Properly model the copy semantics of NSString.stringWithUTF8String and NSString.stringWithString.

Summary:
Both `stringWithUTF8String` and `stringWithString` implements copy semantics that copies the content of their parameter into a newly allocated buffer. We modeled this as pointer assignment in the past, which means that once we write
```
NSString* foo() {
  char buf[...];
  ...
  return [NSString stringWithUTF8String:buf];
}
```
We are going to get a spurious stack variable address escape report because local pointer `buf` is assigned to the newly created string and the string gets returned.
This diff tries to address the issue by heap-allocating a buffer and `memcpy` the contents in `stringWithUTF8String` and `stringWithString`. But this change will create another problem: the allocated buffer will be reported as leaked by the backend, while in reality those buffers won't actually be leaked as they are allocated in a region that will be periodically autoreleased. To suppress spurious memory leak FPs, I added another attribute `Awont_leak` that will suppress the leakage report on any expressions that get tagged with it.

Reviewed By: jeremydubreil

Differential Revision: D5403084

fbshipit-source-id: df6de7f

infer/models/objc/src/NSString.h

@@ -19,4 +19,5 @@
 + (instancetype)localizedStringWithFormat:(NSString*)format, ...;
 - (instancetype)initWithFormat:(NSString*)format, ...;
 - (instancetype)initWithFormat:(NSString*)format arguments:(va_list)argList;
+- (void)dealloc;
 @end

infer/models/objc/src/NSString.m

@@ -10,22 +10,33 @@
 #import "NSString.h"
 #import <stdlib.h>
 
-void __get_array_length(const UInt8);
+size_t __get_array_length(const void* arr);
 
 void __infer_assume(bool cond);
+void __set_wont_leak_attribute(const void* ptr);
 
 @implementation NSString
 
 + (instancetype)stringWithUTF8String:(const char*)bytes {
   // using alloc as the documentation doesn't say it may return nil
   NSString* s = [NSString alloc];
-  s->value = bytes;
+  int len;
+  len = __get_array_length(bytes);
+  s->value = (const char*)malloc(len);
+  // The newly allocated string will be autoreleased by the runtime
+  __set_wont_leak_attribute(s->value);
+  memcpy(s->value, bytes, len);
   return s;
 }
 
 + (instancetype)stringWithString:(NSString*)aString {
   NSString* s = [NSString alloc];
-  s->value = aString->value;
+  int len;
+  len = __get_array_length(aString->value);
+  s->value = (const char*)malloc(len);
+  // The newly allocated string will be autoreleased by the runtime
+  __set_wont_leak_attribute(s->value);
+  memcpy(s->value, aString->value, len);
   return s;
 }
 
@@ -70,4 +81,11 @@ void __infer_assume(bool cond);
   }
 }
 
+- (void)dealloc {
+  if (self != nil && self->value != 0) {
+    free(self->value);
+  }
+  [super dealloc];
+}
+
 @end

infer/src/IR/BUILTINS.mli

@@ -107,6 +107,8 @@ module type S = sig
 
   val __set_untaint_attribute : t
 
+  val __set_wont_leak_attribute : t
+
   val __split_get_nth : t
 
   val __throw : t

infer/src/IR/BuiltinDecl.ml

@@ -123,6 +123,8 @@ let __set_unsubscribed_observer_attribute = create_procname "__set_unsubscribed_
 
 let __set_untaint_attribute = create_procname "__set_untaint_attribute"
 
+let __set_wont_leak_attribute = create_procname "__set_wont_leak_attribute"
+
 let __split_get_nth = create_procname "__split_get_nth"
 
 let __throw = create_procname "__throw"

infer/src/IR/PredSymb.ml

@@ -121,6 +121,7 @@ type t =
   | Aobserver  (** denotes an object registered as an observers to a notification center *)
   | Aunsubscribed_observer
       (** denotes an object unsubscribed from observers of a notification center *)
+  | Awont_leak  (** value do not participate in memory leak analysis *)
   [@@deriving compare]
 
 let equal = [%compare.equal : t]
@@ -158,6 +159,7 @@ type category =
   | ACundef
   | ACretval
   | ACobserver
+  | ACwontleak
   [@@deriving compare]
 
