[cfe-dev] [analyzer] Rough sketch of the algorithm for "Enhancing bug reporting with static backward program slicing"
Kristóf Umann via cfe-dev
cfe-dev at lists.llvm.org
Thu May 30 15:16:27 PDT 2019
On Mon, 27 May 2019 at 19:48, Artem Dergachev <noqnoqneo at gmail.com> wrote:
>
>
> On 5/27/19 10:12 AM, Gábor Horváth wrote:
> > Hi!
> >
> > I wanted to share some of my points as well but had little time to do so.
> >
> > Artem identified two kinds of issues. One of which has all the
> > information available in the bug path (in his terminology:
> > must-have-happened) and the other which has not (
> > should-not-have-happened). The goal is the same in both of the cases,
> > identify all dependencies including control and data dependencies. The
> > main difference is that in one case we could use all the information
> > available in the bug path while in the other we cannot.
> >
> > 1. must-have-happened:
> > Since the bug-path contains all the information we need, I do agree
> > with Artem using bug path visitors might be sufficient. I do not know
> > how reliable `trackExpressionValue` is in identifying data
> > dependencies, but revising it could be one step in GSoC. For example,
> > after quickly skimming through the implementation I am not sure if it
> > is able to track the source of a constant value that was constant
> > folded during the symbolic execution from different values.
>
> It should be able to, because that's how inlined defensive check
> suppressions work.
>
I recently finished (not yet published) my work on implementing control
dependencies for clang's CFG via LLVM's IDFCalculator. Theoretically
speaking, by simply tracking the variables in the block that is a control
dependency of the block that is being tracked, we should be able to get a
proper bug report for Example 1? From afar, it seems reasonable.
> > Only if we had a way to test this functionality in isolation quickly
> > :) I think making that possible would also be a valuable addition.
> >
> > For control dependencies, it is great that LLVM already has some
> > algorithms to calculate dominator sets and it might be possible to
> > make that work for the Clang CFG.
> >
> > In my opinion, if you only tackle these problems by the end of the
> > summer your project already brought great value to the community.
> >
> > 2. should-not-have-happened
> > This category cannot be solved only using visitors for sure. So we
> > might end up with an implementation for this problem that is
> > completely separate from the previous one (maybe except for getting
> > control dependencies?). As Artem pointed out, we might query some
> > information from the analyzer e.g. which functions were inlined. One
> > option is to use a slicing algorithm here. However, we should make
> > sure that the way a function is conservatively evaluated by the
> > slicing algorithm and the analyzer is compatible. E.g.: if the
> > analyzer does not invalidate a memory region during evaluating the
> > call, the statement should not be considered as relevant for the
> > variable in question. We have several challenges here, the analyzer
> > reasons about memory regions while the slicing algorithm will probably
> > not. Also, we need to do something with pointers, since we do not want
> > to implement a full-fledged pointer analysis. We also need to decide
> > how to represent arrays, structures etc.
> >
> > I skimmed through the algorithm in the google docs, and I think there
> > are some other details we need to work out. For example, you never
> > remove anything from the set of relevant variables. Consider the
> > following example:
> > x = 5;
> > y = x + 2;
> > y = 2;
> > z = y + 3;
> >
> > Whit the slicing criterion (z = y + 3). Here, once y is overwritten
> > with 2, it is no longer relevant, sox should not end up in the slice.
> > The algorithm you proposed will not handle this correctly (yet). Also
> > I am not sure if we actually need therelevant_variable_map or having a
> > set will be sufficient.
> >
> > Regards,
> > Gabor
> >
> > On Mon, 27 May 2019 at 12:52, Kristóf Umann <dkszelethus at gmail.com
> > <mailto:dkszelethus at gmail.com>> wrote:
> >
> > + Ádám Balogh
> >
> > On Fri, 24 May 2019 at 23:31, Artem Dergachev <noqnoqneo at gmail.com
> > <mailto:noqnoqneo at gmail.com>> wrote:
> >
> > Ok, i think i have a little bit of clarity.
