[cfe-dev] [analyzer] [GSoC 2019] Apply the Clang Static Analyzer to LLVM-based projects drafts

Artem Dergachev via cfe-dev cfe-dev at lists.llvm.org
Thu Apr 4 19:15:43 PDT 2019


+actual Ravi as he prefers this address. Ravi is this dude - 
https://github.com/ravikandhadai - he's from the Swift universe and he 
has a solid academic background in static analysis (unlike me ^_^") and 
every time i tell him we have a checker for this bug, he gets more and 
more excited :p

On 4/4/19 2:26 PM, Csaba Dabis wrote:
> Hey Clang developers!
>
> I would like to participate in Google Summer of Code this year. I am 
> in my fourth semester BSc student of Computer Science at Eotvos Lorand 
> University, Hungary. I have started to learn C++ parallel with Clang a 
> year and a half ago. Also that was the first time using Linux, Git, 
> VIM…. I love automation so this engine and tools based on Clang like 
> scan-build, CodeChecker, CodeCompass.
>
> I have picked the following project: 
> http://llvm.org/OpenProjects.html#analyze-llvm
> Here is the copy of the problems and their solutions from my 
> near-finished proposal:
>
> Goals
> Eliminate 90% of the false positive findings in LLVM by teaching C++ 
> to the Static Analyzer. Improve the existing debugging facilities so 
> it would be easier to investigate errors. Report and fix the 
> easy-to-fix true positives in LLVM. Report the difficult-to-fix true 
> positives in LLVM so other developers with better experience in that 
> certain area could solve those. Swift is another heavy project as an 
> example to see how an LLVM-related project reports are changing. 
> Measure the quality of the changes in Swift where no direct false 
> positive elimination happen. With these improvements let the LLVM and 
> related project contributors use the Static Analyzer sub-project 
> without any overhead in a continuous integration workflow.

*approves the goals*

> Overview of the debugging facilities
> The Clang Static Analyzer builds the exploded graph which consists of 
> program states as nodes. During the symbolic execution each node 
> represents everything what we know about the program at a certain 
> location.
>
> ExplodedGraph: We could investigate the graph with graphviz as an .SVN 
> file and using Google Chrome. The graph can be so enormous so that 
> Chrome crashes or even cannot load it. If you are able to load it, 
> there is too much information and it is very difficult to use.

I mean, our exploded graph dumps are horrible, but i've been a happy 
user of them for like 5 years. Pretty much every single bugfix that i 
made so far involved an investigation via exploded graph dumps. And on 
top of that, i'm not seeing much information that can be removed from 
them. Yes, viewers are choking immediately, browsers through svg 
conversion are doing better, especially chrome that seems to have the 
most tolerant svg library. But whenever there isn't way to view an 
exploded graph, it becomes 5x times harder to debug anything.

One of the more personal reasons why i've been rooting for this project 
is that i wanted to popularize this systematic debugging workflow of 
narrowing down the bug to the Static Analyzer function in which it's 
happening that consists binary-searching the exploded graph dump for 
invalid values and bindings. I guess i should actually document it some 
day, like, you know, *for once*, 'cause 
http://clang-analyzer.llvm.org/checker_dev_manual.html#visualizing is 
clearly insufficient.

> Alternatively you could use LLDB debugger but because of the such a 
> complex background it is more difficult to gather information which 
> function causes the false positive.
>
> Debug checkers: debug.DumpCalls checker truly writes out every 
> function call, which is too much and too difficult to use. Expression 
> inspection checks[1] are useful for get a feeling what could go wrong 
> by writing out a certain program state, but it cannot be used to 
> compare states due to the graph structure.
>
> Proposed solutions for the debugging facilities
> ExplodedGraph: Create an .HTML frontend for the .SVG graph 
> representation. It could modify the full graph to only show 
> differences between states and it would recolour the current 
> representation for better readability.

It'd be awesome to pull this off, but i suspect this undertaking alone 
might take a few months of your time. I think you should keep your eyes 
open for potential smaller improvements but generally try to familiarize 
yourself with existing tools before building up long-term plans in this 
direction.

> Debug checkers: Create an option for debug.DumpCalls checker to show 
> only a certain variable and if its value is unknown at the location of 
> an error, point out when it became unknown.
>
> Overview of the false positives
> My playground was the LLVM 8.0.0 bug-free release (20 March 2019). 
> With the basic scan-build command 828 bug reports found. Because of 
> our precise review system they are most likely false positive 
> findings, where the half is ‘Memory leak’ (229) and ‘Called C++ object 
> pointer is null’ (217) errors:
> - ‘Memory leak’: Half of the reports (118/229) appears in Error.h on 
> the same function call in different variations.
> - ‘Called C++ object pointer is null’: Third of the reports happen on 
> placement new operations.

Yes! That's what i wanted to hear. Put this on the top :) Great job 
identifying those top issues!

I've been noticing placement-new bugs before, but i didn't ever notice 
leaks in Error.h being a popular FP, nice catch!

> Proposed solutions for the false positives
> One could say creating more assertions could remove the errors and 
> document the code better. Let think about the opposite: removing every 
> assertion like ‘assert()’ and ‘LLVM_DEBUG()’[2] could show the 
> weakness of the Static Analyzer. We cannot force our users to double 
> or triple the number of assertions (even it would be very useful). 
> With that, and the new debug-facilities the door will be open to 
> mitigate the false positives.

That depends. When suppressing Static Analyzer false positives with 
assertions, some assertions are great to add anyway as a means of 
documentation and verification, while others do indeed look like 
ridiculous false positive suppressions that clearly don't belong here.

Regardless of having to add an assertion or not, we should anyway in 
parallel think whether we could have prevented the false positive from 
happening in the first place.

> It is impossible to measure how long does it take to eliminate a false 
> positive.

*wholeheartedly agrees*

> If we think about sets of false positives as the two most common 
> factor is already known, we could define more sets. We have to start 
> the work from the highest set. The workflow is the following: pick the 
> most common false positive, if it is necessary improve the debugging 
> facilities, mitigate the error, document that to LLVM Bugzilla, inject 
> assertions to problematic code, repeat.

Yeah, something like that :)

>
> -------------
> [1] ExprInspection checks: 
> https://clang.llvm.org/docs/analyzer/developer-docs/DebugChecks.html#exprinspection-checks
> [2] LLVM_DEBUG(): 
> http://llvm.org/docs/ProgrammersManual.html#the-llvm-debug-macro-and-debug-option 
>
> -------------
>
> Any feedback would be really appreciated.
>
> Thanks you,
> Csaba.
>

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