 let equal_category = [%compare.equal : category]
@@ -182,9 +184,13 @@ let to_category att =
    -> ACundef
   | Aobserver | Aunsubscribed_observer
    -> ACobserver
+  | Awont_leak
+   -> ACwontleak
 
 let is_undef = function Aundef _ -> true | _ -> false
 
+let is_wont_leak = function Awont_leak -> true | _ -> false
+
 (** convert the attribute to a string *)
 let to_string pe = function
   | Aresource ra
@@ -254,6 +260,8 @@ let to_string pe = function
    -> "OBSERVER"
   | Aunsubscribed_observer
    -> "UNSUBSCRIBED_OBSERVER"
+  | Awont_leak
+   -> "WONT_LEAK"
 
 (** dump an attribute *)
 let d_attribute (a: t) = L.add_print_action (L.PTattribute, Obj.repr a)

infer/src/IR/PredSymb.mli

@@ -96,6 +96,7 @@ type t =
   | Aobserver  (** denotes an object registered as an observers to a notification center *)
   | Aunsubscribed_observer
       (** denotes an object unsubscribed from observers of a notification center *)
+  | Awont_leak  (** value do not participate in memory leak analysis *)
   [@@deriving compare]
 
 val equal : t -> t -> bool
@@ -117,6 +118,7 @@ type category =
   | ACundef
   | ACretval
   | ACobserver
+  | ACwontleak
   [@@deriving compare]
 
 val equal_category : category -> category -> bool
@@ -126,6 +128,8 @@ val to_category : t -> category
 
 val is_undef : t -> bool
 
+val is_wont_leak : t -> bool
+
 val to_string : Pp.env -> t -> string
 (** convert the attribute to a string *)
 

infer/src/backend/Attribute.ml

@@ -107,6 +107,8 @@ let get_observer tenv prop exp = get tenv prop exp ACobserver
 
 let get_retval tenv prop exp = get tenv prop exp ACretval
 
+let get_wontleak tenv prop exp = get tenv prop exp ACwontleak
+
 let has_dangling_uninit tenv prop exp =
   let la = get_for_exp tenv prop exp in
   List.exists

infer/src/backend/Attribute.mli

@@ -65,6 +65,9 @@ val get_taint : Tenv.t -> 'a Prop.t -> Exp.t -> Sil.atom option
 val get_undef : Tenv.t -> 'a Prop.t -> Exp.t -> Sil.atom option
 (** Get the undef attribute associated to the expression, if any *)
 
+val get_wontleak : Tenv.t -> 'a Prop.t -> Exp.t -> Sil.atom option
+(** Get the wontleak attribute associated to the expression, if any *)
+
 val has_dangling_uninit : Tenv.t -> 'a Prop.t -> Exp.t -> bool
 (** Test for existence of an Adangling DAuninit attribute associated to the exp *)
 

infer/src/backend/BuiltinDefn.ml

@@ -636,6 +636,10 @@ let execute___set_unlocked_attribute ({Builtin.pdesc; loc} as builtin_args) : Bu
   in
   execute___set_attr (PredSymb.Aresource ra) builtin_args
 
+(** Set the attibute of the value as wont leak*)
+let execute___set_wont_leak_attribute builtin_args : Builtin.ret_typ =
+  execute___set_attr PredSymb.Awont_leak builtin_args
+
 (** Set the attibute of the value as tainted *)
 let execute___set_taint_attribute {Builtin.tenv; pdesc; args; prop_; path} : Builtin.ret_typ =
   match args with
@@ -1189,6 +1193,9 @@ let __set_unsubscribed_observer_attribute =
 let __set_untaint_attribute =
   Builtin.register BuiltinDecl.__set_untaint_attribute execute___set_untaint_attribute
 