> >
> > Let's first talk about must-have-happened problems. For instance,
> >
> > int *foo() {
> > return coin() ? new int : nullptr; // needs note
> > }
> > void bar() {
> > int *x = foo();
> > *x = 1; // warning: null dereference
> > }
> >
> > In this case trackExpressionValue solves the
> > "must-have-happened" problem of "x must-have been assigned a
> > null pointer value" by displaying a note when foo() returns
> > nullptr. This currently more or less works correctly - there
> > are a lot of bugs but the overall idea behind
> > trackExpressionValue is correct.
> >
> > Example 1 in your document is another example of a
> > must-have-happened problem: in order for the report to be
> > valid, we need to show that 'flag' must-have-changed between
> > line 17 and line 13. That's something that the Static Analyzer
> > currently cannot handle and you plan to improve upon it.
> >
> > Hʏᴘᴏᴛʜᴇsɪs1. All "must-have-happened" problems should be
> > solved with a bug visitor. We don't need any new analysis
> > algorithm for that.
> >
> > In particular, i believe that Example 1 can be solved by
> > extending trackExpressionValue() with a bug visitor that
> > detects control-flow-dependencies via this approach of yours:
> >
> > > "Check whether the statement is a control dependency of
> > Statement 18 with dominator sets: 18 doesn't post dominate 17,
> > but every statement in between them does, meaning that 17 is a
> > control dependency of 18."
> >
> > I.e., while ascending through ExplodedGraph, we pick
> > interesting statements and perform this domination check on
> > their predecessor nodes until we reach the control dependency,
> > then we track the control dependency recursively by adding
> > more visitors.
> >
> > I think this is the first thing we should try on our GSoC.
> >
> > The reason why i believe that Hypothesis 1 is true is that all
> > the information that we need is fully contained in the bug
> > path. If something must have happened on the path, then it's
> > probably in the path and we can figure it out by looking at
> > the path. If something must have happened on the path and it's
> > not in the path, then why did we emit a warning in the first
> > place?
> >
> > ==============
> >
> > Now, to should-not-have-happened problems. The main motivating
> > example is:
> >
> > void foo(int *y) {
> > if (coin()) // needs note
> > *y = 1;
> > }
> > void bar() {
> > int x;
> > foo(&x);
> > use(x); // warning: use of uninitialized value
> > }
> >
> > In order to make the warning comprehensible, we have to
> > explain to the user why do we think that 'x' is uninitialized,
> > as it clearly "should-not-have-been" initialized for the
> > warning to be correct. For that purpose it is absolutely
> > essential to display the execution path within the inlined
> > call to foo().
> >
> > One way to achieve that would be to display *all* execution
> > path and leave it up to the user to see that the event that
> > should-not-have-happened has indeed never happened.
> >
> > That, however, would be overwhelming, so we use "path pruning"
> > that cuts away execution paths in inlined calls that are known
> > to have no interesting events happening. Which, in turn, makes
> > us incapable of solving should-not-have-happened problems, as
> > it's the whole point of the problem to display execution path
> > in inlined functions in which nothing has happened.
> >
> > In order to figure this out, we need to learn how to
> > discriminate between inlined calls in which nothing
> > interesting *could* have happened in the first place and
> > inlined calls in which something interesting could have
> > happened but *didn't*.
> >
> > NoStoreFuncVisitor solves this problem for the example above.
> > The visitor notices that the local variable is passed by
> > non-const pointer into the inlined call and the call
> > syntactically contains assignments through this pointer,
> > therefore this call *could* have initialized the value. The
> > visitor then suppresses pruning of this call by emitting an
> > interesting note within the call ("returning without
> > initializing '*y'"...).
> >
> > However, AST-based analysis in NoStoreFuncVisitor is very
> > primitive and easy to trick. A more sophisticated analysis is
> > clearly necessary.
