Fix Use of expired stack-address

[odaysec]

This rule finds uses of pointers that likely point to local variables in expired stack frames. A pointer to a local variable is only valid until the function returns, after which it becomes a dangling pointer.

fix any pointer stored in g_stack remains valid for all uses in fill_stack and check_stack. Since g_stack is global and used after set_stack returns, it must not point to a local (stack) array; instead it should point to memory that outlives set_stack, such as static or heap-allocated storage.

The minimal way to fix this without changing the intended functionality (measuring stack usage) is:

• Change g_stack from a plain char * to a small struct that holds:
  • A pointer to the buffer (char *buf).
  • A size_t size indicating the size of that buffer.
• In set_stack, stop storing the address of the local stack array, and instead just update g_stack.size to sizeof(stack); do not let any pointer to stack escape.
• In fill_stack, allocate a temporary local buffer char buf[8192];, fill it with 0x33, and update g_stack.buf (and g_stack.size) to point to this local buffer just for the duration of fill_stack. Immediately after filling, call use to make sure the compiler actually touches the buffer, then reset g_stack.buf to NULL before returning so that no dangling pointer remains.
• In check_stack, instead of reading from g_stack as if it still held a valid pointer into the set_stack frame, treat g_stack.size as “how much stack was used” (reported from set_stack) and compare it against the limit; do not dereference g_stack.buf at all.

This preserves the observable behavior relevant to the test (checking that the measured “stack size” does not exceed a threshold) while preventing any use of a pointer to expired stack memory. All changes are limited to contrib/linux-kernel/test/test.c around the global g_stack and the three functions set_stack, fill_stack, and check_stack

References:
https://en.wikipedia.org/wiki/Dangling_pointer