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Jan 21, 2024 - 18 minute read - Comments - semgrep

A Few Fun Semgrep Experiments

I want to use Semgrep as a light code intelligence tool with a few experiments. I will write custom rules to extract info from code and then process the results.

The type of these experiments is inspired by Martin Jambon who is actually a core Semgrep developer. These are supposed to be self-contained but short experiments. You can see his at https://github.com/mjambon/dev-random.

Intro

If you know me, you know I never shut up about Semgrep, see https://parsiya.net/categories/semgrep/.

I use Semgrep if I want something quick that works. For complex uses cases, you need to create your own static analysis tools. Parsing is doable with tree-sitter (same thing Semgrep uses) and tree-sitter queries. For Go, I have also used the ast package.

Requirements

This blog assumes you have this knowledge. Nothing fancy.

  1. Some familiarity with Go.
  2. Writing basic Semgrep rules.

Setup

To interact with Semgrep, I have created a couple of wrapper packages in Go and Rust. The overall concept of post-processing is straightforward:

  1. Create custom rules that use metavariables to extract specific info from code.
  2. Create a wrapper to run Semgrep and deserialize the JSON output.
  3. Draw the rest of the owl. Process the output and apply the logic.

In this blog I will use semgrep_go. You can follow along:

git clone --recurse-submodules https://github.com/parsiya/semgrep-fun
go run main.go 01 code/juice-shop

These are simple applications that just do the job. There's not a lot of error handling.

Gotchas

Hopefully you don't have to repeat my mistakes. These have taken a few hours of my life.

Complete Rule IDs

By default, Semgrep adds some extra text to rule IDs in the results. I call them "complete rule IDs" and (I think) they're added to prevent rule ID collision.

For local rules, the complete text depends on the path in --config. E.g., if I run Semgrep with --config tmp/whatever.yaml and the rule ID is my-rule, the complete rule ID is tmp.whatever.my-rule.

Rules in the Semgrep registry follow a similar pattern. It's based on the path that of the rule in the GitHub repo semgrep/semgrep-rules. E.g., the complete rule ID for the C double-free rule is c.lang.security.double-free.double-free because it's in semgrep-rules/c/lang/security/double-free.yaml. Note the double double-free (har har) in the end because both the file name and the rule ID are the same.

Our options when processing the results are:

  1. Use the complete rule ID. This is doable for registry rules because their ID is predictable, but not practical for local rules because we might have no control over their path.
  2. Check if the rule ID in the results ends with the rule ID from our files with strings.HasSuffix.
    1. Don't split by . and compare the last part. I've been bit by this when the rule ID had periods.
  3. Run Semgrep with --no-rewrite-rule-ids to disable the complete rule ID generation. We will only the rule ID in the output, but this might lead to collisions.

Reading the JSON Output

There are two ways to read the JSON output.

  1. Tell Semgrep to store the results in a file with the --output switch.
    1. This might be removed in the future according to the developers.
  2. Read it from process output.
    1. The package uses this option.

Output Structs

The structure of the output is defined in semgrep/semgrep-interfaces. The source of truth is the atd file, but it's an OCaml thing and I don't know how to parse it. I rely on the automatically generated JSON schema in semgrep_output_v1.jsonschema.

I used omissis/go-jsonschema (formerly atombender/go-jsonschema) to generate the Go structs from the JSON schema. From time to time, the schema might break backwards compatibility.

Don't upgrade your Semgrep version without checking if the format has changed. Generate the structs and then do a simple compare to see if anything major has changed.

Generating Structs

I use these commands:

$ git clone https://github.com/semgrep/semgrep-interfaces && cd semgrep-interfaces
# optional: check out the tag for a specific Semgrep version
$ git checkout v1.52.0

# install go-jsonschema
$ go install github.com/omissis/go-jsonschema/cmd/gojsonschema@latest

# generate the output
# -p output: package name is output
# -o output.go: write the structs to output.go
$ gojsonschema -p output -o output.go --verbose semgrep-interfaces/semgrep_output_v1.jsonschema

What I tried and didn't work in Go: https://parsiya.io/abandoned-research/semgrep-output-json/.

Similar article for Rust: https://parsiya.net/blog/2022-10-16-yaml-wrangling-with-rust/.

Extracting Metavariables via the Message Field

In these examples, I have smuggled the value of metavariables from the message field. It's convenient and with a bit of smart placement and text processing, you can get structured data out of it.

