Re: An overview of the analyzer support of the operator new

[David Malcolm <dmalcolm@redhat.com>]

On Wed, 2023-06-07 at 19:19 +0200, Benjamin Priour wrote:
> Hi,
> 
> I've been mapping where the analyzer is lacking support of the
> operator new
> different variants.
> I've written a bunch of test cases already to demonstrate it, you can
> find
> them below.
> They are not yet formatted for a patch submission, and as some of
> them may
> require new warnings, I didn't use dg-* directives either.

You can always mark the dg-directives with "xfail" and a comment if the
warning isn't implemented yet.

> You will notice I included true positives and negatives as well, as I
> think
> they might spur ideas on some edge cases that may fail.
> All that to say I would greatly appreciate your comments if any test
> is
> wrong, or if you have pointers on additional test cases.

Looks great.

Note that the results might be affected by exceptions; do any results
change for -fexceptions versus -fno-exceptions?

> You can also find a godbolt <https://godbolt.org/z/dxj87fxG7> here.
> 
> The most annoying one is the recurrent noisy false positive
> -Wanalyzer-possible-null-argument on usage of a new expression.
> Although a placement new on a static buffer too short is flagged by
> the
> middle-end, the analyzer stay quiet.
> A placement on a dynamic buffer too short to contain the placement is
> never
> reported however. See PR105948
> <https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105948>

Yeah; looks like that will need some extra code in the analyzer to
implement; you can ask the region_model what the capacity of the region
is; do you have access to the required size of the region at the
placement new call?  If so then the implementation should be very
similar as -Wanalyzer-out-of-bounds (or reuse it?)

Dave

> 
> Thanks,
> Benjamin
> 
> #include <new>
> 
> struct A
> {
> int x = 4;
> int y = 6;
> };
> 
> void test1()
> {
> int *x = ::new int; // true negative on -Wanalyzer-possible-null-
> argument
> int *arr = ::new int[3]; // true negative on
> -Wanalyzer-possible-null-argument
> A *a = ::new A(); // false positive -Wanalyzer-possible-null-argument
> (a
> throwing new cannot returns null)
> ::delete a;
> ::delete x;
> ::delete[] arr;
> }
> 
> void test_allocators_mismatch()
> {
> int *a = ::new int;
> int *b = ::new int[3];
> 
> ::delete[] a; /* true positive -Wanalyzer-mismatching-deallocation
> flagged
> */
> ::delete b; /* true positive -Wanalyzer-mismatching-deallocation
> flagged */
> }
> 
> // From clang core.uninitialized.NewArraySize
> void test_garbage_new_array()
> {
> int n;
> int *arr = ::new int[n]; /* true positive
> -Wanalyzer-use-of-uninitialized-value reported for 'n' */
> /* however nothing is reported for 'arr', even with
> '-fno-analyzer-suppress-followups', one could expect a specific
> warning */
> ::delete[] arr; /* no warnings here either */
> }
> 
> void test_placement()
> {
> void *chunk = ::operator new(20); // true negative
> -Wanalyzer-possible-null-dereference
> A *a = ::new (chunk) A();
> a->~A();
> ::operator delete(chunk);
> }
> 
> void test_delete_placement()
> {
> A *a = ::new A; // false positive -Wanalyzer-possible-null-argument
> (throwing new)
> int *z = ::new (&a->y) int;
> a->~A(); // deconstruct properly
> ::operator delete(a);
> ::operator delete(z); // nothing from analyzer but got
> -Wfree-nonheap-object, even though analyzer also has
> Wanalyzer-free-of-non-heap
> }
> 
> void test_write_placement_after_delete()
> {
> short *s = ::new short;
> long *lp = ::new (s) long;
> ::delete s;
> *lp = 12; // true positive -Wanalyzer-use-after-free flagged, as well
> as a
> wrong -Wanalyzer-null-dereference of lp
> }
> 
> void test_read_placement_after_delete()
> {
> short *s = ::new short;
> long *lp = ::new (s) long;
> ::delete s;
> long m = *lp; // true positive -Wanalyzer-use-after-free flagged, as
> well
> as a wrong -Wanalyzer-null-dereference of lp
> }
> 
> void test_use_placement_after_destruction()
> {
> A a;
> int *lp = ::new (&a.y) int;
> a.~A();
> int m = *lp; /* true positive -Wanalyzer-use-of-uninitialized-value,
> nothing about use-after-delete though */
> }
> 
> // From clang cplusplus.PlacementNewChecker
> void test_placement_size_static()
> {
> short s;
> long *lp = ::new (&s) long; /* nothing from analyzer, but still got
> -Wplacement-new= */
> }
> 
> void test_placement_size_dynamic()
> {
> short *s = ::new short;
> long *lp = ::new (s) long; // Nothing reported here at all, would
> expect a
> -Wanalyzer-placement-new=
> ::delete s;
> }
> 
> void test_placement_null()
> {
> int *x = nullptr;
> int *p = ::new (x) int; // Placement new on NULL is undefined, yet
> nothing
> is reported.
> ::operator delete(x);
> }
> 
> void test_initialization_through_placement()
> {
> int x;
> int *p = ::new (&x) int;
> *p = 10;
> int z = x + 2; // Everything is fine, no warning emitted
> }