falho marked 5 inline comments as done.
================
Comment at: clang-tidy/cert/LimitedRandomnessCheck.cpp:22-23
@@ +21,4 @@
+ Finder->addMatcher(
+ declRefExpr(hasDeclaration(functionDecl(namedDecl(hasName("::rand")),
+ parameterCountIs(0))))
+ .bind("randomGenerator"),
----------------
xazax.hun wrote:
> aaron.ballman wrote:
> > xazax.hun wrote:
> > > aaron.ballman wrote:
> > > > xazax.hun wrote:
> > > > > aaron.ballman wrote:
> > > > > > Prazek wrote:
> > > > > > > aaron.ballman wrote:
> > > > > > > > Prazek wrote:
> > > > > > > > > aaron.ballman wrote:
> > > > > > > > > > This should be looking at a callExpr() rather than a
> > > > > > > > > > declRefExpr(), should it not?
> > > > > > > > > I was also thinking about this, but this is actually better,
> > > > > > > > > because it will also match with binding rand with function
> > > > > > > > > pointer.
> > > > > > > > True, but a DeclRefExpr doesn't mean it's a function call.
> > > > > > > > Binding the function is not contrary to the CERT rule, just
> > > > > > > > calling it. For instance, the following pathological case will
> > > > > > > > be caught by this check:
> > > > > > > > ```
> > > > > > > > if (std::rand) {}
> > > > > > > > ```
> > > > > > > > The behavior of this check should be consistent with
> > > > > > > > cert-env33-c, which only looks at calls. (If we really care
> > > > > > > > about bound functions, we'd need flow control analysis, and I
> > > > > > > > think that's overkill for both of those checks, but wouldn't be
> > > > > > > > opposed to someone writing the flow analysis if they really
> > > > > > > > wanted to.)
> > > > > > > It would indeed fire on this pathological case, but I don't think
> > > > > > > we should care about cases like this, because no one is writing
> > > > > > > code like this (and if he would then it would probably be a bug).
> > > > > > > I don't think that there is much code that binds pointer to
> > > > > > > std::rand either, but I think it would be good to display warning
> > > > > > > for this, because even if the function would be never called,
> > > > > > > then it means that this is a bug, and if it would be called then
> > > > > > > it would be nice to tell user that rand might be used here.
> > > > > > >
> > > > > > > Anyway I don't oppose for changing it to callExpr, but I think it
> > > > > > > is better this way.
> > > > > > > It would indeed fire on this pathological case, but I don't think
> > > > > > > we should care about cases like this, because no one is writing
> > > > > > > code like this (and if he would then it would probably be a bug).
> > > > > >
> > > > > > It would be a known false-positive for a check designed to conform
> > > > > > to a particular coding standard. When deviations have come up in
> > > > > > the past for various coding standards, we've added an option to
> > > > > > enable the additional functionality, which I don't think would be
> > > > > > reasonable in this case. Alternatively, the CERT guideline could be
> > > > > > modified, but that is unlikely to occur because binding the
> > > > > > function pointer is not a security concern (only calling the
> > > > > > function).
> > > > > In case you let binding to function pointer you introduce potential
> > > > > false negatives which is worse in this case in my opinion.
> > > > Basically: this half-measure is sufficient for the CERT coding rule,
> > > > but isn't ideal. The ideal check isn't likely to uncover many more
> > > > cases than the half-measure, which is why it was not implemented in the
> > > > past. If someone wants to implement the whole-measure, that's great!
> > > > But implementing a half, half-measure that isn't consistent with other,
> > > > similar checks is the wrong thing to do.
> > > You can not implement an ideal checker. In general, it is undecidable
> > > whether std::rand is called or not. (You can easily create an example
> > > where you would need to solve the halting problem in order to decide
> > > whether std::rand is called.)
> > >
> > > Since the ideal checker is infeasible the question is whether you are OK
> > > with false positives or false negatives. The approach you are suggesting
> > > result in false negatives. The current approach results in false
> > > positives. I think, for this security checker, a false positive is much
> > > less serious to have than a false negative. Moreover, I doubt that people
> > > write code where such false positives are intended and the code should
> > > not be changed. But in case you can think of an example, please let us
> > > know.
> > > You can not implement an ideal checker. In general, it is undecidable
> > > whether std::rand is called or not. (You can easily create an example
> > > where you would need to solve the halting problem in order to decide
> > > whether std::rand is called.)
> >
> > I said "ideal", not "perfect", but we're splitting hairs. You are correct,
> > you're never going to get perfect clarity without runtime instrumentation.
> > By "ideal", I meant "something that actually pays attention to the bound
> > value from the point it is bound to the point the function pointer is
> > called." Simply having the address of the function is not a security
> > concern; calling it is.
> >
> > > I think, for this security checker, a false positive is much less serious
> > > to have than a false negative.
> >
> > We'll have to agree to disagree. As the person who maintains the CERT rules
> > (and their checkers), my preference for right now is to use callExpr().
> >
> I think consisteny with other checks are not always a good argument. You
> might want to ask what is the expected false positive and false nagtive rate
> from a check, and what is the guarantee that a user expects from a check. And
> I think base on that it is a unique decision that should be considered
> independently for each check. In this case I think it is more valuable to
> have a guarantee that in case all of the code is covered, std::rand() is not
> invoked. Using a callExpr instead of declRefExpr we would loose this
> guarantee at the cost of not reporting some false positive cases that are
> unlikely to annoy anyone anyways.
Thank you for the reviews! So what is the conclusion? Should I change to
callExpr()?
Repository:
rL LLVM
https://reviews.llvm.org/D22346
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