You could also access the values of metavariables from the struct. They can be accessed via result.Extra.Metavars["$METAVARNAME"]. There are two fields, AbstractContent and PropagatedValue. Generally, you want the propagated value (the equivalent of having value($METAVARNAME) in the message field). For more information please see https://semgrep.dev/docs/writing-rules/experiments/display-propagated-metavariable/.

The Experiments

The code subdirectory contains the test data as git submodules. Be sure to populate them with before running the examples. They are:

00. Running Semgrep

The "official" way to run Semgrep is via the Semgrep command. So my package uses a wrapper. You can also use the osemgrep binary directly or look into how the Python wrapper does it.

I am going to run it on OWASP Juice Shop with the p/default ruleset and then use it in some other examples.

The semgrep_go package allows us to use some common switches. The Extra field is a []string that allows us to pass the rest of the switches. Because we want to store the output in a file, we will just pass --output. But we can also have multiple parameters like []string{"--no-rewrite-rule-ids", "--severity", "WARN"}.

// Setup Semgrep switches.
opts := run.Options{
    Output:    run.JSON,       // Output format is JSON.
    Paths:     []string{path}, // "code/juice-shop"
    Rules:     []string{"p/default"},
    Verbosity: run.Debug,
    Extra:     []string{"--output=output/juice-shop.json"},
}

Let's run Semgrep. We don't care about the output so we will use Run instead of RunJSON. We will also ignore the output.

log.Print("Running Semgrep, this might take a minute.")
// Run Semgrep and ignore the output.
_, err := opts.Run()
return err

Read output/juice-shop.json and parse it with jq. This example shows the ruleID and path of every hit.

$ jq '.results[] | "ruleid: " + .check_id + " - path: " + .path' output/juice-shop.json
See the complete rule IDs See the complete rule IDs

Exclude Rules

Objective: Remove results for specific rules.

Let's start with something easy. We've used a ruleset/rulepack, but we want to ignore results from certain rules instead of modifying it directly to remove those rules. We can do it in two ways:

  1. Use multiple --exclude-rule switches.
  2. Process the results and delete the hits from excluded rules.

Our example is running Semgrep with the p/default ruleset on OWASP Juice Shop in code/juice-shop.

01. Passing Multiple exclude-rule Switches

The semgrep-go package allows us to pass command line switches to Semgrep. We could have used jq to process JSON on the command line, too, but I rather use Go in a proper program especially if the list of excluded rules is big or dynamic.

// fun/01_exclude_switch.go

// In the real world we will get a long list from somewhere.
excludedRules := []string{
    "javascript.audit.detect-replaceall-sanitization.detect-replaceall-sanitization",
}
var extra []string
// Add all the excluded rules like a `--exclude-rule=[ID]` argument.
for _, r := range excludedRules {
    extra = append(extra, "--exclude-rule="+r)
}

log.Printf("Excluding results for: %s", excludedRules)

Add the extra switches in the Extra field.

// Setup Semgrep switches.
opts := run.Options{
    Output:    run.JSON,       // Output format is JSON.
    Paths:     []string{path}, // "code/juice-shop"
    Rules:     []string{"p/default"},
    Verbosity: run.Debug,
    Extra:     extra, // Items in Extra will be added to the CLI as-is.
}

Run Semgrep and deserialize the output.

log.Print("Running Semgrep, this might take a minute.")
// Run Semgrep and get the deserialized output.
out, err := opts.RunJSON()
if err != nil {
    return err
}

Loop through the hits/findings/matches in the deserialized output and check ruleIDs against the exclusions.

// Check if any of the ruleIDs match what we wanted to exclude.
for _, hit := range out.Results {
    if strings.Contains(hit.RuleID(), "detect-replaceall-sanitization") {
        return fmt.Errorf("Found a rule that should have been excluded.")
    }
}

The field name for ruleID in the output is actually CheckId. So I have a RuleID() method. Finally, we use the package to create an ASCII table of all ruleIDs and their number of hits.

If you've cloned the semgrep-fun repo, run go run main.go 01 code/juice-shop.

Excluding rules with CLI switches Excluding rules with CLI switches

02. Removing Specific Results from the Output

The results might come from a pipeline where cannot modify the Semgrep command. In this experiment, I will process the deserialized output and remove all hits with the specific rule IDs.

Instead of running Semgrep again, I will use the output from section 00 in output/juice-shop.json.

