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132 lines
9.4 KiB
ReStructuredText
132 lines
9.4 KiB
ReStructuredText
.. _basic-query-for-rust-code:
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Basic query for Rust code
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==========================
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Learn to write and run a simple CodeQL query using Visual Studio Code with the CodeQL extension.
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.. include:: ../reusables/vs-code-basic-instructions/setup-to-run-queries.rst
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About the query
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---------------
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The query we're going to run performs a basic search of the code for ``if`` expressions that are redundant, in the sense that they have an empty ``then`` branch. For example, code such as:
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.. code-block:: rust
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if error {
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// we should handle the error
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}
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.. include:: ../reusables/vs-code-basic-instructions/find-database.rst
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Running a quick query
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---------------------
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.. include:: ../reusables/vs-code-basic-instructions/run-quick-query-1.rst
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#. In the quick query tab, delete the content and paste in the following query.
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.. code-block:: ql
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import rust
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from IfExpr ifExpr
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where ifExpr.getThen().(BlockExpr).getStmtList().getNumberOfStmtOrExpr() = 0
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select ifExpr, "This 'if' expression is redundant."
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.. include:: ../reusables/vs-code-basic-instructions/run-quick-query-2.rst
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.. image:: ../images/codeql-for-visual-studio-code/basic-rust-query-results-1.png
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:align: center
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If any matching code is found, click a link in the ``ifExpr`` column to open the file and highlight the matching ``if`` expression.
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.. image:: ../images/codeql-for-visual-studio-code/basic-rust-query-results-2.png
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:align: center
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.. include:: ../reusables/vs-code-basic-instructions/note-store-quick-query.rst
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About the query structure
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~~~~~~~~~~~~~~~~~~~~~~~~~
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After the initial ``import`` statement, this simple query comprises three parts that serve similar purposes to the FROM, WHERE, and SELECT parts of an SQL query.
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+----------------------------------------------------------------------------------+-------------------------------------------------------------------------------------------------------------------+------------------------------------------------------------------------------------------------------+
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| Query part | Purpose | Details |
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+==================================================================================+===================================================================================================================+======================================================================================================+
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| ``import rust`` | Imports the standard CodeQL AST libraries for Rust. | Every query begins with one or more ``import`` statements. |
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+----------------------------------------------------------------------------------+-------------------------------------------------------------------------------------------------------------------+------------------------------------------------------------------------------------------------------+
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| ``from IfExpr ifExpr`` | Defines the variables for the query. | We use: an ``IfExpr`` variable for ``if`` expressions. |
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| | Declarations are of the form: | |
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| | ``<type> <variable name>`` | |
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+----------------------------------------------------------------------------------+-------------------------------------------------------------------------------------------------------------------+------------------------------------------------------------------------------------------------------+
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| ``where ifExpr.getThen().(BlockExpr).getStmtList().getNumberOfStmtOrExpr() = 0`` | Defines a condition on the variables. | ``ifExpr.getThen()``: gets the ``then`` branch of the ``if`` expression. |
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| | | ``.(BlockExpr)``: requires that the ``then`` branch is a block expression (``{ }``). |
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| | | ``.getStmtList()``: gets the list of things in the block. |
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| | | ``.getNumberOfStmtOrExpr() = 0``: requires that there are no statements or expressions in the block. |
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+----------------------------------------------------------------------------------+-------------------------------------------------------------------------------------------------------------------+------------------------------------------------------------------------------------------------------+
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| ``select ifExpr, "This 'if' expression is redundant."`` | Defines what to report for each match. | Reports the resulting ``if`` expression with a string that explains the problem. |
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| | | |
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| | ``select`` statements for queries that are used to find instances of poor coding practice are always in the form: | |
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| | ``select <program element>, "<alert message>"`` | |
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+----------------------------------------------------------------------------------+-------------------------------------------------------------------------------------------------------------------+------------------------------------------------------------------------------------------------------+
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Extend the query
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----------------
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Query writing is an inherently iterative process. You write a simple query and then, when you run it, you discover examples that you had not previously considered, or opportunities for improvement.
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Remove false positive results
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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Browsing the results of our basic query shows that it could be improved. Among the results you are likely to find examples of ``if`` expressions with an ``else`` branch, where an empty ``then`` branch does serve a purpose. For example:
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.. code-block:: rust
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if (option == "-verbose") {
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// nothing to do - handled earlier
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} else {
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handleError("unrecognized option")
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}
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In this case, identifying the ``if`` expression with the empty ``then`` branch as redundant is a false positive. One solution to this is to modify the query to select ``if`` expressions where both the ``then`` and ``else`` branches are missing.
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To exclude ``if`` expressions that have an ``else`` branch:
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#. Add the following to the where clause:
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.. code-block:: ql
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and not exists(ifExpr.getElse())
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The ``where`` clause is now:
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.. code-block:: ql
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where
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ifExpr.getThen().(BlockExpr).getStmtList().getNumberOfStmtOrExpr() = 0 and
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not exists(ifExpr.getElse())
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#. Re-run the query.
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There are now fewer results because ``if`` expressions with an ``else`` branch are no longer included.
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Further reading
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---------------
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.. include:: ../reusables/rust-further-reading.rst
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.. include:: ../reusables/codeql-ref-tools-further-reading.rst
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.. Article-specific substitutions for the reusables used in docs/codeql/reusables/vs-code-basic-instructions
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.. |language-text| replace:: Rust
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.. |language-code| replace:: ``rust``
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.. |example-url| replace:: https://github.com/rust-lang/rustlings
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.. |image-quick-query| image:: ../images/codeql-for-visual-studio-code/quick-query-tab-rust.png
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.. |result-col-1| replace:: The first column corresponds to the expression ``ifExpr`` and is linked to the location in the source code of the project where ``ifExpr`` occurs.
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