[llvm-branch-commits] [compiler-rt] [TySan] Fix struct access with different bases (PR #108385)

[Tavian Barnes via llvm-branch-commits]

tavianator wrote:

Here's the new testcase.  Not sure if this bug is related or not.  It has to do with `memcpy()`; if you replace the call with the commented-out line above it, it works.

```c
struct node {
	struct node *next;
};

struct list {
	struct node *head, **tail;
};

int main(void) {
	struct list *list = __builtin_malloc(sizeof(*list));
	list->head = 0;
	list->tail = &list->head;

	struct node *node = __builtin_malloc(sizeof(*node));
	node->next = 0;

	*list->tail = node;
	list->tail = &node->next;

	while (list->head) {
		struct node *node = list->head;
		// list->head = node->next;
		__builtin_memcpy(&list->head, &node->next, sizeof(list->head));
		node->next = 0;
	}

	return 0;
}
```

```console
tavianator at tachyon $ ~/code/llvm/llvm-project/build/bin/clang -Wall -g -fsanitize=type foo.c -o foo
tavianator at tachyon $ ./foo
==5885==ERROR: TypeSanitizer: type-aliasing-violation on address 0x55af02a8c2a0 (pc 0x55aef600fb36 bp 0x7ffcbf810cf0 sp 0x7ffcbf810c90 tid 5885)
READ of size 8 at 0x55af02a8c2a0 with type any pointer (in list at offset 0) accesses an existing object of type any pointer (in node at offset 0)
    #0 0x55aef600fb35 in main /home/tavianator/code/bfs/foo.c:20:15

```

https://github.com/llvm/llvm-project/pull/108385