// Read the data in "output/juice-shop.json".
data, err := os.ReadFile("output/juice-shop.json")
if err != nil {
    return err
}
// Deserialize the data.
out, err := output.Deserialize(data)
if err != nil {
    return err
}

out has our deserialized results.

// Create a new slice to hold the modified results.
var modifiedResults []output.CliMatch

// Loop through the results.
for _, hit := range out.Results {
    // Check if the hit's ruleID matches any of the excluded rules.
    if slices.Contains(excludedRules, hit.RuleID()) {
        // If it does, skip it.
        continue
    }
    // If the ruleID is not excluded, add it to modifiedResults.
    modifiedResults = append(modifiedResults, hit)
}

// Replace the results with the modified results.
out.Results = modifiedResults

Note how we are replacing the results in the original object. We can serialize the modified object back to JSON and pass it to the next step of the pipeline.

js, err := out.Serialize(true)
if err != nil {
    return err
}

Run this command to see it in action: go run main.go 02 code/juice-shop.

Summary of results Summary of results

03. Unit Test Coverage in Go

Objective: Which functions in a package have unit tests.

Go Unit Tests

TL;DR:

  1. Assume function is Func1 in file func1.go.
  2. Test is function TestFunc1 in file func1_test.go in the same package.

I usually use the vscode-go extension to generate test skeletons. Works better than AI, tbh.

Logic

  1. Extract all function names with their package and file names.
    1. The package name can reduce false positives for identical names in different packages.
  2. Go through the list of functions and filter the noise.
    1. (optional) Select functions that are not in files that end in _test.go.
    2. (optional) Select functions that are exported (start with a capital letter).
  3. Check each function has a test. E.g., for Func1 do:
    1. Does TestFunc1 exist in the same package?
      1. If not, log there's no test.
    2. If so, is it in the func1_test.go?
      1. If not, log the test is in the wrong place.
    3. Log Func1 has a test in the correct location.

.semgrepignore

Semgrep ignores certain test files and paths (among other things). See everything in the default .semgrepignore file. We want to scan the test files.

To make Semgrep scan test files we create an empty .semgrepignore file in the current working directory before we run Semgrep and delete it after.

The Custom Rule

This is a custom rule that collects:

  1. Package name
  2. Function name

We also need the file name but that's done in the post-processing part.

The rule will look like this. This is the experimental rule syntax. Here's a tutorial. I have also included the old rule syntax.

rules:
  - id: blog-2023-11-go-function-extract
    match:
      all:
        - inside: |
            package $PKG
            ...            
        - func $FUNC(...)
    # patterns:
    #   - pattern-inside: |
    #       package $PKG
    #       ...
    #   - pattern: func $FUNC(...)
    message: $PKG - $FUNC
    languages:
      - go 
    severity: WARNING

You can see the rule in action at: https://semgrep.dev/playground/r/zdUKA4D/parsiya.blog-2023-11-go-function-extract

Running Semgrep and Extracting the Info

Rule messages are in the form of $PKG - $FUNC. We will need three pieces of information:

  1. Package name: In message.
  2. Function name: In message.
  3. File path: In .Path or using the helper method .FilePath().

Then it becomes a DSA problem and you can skip reading the rest of the section if you want to.

First, we create a FuncInfo object for each function:

// FuncInfo contains information about a function.
type FuncInfo struct {
	Package string
	Name    string
	Path    string
}

And create a map where the key is a package name and the members are the functions in that package.

// FuncList is a map where key is the package name and the value is FuncMap.
type FuncList map[string]FuncMap

Then we can easily populate these from the results:

// Loop through the results.
for _, hit := range out.Results {
    // $PKG - $FUNC
    msg := strings.Split(hit.Message(), " - ")
    // msg[0]: $PKG
    // msg[1]: $FUNC

    // Note we're not doing a lot of error checking here.
    if len(msg) != 2 {
        log.Printf("Wrong message, got: %s", hit.Message())
        continue
    }
    // Store the function info in a FuncInfo struct.
    fn := FuncInfo{
        Package: msg[0],
        Name:    msg[1],
        Path:    hit.FilePath(),
    }
    if _, ok := funcList[fn.Package]; !ok {
        funcList[fn.Package] = make(FuncMap)
    }
    funcList[fn.Package][fn.Name] = fn
}

We could use this map to extract any info we want. We can also use another trick here. In Go, the tests belong to the same package so we can just create a list of functions in each package and then do a search in an array of strings.

