocation expression such as call to `malloc` or a `new` expression.
 */
abstract class AllocationExpr extends Expr {
  /**
   * Gets an expression for the allocation size, if any. The actual allocation
   * size is the value of this expression multiplied by the result of
   * `getSizeMult()`, in bytes.
   */
  Expr getSizeExpr() { none() }

  /**
   * Gets a constant multiplier for the allocation size given by `getSizeExpr`,
   * in bytes.
   */
  int getSizeMult() { none() }

  /**
   * Gets the size of this allocation in bytes, if it is a fixed size and that
   * size can be determined.
   */
  int getSizeBytes() { none() }

  /**
   * Gets the expression for the input pointer argument to be reallocated, if
   * this is a `realloc` function.
   */
  Expr getReallocPtr() { none() }

  /**
   * Gets the type of the elements that are allocated, if it can be determined.
   */
  Type getAllocatedElementType() { none() }

  /**
   * Whether or not this allocation requires a corresponding deallocation of
   * some sort (most do, but `alloca` for example does not).  If it is unclear,
   * we default to no (for example a placement `new` allocation may or may not
   * require a corresponding `delete`).
   */
  predicate requiresDealloc() { any() }
}

Which also doesn't help us at all. But now I know that AllocationExpr is an abstract class, so I suppose that when I select it it will matches all its subclasses ? So I guess that I need to look at the definition of all the subclasses ? The problem is AllocationExpr does not have known subtype. So I don't really know what can I do except finding every string "extends AllocationExpr" in the .qll files on my computer but that seems like a terrible idea.

After trying a simple :

from AllocationExpr ae
select ae, ae.getPrimaryQlClasses()

It returns relevant results (proof that AllocationExpr has subclass because otherwise it would return the result than if I selected Expr), such as NewExpr, FunctionCall, however, both of this CodeQL classes, neither in their online doc on in the .qll file where they're defined inherit from AllocationExpr ... I was lost so I read once more this page, which also didn't helped me understood any of this.
During my searchs I also found about "Database type" (prefixed with an "@") but this seems to be unrelated.

I would be grateful if someone could redirect me to relevant documentation or explain me where am I mistaken.

[smowton]

The problem is that all the all the subclasses are private. For example, in Allocation.qll we have

private class NewAllocationExpr extends AllocationExpr, NewExpr { ... }

A private class like that can contribute to the abstract type AllocationExpr without itself being a publicly-accessible (and therefore documented) subtype of AllocationExpr.

It isn't even necessary for the other classes brought into AllocationExpr to be mentioned in the extends line-- for example, I could have

private class SomeExprs extends AllocationExpr {

  SomeExprs() { this.toString() = "Hello world" }

}

That would bring any expression (the supertype of AllocationExpr) that happens to stringify to "Hello world" into the type AllocationExpr, and wouldn't be documented as a subtype since it's private -- and all we know about the expression is it's an Expr, though it might satisfy other subtypes too; in this case surely it would specifically be a string literal.

As a result, looking for "extends ... AllocationExpr" really is your best bet, or looking for overrides of a member predicate you're interested in.

[Sixelayo]

Oh, ok thank you now I sort of understand. (Just to be sure I don't see NewAllocationExpr in my Allocation.qll file, do I have a wrong version or did you meant another file ?)

That would bring any expression (the supertype of AllocationExpr) that happens to stringify to "Hello world" into the type AllocationExpr, and wouldn't be documented as a subtype since it's private

I didn't think about that this is clearly not a wanted behavior right, so what is the fix for this ? Every class extending AllocationExpr must have a correct constructor, and this is just a misuse ?

[smowton]

If you wrote that SomeExprs class, the meaning of what you've written is "So long as SomeExprs is in scope, I want anything matching the constructor (characteristic predicate, technically) to be considered part of AllocationExpr". Note that is particular to abstract classes -- for concrete classes, an extends always expresses a subset of its parent. You can use instanceof Blah instead of extends Blah to obtain that restriction-of-class rather than contribution-to-class meaning even for abstract classes.