CodeQL documentation

Missing error check

ID: go/missing-error-check
Kind: problem
Severity: warning
Precision: high
Tags:
   - reliability
   - correctness
   - logic
Query suites:
   - go-security-and-quality.qls

Click to see the query in the CodeQL repository

When a function call returns two values, a pointer and a (subtype of) error, it is conventional to assume that the pointer might be nil until either the pointer or error value has been checked.

If the pointer is dereferenced without a check, an unexpected nil pointer dereference panic may occur.

Recommendation

Ensure that the returned pointer is either directly checked against nil, or the error value is checked before using the returned pointer.

Example

In the example below, user dereferences ptr without checking either ptr or err. This might lead to a panic.

package main

import (
	"fmt"
	"os"
)

func user(input string) {

	ptr, err := os.Open(input)
	// BAD: ptr is dereferenced before either it or `err` has been checked.
	fmt.Printf("Opened %v\n", *ptr)
	if err != nil {
		fmt.Printf("Bad input: %s\n", input)
	}

}

The corrected version of user checks err before using ptr.

package main

import (
	"fmt"
	"os"
)

func user(input string) {

	ptr, err := os.Open(input)
	if err != nil {
		fmt.Printf("Bad input: %s\n", input)
		return
	}
	// GOOD: `err` has been checked before `ptr` is used
	fmt.Printf("Result was %v\n", *ptr)

}

References