data := make([][]string, 0)

// Now, we have a map of all functions in code, we can go through the
// functions in each package and check if they have a test.
for _, funcs := range funcList {

    // It's easier to create a slice of all functions in a package for
    // searching.
    var funcNames []string
    for _, fn := range funcs {
        funcNames = append(funcNames, fn.Name)
    }

    // [next section with the checks]
}

Then we can go through each function and check if it has a test in the same package by using string.Contains. We're also saving a few cycles by skipping test functions (in files ending in _tests.go).

for _, fn := range funcs {
    // Skip functions in `*_test.go` files.
    if strings.HasSuffix(fn.Path, "_test.go") {
        continue
    }

    // Check if the function has a test. AKA "Test"+fn.Name is in the
    // package.
    if slices.Contains(funcNames, "Test"+fn.Name) {
        continue
    }
    // Add the functions with missing tests to the data slice.
    data = append(data, []string{fn.Name, fn.Package, fn.Path})
}

Running it on code/logrus gives us this table. But you can pass the path to any other Go code to the command line.

List of functions in logrus List of functions in logrus

Summary Reports

You run Semgrep and get a bunch of results. If you want to review the results, you should use the --sarif switch and use a SARIF viewer. I usually use the VS Code SARIF plugin. It allows me to look at the issue in the editor.

But we can also process the output and create reports with specific items. The semgrep_go package supports creating two types of tables:

  1. Count by rule ID
  2. Count by file path

We're going to use the output of running p/default on Juice Shop from before.

04. Text Summary Report

Let's create a simple ASCII report. We will create two tables. First one shows us high impact rules and the second one, files with highest number of findings. Generally, we want to have the rule IDs and files with the highest number of findings at the top, this is done by passing true to the two functions below. If we pass false, the tables will be sorted by rule ID or file path alphabetically.

// fun/04_text_report.go

// [Removed]
// The deserialized results are in `out`.

// Create the reports.
ruleIDTextReport := out.RuleIDTextReport(true)
filePathTextReport := out.FilePathTextReport(true)

// Print the reports.
log.Print("Rule ID report:")
log.Print(ruleIDTextReport)

log.Print("File Path Report:")
log.Print(filePathTextReport)

This time instead of passing the code base path, we will pass the output file from section 00. The program will print two text tables. go run main.go 04 output/juice-shop.json

Summary report Summary report

05. HTML Summary Report

For the HTML summary report, I wanted to make something similar to the Semgrep outline. We can create a template using the Go html/template package and apply it to the output object. This is going to look ugly, but good enough for an example.

The code is similar to the previous section. We will deserialize the result and create a report struct.

// fun/05_html_report.go

// Report contains the information in the HTML report.
type Report struct {
	NumberOfFindings int
	ByRuleID         []output.HitMapRow
	ByFilePath       []output.HitMapRow
}

func HTMLReport(path string) error {
    // [Removed]
    // The deserialized results are in `out`.

    // Create the report object.
    rep := Report{
        NumberOfFindings: len(out.Results),
        ByRuleID:         out.RuleIDHitMap(true),
        ByFilePath:       out.FilePathHitMap(true),
    }
    // removed
}

Then we will pass to an HTML template which uses the built-in Go template engine.

// embed the 05-report-html-tmpl.html in a string.
//
//go:embed 05-report-html-template.html
var tmpl string

func HTMLReport(path string) error {
    // removed

	// Apply the template.
	t, err := template.New("report").Parse(tmpl)
	if err != nil {
		return err
	}
	var data bytes.Buffer
	if err = t.Execute(&data, rep); err != nil {
		return err
	}
    // removed
}

We will write the resulting HTML report to output/05-report.html. You can also see it in the cloned repo.

go run main.go 05 output/juice-shop.json

Graphic design is my passion Graphic design is my passion

06. Go Function Call Chain

This is a simple function call chain experiment.

