[cfe-dev] Clang Static Analyzer: False Positive Suppression Support

[Gábor Horváth via cfe-dev]

Hi!

In the CodeChecker tool (https://github.com/Ericsson/codechecker) we
support suppressing false positive bugs as a post processing step. After
all the issues were reported, CodeChecker filters out the suppressed ones,
and those will not be displayed to the user.

In our experience, however, and external tool only suppression suppport is
not sufficient for certian reasons:
* When a checker generates a sink node during analysis, the rest of the
path will not be covered by the Static Analyzer. Unfortunately, if the user
suppress the bug in an external tool, the coverage will not get better.
This way false positive results can hide true positive results regardless
of suppression.
* The compiler could do better job diagnosing ill formed suppressions
(invalid source range, typo in checker name etc).
* Tools that are developed on top of the compiler need not to introduce
their own customized solution for suppression.

It is beneficial to have a suppression format that is standard across all
clang tools. So the same format could be used to:
* Suppress clang warnings
* Suppress clang static analyzer warnings
* Suppress clang tidy warnings

There are two main approaches to suppress warnings that I can think of
right now:

Suppress using comments (or pragma):
* This is a good solution for code evolution. The comments are moving
together with the code. Edits are less likely to invalidate comments.
* This is an intrusive solution, the source code needs to be changed to
suppress an issue. This is both good, because the suppressions are version
controlled and bad, because suppressing in 3rd party libraries might be
problematic.
* For path sensitive checks, it is not always self evident at which point
of the path should a suppression comment be added.


Suppress using hashes:
* It is hard to come up with a reliable hash to identify bugs. There always
might be corner cases when the hashes of different bugs collide or the hash
of a bug changes due to an edit that in fact should not affect the bug.
* It is non intrusive.
* The user do not need to think about at which point should the suppression
comment be inserted.


I would suggest the suppress comment road.
The syntax could be something like:
// clang suppress warning-or-checker-name [optional line offset][optional
column range] [comment]

The warning-or-checker-name could be a regex, so multiple checks can be
suppressed at the same time.
Column ranges are useful when multiple errors are reported for the same
line.
Line offsets are useful when one do not want to break the flow of a multi
line code snippet with a comment, so this way warnings could be suppressed
without too much negative effects on the readability.
Comment is for documentation purposes, the user can document why does she
think that this is a false positive.

What do you think?

Regards,
Gábor
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