> >
> > The second thing we can try in our GSoC is to come up with a
> > better analysis specifically for the uninitialized value
> > checker, because we already have a place to stick it into and
> > we know how exactly do we want to consume the result of such
> > analysis and evaluate it.
> >
> > The third thing we can try in our GSoC is to come up with more
> > such analysis for other checkers and other
> > should-have-happened problems and see if a reusable framework
> > for creating such analyses can be implemented.
> >
> >
> > On 5/23/19 1:25 PM, Kristóf Umann wrote:
> >>
> >>
> >> On Thu, 23 May 2019 at 21:58, Kristóf Umann
> >> <dkszelethus at gmail.com <mailto:dkszelethus at gmail.com>> wrote:
> >>
> >> Please let me know if I didn't address something you
> >> mentioned!
> >>
> >> On Thu, 23 May 2019 at 02:29, Artem Dergachev
> >> <noqnoqneo at gmail.com <mailto:noqnoqneo at gmail.com>> wrote:
> >>
> >> My primary question is, how do you plan to use the
> >> data that you obtain via your analysis?
> >>
> >>
> >> Gather relevant statements. Keep ExplodedNodes in the
> >> bugpath for which PathDiagnosticLocation::getStmt() is in
> >> the set (or, something like that at least). But honestly,
> >> I didn't think too much about that yet :^) I guess we
> >> could just gather ExplodedNodes rather than statements,
> >> we'll see.
> >>
> >> Like, do you plan to backtrack the value of all
> >> relevant variables and/or expressions in the final
> >> bug report that were also encountered along the bug
> >> path? If yes, then is it possible that the slicing
> >> criterion gets updated in the process and you'll have
> >> to re-run the CFG-based analysis to take it into
> account?
> >>
> >>
> >> No, the slicing criterion is what it is, and will not
> >> change. The set of relevant statements to the slicing
> >> criterion define the actual program slice, which is
> >> basically the thing we're going for in this project. When
> >> it turns out that a value of a variable directly affects
> >> a variable in the slicing criterion (either through data
> >> or control dependency), we just add it to the set of
> >> relevant variables, and then if something in the set of
> >> relevant variables is affected (some statements or
> >> variables turn out to be a control/data dependencies to
> >> them), then it's added to the respective relevancy sets.
> >> Should we only traverse the basic blocks the analyzer
> >> visited, I think we can pull off the slicing with a
> >> single pass.
> >>
> >>
> >> We'll see about that single pass thing though. I'm gathering
> >> some tricky examples in the document I shared and working on
> >> a neat algorithm.
> >>
> >> > What would also be really great is to assist this
> >> traversal with the information the analyzer already
> >> has, essentially only inspecting basic blocks the
> >> analyzer actually visits.
> >>
> >> You mean visits on the current bug path or visits on
> >> the equivalence class of bugs or visits in general
> >> during analysis?
> >>
> >> Regardless, for many problems ideally we should
> >> traverse basic blocks that the analyzer *didn't*
> >> visit (eg., if the function didn't initialize the
> >> variable on the current path, we're interested in
> >> parts of the code in which it *did* initialize the
> >> variable, even though we didn't necessarily have time
> >> to visit them).
> >>
> >>
> >> Well I mean slicing by definition isn't intended to do
> >> that. The entire point of it is to gather the smallest
> >> slice related to the slicing criterion, and my project
> >> aims to fill in the gaps where we don't provide enough
> >> information.
> >>
> >> It actually sounds as if all problems that we're
> >> trying to solve here can be classified into
> >> "must-have-happened" problems (eg., "variable
> >> must-have been declared without initialization",
> >> "variable must-have been set to nullptr", "memory
> >> must-have been allocated") and
> >> "should-not-have-happened" problems (eg., "variable
> >> should-not-have been initialized", "null value
> >> should-not-have been overwritten", "pointer
> >> should-not-have escaped").
> >>
> >> For must-have-happened problems, i'm recently under
> >> an impression that we should suppress the bug report
> >> entirely when we fail to solve them (i.e., if fail to
> >> explain to the user where exactly does this happen,
> >> then the report is impossible to understand anyway).
