[cfe-dev] RFC: Nullability qualifiers

[Douglas Gregor]

> On Mar 2, 2015, at 5:11 PM, Richard Smith <richard at metafoo.co.uk> wrote:
> 
> On Mon, Mar 2, 2015 at 4:20 PM, Douglas Gregor <dgregor at apple.com <mailto:dgregor at apple.com>> wrote:
> 
>> On Mar 2, 2015, at 3:34 PM, Richard Smith <richard at metafoo.co.uk <mailto:richard at metafoo.co.uk>> wrote:
>> 
>> On Mon, Mar 2, 2015 at 1:22 PM, Douglas Gregor <dgregor at apple.com <mailto:dgregor at apple.com>> wrote:
>> Hello all,
>> 
>> Null pointers are a significant source of problems in applications. Whether it’s SIGSEGV taking down a process or a foolhardy attempt to recover from NullPointerException breaking invariants everywhere, it’s a problem that’s bad enough for Tony Hoare to call the invention of the null reference his billion dollar mistake [1]. It’s not the ability to create a null pointer that is a problem—having a common sentinel value meaning “no value” is extremely useful—but that it’s very hard to determine whether, for a particular pointer, one is expected to be able to use null. C doesn’t distinguish between “nullable” and “nonnull” pointers, so we turn to documentation and experimentation. Consider strchr from the C standard library:
>> 
>> 	char *strchr(const char *s, int c);
>> 
>> It is “obvious” to a programmer who knows the semantics of strchr that it’s important to check for a returned null, because null is used as the sentinel for “not found”. Of course, your tools don’t know that, so they cannot help when you completely forget to check for the null case. Bugs ensue.
>> 
>> Can I pass a null string to strchr? The standard is unclear [2], and my platform’s implementation happily accepts a null parameter and returns null, so obviously I shouldn’t worry about it… until I port my code, or the underlying implementation changes because my expectations and the library implementor’s expectations differ. Given the age of strchr, I suspect that every implementation out there has an explicit, defensive check for a null string, because it’s easier to add yet more defensive (and generally useless) null checks than it is to ask your clients to fix their code. Scale this up, and code bloat ensues, as well as wasted programmer effort that obscures the places where checking for null really does matter.
>> 
>> In a recent version of Xcode, Apple introduced an extension to C/C++/Objective-C that expresses the nullability of pointers in the type system via new nullability qualifiers . Nullability qualifiers express nullability as part of the declaration of strchr  [2]:
>> 
>> 	__nullable char *strchr(__nonnull const char *s, int c);
>> 
>> With this, programmers and tools alike can better reason about the use of strchr with null pointers. 
>> 
>> We’d like to contribute the implementation (and there is a patch attached at the end [3]), but since this is a nontrivial extension to all of the C family of languages that Clang supports, we believe that it needs to be discussed here first.
>> 
>> Goals
>> We have several specific goals that informed the design of this feature. 
>> 
>> Allow the intended nullability to be expressed on all pointers: Pointers are used throughout library interfaces, and the nullability of those pointers is an important part of the API contract with users. It’s too simplistic to only allow function parameters to have nullability, for example, because it’s also important information for data members, pointers-to-pointers (e.g., "a nonnull pointer to a nullable pointer to an integer”), arrays of pointers, etc.
>> Enable better tools support for detecting nullability problems: The nullability annotations should be useful for tools (especially the static analyzer) that can reason about the use of null, to give warnings about both missed null checks (the result of strchr could be null…) as well as for unnecessarily-defensive code.
>> Support workflows where all interfaces provide nullability annotations: In moving from a world where there are no nullability annotations to one where we hope to see many such annotations, we’ve found it helpful to move header-by-header, auditing a complete header to give it nullability qualifiers. Once one has done that, additions to the header need to be held to the same standard, so we need a design that allows us to warn about pointers that don’t provide nullability annotations for some declarations in a header that already has some nullability annotations.
>> Zero effect on ABI or code generation: There are a huge number of interfaces that could benefit from the use of nullability qualifiers, but we won’t get widespread adoption if introducing the nullability qualifiers means breaking existing code, either in the ABI (say, because nullability qualifiers are mangled into the type) or at execution time (e.g., because a non-null pointer ends up being null along some error path and causes undefined behavior).
>> A sanitizer for this feature would seem very useful, but this bullet point suggests that such a sanitizer would violate the model. Likewise, I don't see why we should rule out the option of optimizing on the basis of these qualifiers (under a -fstrict-nonnull flag or similar).
> 
> I agree that a sanitizer would be useful as a debugging aid. My primary concern here is that optimizing based on this information not be a part of any normal optimization flag (-O2, -Os, whatever), because it will hamper widespread adoption of this feature if adding the annotations to indicate the API contract suddenly starts breaking existing clients by, e.g., optimizing out existing, defensive null checks.
> 
>>  
>> Why not __attribute__((nonnull))?