+let __set_wont_leak_attribute =
+  Builtin.register BuiltinDecl.__set_wont_leak_attribute execute___set_wont_leak_attribute
+
 (* splits a string given a separator and returns the nth string *)
 let __split_get_nth = Builtin.register BuiltinDecl.__split_get_nth execute___split_get_nth
 

infer/src/backend/abs.ml

@@ -1232,6 +1232,11 @@ let check_junk ?original_prop pname tenv prop =
               let res = ref None in
               let do_entry e =
                 check_observer_is_unsubscribed_deallocation tenv prop e ;
+                match Attribute.get_wontleak tenv prop e with
+                | Some Apred ((Awont_leak as a), _)
+                 -> L.d_strln "WONT_LEAK" ;
+                    res := Some a
+                | _ ->
                   match Attribute.get_resource tenv prop e with
                   | Some Apred ((Aresource {ra_kind= Racquire} as a), _)
                    -> L.d_str "ATTRIBUTE: " ;
@@ -1241,7 +1246,8 @@ let check_junk ?original_prop pname tenv prop =
                   | _ ->
                     match Attribute.get_undef tenv prop e with
                     | Some Apred ((Aundef _ as a), _)
-                   -> res := Some a
+                     -> L.d_strln "UNDEF" ;
+                        res := Some a
                     | _
                      -> ()
               in
@@ -1285,6 +1291,8 @@ let check_junk ?original_prop pname tenv prop =
             in
             let ignore_resource, exn =
               match (alloc_attribute, resource) with
+              | Some PredSymb.Awont_leak, Rmemory _
+               -> (true, exn_leak)
               | Some _, Rmemory Mobjc when hpred_in_cycle hpred
                -> (* When there is a cycle in objc we ignore it
                         only if it's empty or it has weak or unsafe_unretained fields.

infer/tests/codetoanalyze/objc/errors/issues.exp

@@ -91,7 +91,6 @@ codetoanalyze/objc/shared/memory_leaks_benchmark/TollBridgeExample.m, TollBridge
 codetoanalyze/objc/shared/memory_leaks_benchmark/TollBridgeExample.m, TollBridgeExample_bridge, 2, MEMORY_LEAK, [start of procedure bridge]
 codetoanalyze/objc/shared/memory_leaks_benchmark/TollBridgeExample.m, bridgeDictionaryNoLeak, 1, UNINITIALIZED_VALUE, [start of procedure bridgeDictionaryNoLeak()]
 codetoanalyze/objc/errors/memory_leaks_benchmark/NSData_models_tests.m, NSData_models_tests_macForIV:, 2, MEMORY_LEAK, [start of procedure macForIV:]
-codetoanalyze/objc/errors/memory_leaks_benchmark/NSString_models_tests.m, createURLQueryStringBodyEscaping, 6, PRECONDITION_NOT_MET, [start of procedure createURLQueryStringBodyEscaping(),Condition is true]
 codetoanalyze/objc/errors/memory_leaks_benchmark/NSString_models_tests.m, createURLQueryStringBodyEscaping, 11, UNINITIALIZED_VALUE, [start of procedure createURLQueryStringBodyEscaping(),Condition is false]
 codetoanalyze/objc/errors/memory_leaks_benchmark/RetainReleaseExampleBucketing.m, RetainReleaseTest, 0, RETURN_VALUE_IGNORED, [start of procedure RetainReleaseTest()]
 codetoanalyze/objc/errors/npe/Fraction.m, test_virtual_call, 7, NULL_DEREFERENCE, [start of procedure test_virtual_call(),start of procedure setNumerator:,return from a call to Fraction_setNumerator:,start of procedure getNumerator,return from a call to Fraction_getNumerator,Condition is true]

infer/tests/codetoanalyze/objc/errors/memory_leaks_benchmark/NSString_models_tests.m

@@ -30,6 +30,17 @@ NSString* createURLQueryStringBodyEscaping(NSDictionary* parameters,
   return resultString;
 }
 
+NSString* stringWithUTF8StringNoLeakOk(const char* buf) {
+  return [NSString stringWithUTF8String:buf];
+}
+NSString* stringWithStringNoLeakOk(const char* buf) {
+  return [NSString stringWithString:buf];
+}
+
+- (void)NSAssertNoLeakOk {
+  NSAssert(true, @"This string does not create any memory leak");
+}
+
 - (NSString*)hexStringValue {
   size_t hexLen = 2 * 10 * sizeof(char);
   char* outString = (char*)malloc(hexLen + 1);