<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/110385>110385</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            __builtin_dynamic_object_size() fails to return correct size depending on depth of flexible array
        </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
            new issue
      </td>
    </tr>

    <tr>
      <th>Assignees</th>
      <td>
      </td>
    </tr>

    <tr>
      <th>Reporter</th>
      <td>
          kees
      </td>
    </tr>
</table>

<pre>
    As seen in the Linux kernel:

- https://lore.kernel.org/all/202409170436.C3C6E7F7A@keescook/
- https://lore.kernel.org/all/CAGG=3QVWCQB-3sM=iwgTmX8zrU81H+F_A1icJwROvW_DSvsBeA@mail.gmail.com

we're having erroneous size reporting from `__builtin_dynamic_object_size()`, where the depth of dereference for the flexible array causes a 0 size report:

```
https://godbolt.org/z/qohGd5xh1


#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

struct variable {
        int a;
        int b;
        int length;
 short array[] __attribute__((counted_by(length)));
};

struct bucket {
        int a;
        struct variable *growable;
 int b;
};

int main(int argc, char *argv[])
{
        struct bucket *p;
        struct variable *v;

        p = malloc(sizeof(*p));
        v = malloc(sizeof(*p->growable) + sizeof(*p->growable->array) * 32);
        v->length = 32;


 printf("%zu\n", __builtin_dynamic_object_size(v->array, 1));

 p->growable = v;
        printf("%zu\n", __builtin_dynamic_object_size(p->growable->array, 1));

        return 0;
}


GCC shows 64 64, but Clang shows 64 0.
```

cc @isanbard @bwendling 
</pre>
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