> >> This is currently a very big problem for our null
> >> dereference checker on some codebases, especially
> >> because it uses this tracking for suppressions as
> >> well (aka inlined defensive checks), so when it fails
> >> to track the variable it's likely a legit false
> >> positive as well, not simply a hard-to-understand
> report.
> >>
> >>
> >> I think this set calculating approach inherently gathers
> >> far more information, allowing us to make better
> >> judgement on whether we should suppress the report.
> >>
> >> For should-not-have-happened problems i'm much more
> >> confused. We're talking about looking for places
> >> where it *could* have happened and then trying to
> >> explain to the user why none of them have actually
> >> happened. I'm not sure what are the consequences of
> >> failing to explain to the user why didn't a
> >> particular piece of code do something, because i've
> >> no idea how do users intuitively figure out which
> >> code *could* have done these things and which clearly
> >> couldn't.
> >>
> >>
> >> Im really at a loss here :) Can you provide some example
> >> as to what a problematic "must-have-happened" bug report
> >> would look like, and how would you like to see it
> >> improved? Same for "should-not-have-happened". Because as
> >> I understand it, and I might be wrong, you have problems
> >> with this:
> >>
> >> int a; // declaring a without initializing
> >>
> >> if (rand()) // assuming that the condition is false
> >> a = 5;
> >>
> >> print(a); // a is undef
> >>
> >> And prefer to see this:
> >>
> >> int a; // declaring a without initializing
> >>
> >> if (rand()) // should this condition be false, a's value
> >> will be indeterministic
> >> a = 5; // writing to a skipped
> >>
> >> print(a); // a is undef
> >>
> >> and I just don't see how this would be possible with
> >> slicing at all. Also, I can't see how this would scale to
> >> real-world production code.
> >>
> >>
> >> On 5/22/19 4:11 PM, Kristóf Umann wrote:
> >>>
> >>> Hi!
> >>>
> >>>
> >>> I'd like to share some thoughts about my GSoC
> >>> project, "Enhancing bug reporting with static
> >>> backward program slicing"[1].
> >>>
> >>>
> >>> My proposal is very specific about whatI'm aiming to
> >>> improve on, but vague on the howpart of it. This
> >>> mail aims to add clarify some of this.
> >>>
> >>>
> >>> Program slicing is essentially a technique of
> >>> discovering data and control dependencies to the
> >>> slicing criterion, which is a (statement, {set of
> >>> variables}) pair. Fortunately, tools for control
> >>> dependencies, namely, dominator set calculations are
> >>> already implemented, but seems to be unstable with
> >>> clang's CFG. It would be a great tool if I were able
> >>> to fix it.
> >>>
> >>>
> >>> While my proposal states that I plan to implement an
> >>> AST-based solution, I'm actually not sure that this
> >>> is the optimal approach. We could instead inspect
> >>> CFG blocks in a backwards manner (coupled with the
> >>> analyzer's call stack), and gather relevant variable
> >>> in the meanwhile.
> >>>
> >>>
> >>> What would also be really great is to assist this
> >>> traversal with the information the analyzer already
> >>> has, essentially only inspecting basic blocks the
> >>> analyzer actually visits.
> >>>
> >>>
> >>> Here’s my idea for an algorithm (from the point of
> >>> the slicing criterion already being constructed):
> >>>
> >>>
> https://docs.google.com/document/d/1Lx867o3meyQsj0WKOSWMdosSBdw2MUq1aIxyPJM6iLU/edit?usp=sharing
> >>>
> >>>
> >>>
> >>> Please note that this is a veeery rough sketch, I
> >>> didn't think about every edge case that exists, but
> >>> I think it's an okay start.
> >>>
> >>>
> >>> [1]
> >>>
> https://docs.google.com/document/d/1ci1BCAKojPlqIxIw1J_K2dnATA3z01PuwR_vHJS55TI/edit
> >>>
> >>
> >
>
>
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