Probably the most straightforward way of doing this would be with a rule like this rule.

rules:
- id: go-function-chain
  patterns:
    - pattern-inside: |
        package $PKG
        ...        
    - pattern-inside: |
        func $CALLER(...) {...}        
    - pattern-either:
        - pattern: $CALLEE(...)
        - pattern: $IMP.$CALLEE(...)
    - metavariable-regex:
        metavariable: $CALLEE
        regex: ^[^.]*$
    - focus-metavariable: $CALLEE
  message: $PKG - $CALLER - $CALLEE - $IMP
  languages:
    - go
  severity: WARNING

We're looking for two types of calls:

  1. Local package functions: $CALLEE(...)
  2. Imported functions: $IMP.$CALLEE(...)

Playground link: https://semgrep.dev/playground/r/oqUgbKy/parsiya.go-function-chain

I had to fix two issues with this pattern:

First, Imported functions like strings.Contains are captured twice, one as $CALLEE: strings.Contains and the other as $IMP: strings, $CALLEE = Contains. The metavariable-regex looks for the literal dot in $CALLEE and drops the first match.

Second, we're only capturing the top-level package name instead of the complete name. So if two different packages have the same top-level package and function names, we have a collision.

We can track all imports in each file. The pattern will be import $IMPORT and we will group the results by file path. Now, we can create the complete package name for each file.

rules:
- id: go-import-collection
  patterns:
    - pattern-inside: |
        package $PKG
        ...        
    - pattern-either:
      - patterns:
          - pattern: import "$IMPORT"
          - pattern-not: import $ALIAS "$IMPORT"
      - pattern: import $ALIAS "$IMPORT"
  message: $PKG - $ALIAS - $IMPORT
  focus-metavariable: $IMPORT
  languages:
    - go
  severity: WARNING

Playground link: https://semgrep.dev/playground/r/2ZUzvdK/parsiya.go-import-collection When processing the results from this rule, we have to filter out the text $ALIAS in the message which happens when the package is not imported as an alias. Alternatively, we could have two rules. One for each pattern.

The rest of the processing is kind of straightforward.

The go-import-collection rule returns each package's alias (if it exists) and its complete name. We create an ImportMap for each file. It's a map[string]string where the key is the top-level package name (e.g., bar for github.com/foo/bar) or its import alias (e.g., bar for import bar "github.com/foo/bar").

All of these go into a different map where the key is the file path and value is the import map.

// ImportMap is map of one file's imports.
//
// Key: Alias if it exists or the top-level package if not (e.g., "bar" for
// "github.com/foo/bar").
//
// Value: Complete package name.
type ImportMap map[string]string

// Imports is a map of file paths to their imports. Key is file path.
type Imports map[string]ImportMap

go-function-chain returns the function name, package name, and file path for each function. This is enough to identify each function in code. The file path helps prevent collisions when two packages have the same name in different parent packages. E.g., github.com/foo/bar and github.com/baz/bar.

// Function represents one function.
type Function struct {
	Package  string
	Name     string
	FilePath string
}

The final result is still undercooked and has two issues:

  1. It only goes down one level. E.g., caller to callee.
  2. Object methods are not recognized properly. E.g., runtime.FuncForPC(pcs[i]).Name() has package name of runtime.FuncForPC(pcs[i]) which is wrong.

But we have enough information to create a decent call chain.

Bonus Go Package Shenanigans

Go packages are special. All files in the same package can be merged into one big file without parsing issues. When dealing with Go packages, we can use this to our advantage and treat each package as one big file. This helps with intra-package analysis because we can simplify our rules.

There's one issue. When you merge everything under the hood before Semgrep analysis, the results will have points to the merged file (e.g., line 15 of the merged.go). We have to modify the results and translate them back to the original files because the user did not see our pre-processing.

This has a simple solution. We have the code snippet with the result, we can just search it in the original files. I did not expect it to be this simple. I was creating a map of offsets to track which offset is from which file, but this solution is just easier.

07. Misc Tips and Tricks

At this point I have been sporadically writing this blog for almost two months so I just wanna finish it. Here are some other tips and tricks, you can read more at https://parsiya.io/research/semgrep-tips/.

# Download a ruleset yaml file.
$ wget https://semgrep.dev/c/p/{ruleset-name} -O file.yaml
# Example
$ wget https://semgrep.dev/c/p/default -O default.yaml

Run all rules --config r/all.

Check if a specific file exists. E.g., /path/to/badfile.ext.

rules:
- id: detect-file
  patterns:
    - pattern-regex: .*
  message: Semgrep found the file
  languages:
    - generic
  severity: WARNING
  paths:
    include:
      - /path/to/badfile*
    exclude:
      - /paths/to/exclude/*

What Did We Learn Here Today?

We learned to process Semgrep output results. With handcrafted rules, we can extract so much information from Semgrep results.

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