>> Clang already has an attribute to express nullability, “nonnull”, which we inherited from GCC [4]. The “nonnull” attribute can be placed on functions to indicate which parameters cannot be null: one either specifies the indices of the arguments that cannot be null, e.g.,
>> 	extern void *my_memcpy (void *dest, const void *src, size_t len) __attribute__((nonnull (1, 2)));
>> or omits the list of indices to state that all pointer arguments cannot be null, e.g.,
>> 	extern void *my_memcpy (void *dest, const void *src, size_t len) __attribute__((nonnull));
>> More recently, “nonnull”  has grown the ability to be applied to parameters, and one can use the companion attribute returns_nonnull to state that a function returns a non-null pointer:
>> 	extern void *my_memcpy (__attribute__((nonnull)) void *dest, __attribute__((nonnull)) const void *src, size_t len) __attribute__((returns_nonnull));
>> There are a number of problems here. First, there are different attributes to express the same idea at different places in the grammar, and the use of the “nonnull” attribute on the function actually has an effect on the function parameters can get very, very confusing. Quick, which pointers are nullable vs. non-null in this example?
>> 
>> 	__attribute__((nonnull)) void *my_realloc (void *ptr, size_t size);
>> 
>> According to that declaration, ptr is nonnull and the function returns a nullable pointer… but that’s the opposite of how it reads (and behaves, if this is anything like a realloc that cannot fail). Moreover, because these two attributes are declaration attributes, not type attributes, you cannot express that nullability of the inner pointer in a multi-level pointer or an array of pointers, which makes these attributes verbose, confusing, and not sufficiently generally. These attributes fail the first of our goals.
>> 
>> These attributes aren’t as useful as they could be for tools support (the second and third goals), because they only express the nonnull case, leaving no way to distinguish between the unannotated case (nobody has documented the nullability of some parameter) and the nullable case (we know the pointer can be null). From a tooling perspective, this is a killer: the static analyzer absolutely cannot warn that one has forgotten to check for null for every unannotated pointer, because the false-positive rate would be astronomical.
>> 
>> Finally, we’ve recently started considering violations of the __attribute__((nonnull)) contract to be undefined behavior, which fails the last of our goals. This is something we could debate further if it were the only problem, but these declaration attributes fall all of our criteria, so it’s not worth discussing.
>> 
>> Nullability Qualifiers
>> We propose the addition of a new set of type qualifiers,  spelled __nullable, __nonnull, and __null_unspecified, to Clang. These are collectively known as nullability qualifiers and may be written anywhere any other type qualifier may be written (such as const) on any type subject to the following restrictions:
>> 
>> Two nullability qualifiers shall not appear in the same set of qualifiers.
>> A nullability qualifier shall qualify any pointer type, including pointers to objects, pointers to functions, C++ pointers to members, block pointers, and Objective-C object pointers.
>> A nullability qualifier in the declaration-specifiers applies to the innermost pointer type of each declarator (e.g., __nonnull int * is equivalent to int * __nonnull).
>> What happens if there's a mixture of different kinds of declarator? (Can I have '__nonnull int (*p)[3]'? Can I have '__nonnull int *p[3];'?)
>> 
>> I think you're saying that this decision is made based on the syntax of the declarator and not based on the underlying type, right? (So in
>> 
>>   __nonnull T *
>> 
>> the __nonnull appertains to the *, even if T names a pointer type.) 
> 
> I’ve said it poorly. It is based on the type, so
> 
> 	__nonnull T *
> 
> applies __nonnull to T unless T is a known type that is not a pointer type. In the patch Type::canHaveNullability() computes the operation, and essentially we apply the __nonnull to T when:
> 	- T is of pointer, C++ member pointer, block pointer, or Objective-C object pointer
> 	- T is a dependent type that could instantiate to some kind of pointer type
> 
> That seems like it could be very confusing:
> 
>   void f(__nonnull int *p);
> 
>   template<typename Integral>
>   void f(__nonnull Integral *p);
> 
> ... would apply the __nonnull to different components of the type (and I don't even want to think about what happens when T is a member of the current instantiation). The outcome seems to be that people writing templates need to know about both ways of writing this, and they need to know the gotchas and the minutiae of the rules, and they need to be able to reason with precision about which types are dependent.
> 
> This is a problem in C too. Consider:
> 
>   #include <some_library.h> // vends an opaque_t typedef
>   void use_library(__nonnull opaque_t *handle);
> 
> We cannot know what this program means without depending on an implementation detail of some_library.h. And this is not an exotic problem; consider, for instance, some_library == stdio and opaque_t == FILE.

So, we're going to remove the ability for a nullability qualifier written in the decl-specifiers to move to a pointer/block/member pointer declarator. To aid our own transition, I'll commit it as a warning (in its own, unique warning group) set to DefaultError that has a proper Fix-It, and once the transition is complete, we'll change it to an error.

Thanks for all the helpful feedback!

	- Doug

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