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99 lines
2.5 KiB
99 lines
2.5 KiB
Infer reports null dereference bugs in Java, C, C++, and Objective-C
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when it is possible that the null pointer is dereferenced, leading to
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a crash.
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### Null dereference in Java
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Many of Infer's reports of potential Null Pointer Exceptions (NPE) come from code of the form
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```java
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p = foo(); // foo() might return null
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stuff();
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p.goo(); // dereferencing p, potential NPE
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```
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If you see code of this form, then you have several options.
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**If you are unsure whether or not `foo()` will return null**, you should
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ideally either
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1. Change the code to ensure that `foo()` can not return null, or
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2. Add a check that `p` is not `null` before dereferencing `p`.
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Sometimes, in case (2) it is not obvious what you should do when `p`
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is `null`. One possibility is to throw an exception, failing early but
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explicitly. This can be done using `checkNotNull` as in the following
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code:
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```java
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// code idiom for failing early
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import static com.google.common.base.Preconditions.checkNotNull;
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//... intervening code
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p = checkNotNull(foo()); // foo() might return null
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stuff();
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p.goo(); // p cannot be null here
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```
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The call `checkNotNull(foo())` will never return `null`: if `foo()`
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returns `null` then it fails early by throwing a Null Pointer
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Exception.
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Facebook NOTE: **If you are absolutely sure that foo() will not be
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null**, then if you land your diff this case will no longer be
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reported after your diff makes it to master.
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### Null dereference in C
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Here is an example of an inter-procedural null dereference bug in C:
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```c
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struct Person {
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int age;
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int height;
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int weight;
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};
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int get_age(struct Person *who) {
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return who->age;
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}
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int null_pointer_interproc() {
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struct Person *joe = 0;
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return get_age(joe);
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}
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```
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### Null dereference in Objective-C
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In Objective-C, null dereferences are less common than in Java, but they still
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happen and their cause can be hidden. In general, passing a message to nil does
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not cause a crash and returns `nil`, but dereferencing a pointer directly does
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cause a crash as well as calling a `nil` block.C
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```objectivec
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-(void) foo:(void (^)())callback {
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callback();
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}
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-(void) bar {
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[self foo:nil]; //crash
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}
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```
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Moreover, there are functions from the libraries that do not allow `nil` to be
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passed as argument. Here are some examples:
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```objectivec
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-(void) foo {
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NSString *str = nil;
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NSArray *animals = @[@"horse", str, @"dolphin"]; //crash
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}
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-(void) bar {
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CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB(); //can return NULL
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...
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CFRelease(colorSpace); //crashes if called with NULL
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}
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```
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