Unraveling the Nuances: A Deep Dive into Stack Behavior with clone()

We at revWhiteShadow frequently delve into the intricate workings of system programming, particularly concerning process and thread management within Linux. Our exploration today addresses a pertinent question regarding the behavior of memory stacks when utilizing the clone() system call, a powerful tool for creating new processes or threads with fine-grained control over shared resources. The core of the inquiry revolves around the lifecycle of the stack memory allocated for a clone()d child, especially in scenarios where this memory is managed by the parent process after the clone() call.

We understand the challenge: you require a method to collect data from disparate namespaces, but the direct application of setns() is precluded by the multithreaded nature of your primary application. Your subsequent approach involved fork() to isolate the namespace operations within a single-threaded child, subsequently communicating the collected data back to the parent via a pipe. However, you pivoted to clone() to mitigate the substantial default stack size (typically 8MB) typically associated with traditional fork()ed processes, aiming for a more memory-efficient solution with a custom, smaller stack. Your current implementation appears functional, but a fundamental question regarding the lifetime of this custom stack memory persists.

Let us meticulously examine the provided code snippet and the underlying assumptions:

const int stack_size = 65536;
void * stack = malloc(stack_size);
clone(my_func, stack + stack_size, CLONE_FILES);
free(stack);

Your core hypothesis is that upon calling clone(), the child process inherits the complete virtual memory space of the parent. Furthermore, you surmise that the stack memory is effectively “copied” or “materialized” for the child only upon its first access (a concept akin to copy-on-write for memory segments). This would logically imply that freeing the stack pointer in the parent process after the clone() call should not negatively impact the child’s ability to utilize its own stack.

Understanding clone() and Memory Inheritance

At its heart, clone() is a versatile system call that allows for the creation of new processes or threads. The key differentiator of clone() from fork() lies in its ability to selectively share resources between the parent and the child. The flags passed to clone() dictate precisely what is duplicated and what is shared.

When clone() is invoked without the CLONE_VM flag, the child process receives a copy of the parent’s memory space. This is a crucial distinction. It doesn’t mean the entire memory is physically copied at the moment of the clone() call. Instead, the operating system typically employs techniques like copy-on-write (COW).

Copy-on-Write (COW) Explained

With COW, the page tables of the child process are initialized to point to the same physical memory pages as the parent. The pages are marked as read-only for both processes. Only when either the parent or the child attempts to write to a shared page does the operating system intercept this operation. A page fault occurs, and the OS then creates a private copy of that specific page for the process that attempted the write. This copy is then updated to be writable, and the process can proceed without affecting the original page held by the other process.

Implications for the Stack

Your intuition about the stack being “touched” and subsequently copied is largely accurate in the context of COW. The stack, being a region of memory that is actively written to by a running function (e.g., pushing arguments, local variables, return addresses), will indeed trigger COW behavior if it’s shared. However, the initial allocation of the stack memory in the parent for the clone() call is an explicit act by the parent.

When you malloc memory for the stack and then pass a pointer to the beginning of this memory region (adjusted to point to the top of the stack for clone()’s convention) to clone(), you are essentially designating that memory area for the child’s stack.

The CLONE_FILES Flag

The CLONE_FILES flag you are using instructs clone() to share the parent’s file descriptor table. This means that file descriptors opened by the parent remain accessible to the child, and vice versa. This flag is critical for resource sharing but is distinct from memory management.

Analyzing the free(stack) Operation

Let’s return to your code:

void * stack = malloc(stack_size);
clone(my_func, stack + stack_size, CLONE_FILES);
free(stack);

Your understanding that freeing stack in the parent immediately after calling clone() should be safe is generally correct under the standard behavior of clone() without CLONE_VM. Here’s why:

  1. Explicit Allocation: You explicitly allocated the memory for the stack using malloc in the parent. This memory belongs to the parent’s address space.
  2. No CLONE_VM: Since you are not using CLONE_VM, the child process receives a copy of the parent’s memory mappings, but it’s not a direct sharing of the same physical pages for all memory segments from the outset. The stack memory you provided is a user-allocated buffer.
  3. Child’s Stack Initialization: When clone() is called with a user-provided stack, the operating system uses this memory region for the child’s initial stack. The child’s initial execution context (registers, stack pointer) is set up to point to the top of this region.
  4. COW on Stack Access: When my_func begins execution in the child, it will start pushing data onto its stack. If the underlying memory pages holding your stack buffer are not yet fully materialized or COW-protected for the child, the first write operations will trigger COW. A private copy of the necessary stack pages will be made for the child.
  5. Parent’s free(): When you call free(stack) in the parent, you are releasing the memory managed by the parent’s malloc allocator. This deallocates the memory from the parent’s perspective. Crucially, this operation does not invalidate the physical memory pages that may have been copied to the child via COW. The child continues to operate on its own distinct copies of any stack pages it has accessed.

Therefore, your understanding that freeing the memory after clone() is valid is correct because the child process, due to COW, will have its own independent copies of the stack pages it actively uses, regardless of the parent’s subsequent management of the original malloc’d buffer.

The Curious Case of CLONE_VM

Now, let’s address the more intriguing scenario:

clone(my_func, stack + stack_size, CLONE_FILES | CLONE_VM);

This is where your observation becomes particularly insightful. The CLONE_VM flag is a powerful, and often less commonly used, option. It instructs clone() to share the entire virtual memory space between the parent and the child. This means that the memory mappings, page tables, and the actual physical memory pages are identical for both processes. There is no copy-on-write for memory segments when CLONE_VM is active; they are truly sharing the same underlying memory.

Implications of Shared Memory with CLONE_VM

When you share the entire virtual memory space, any modification or deallocation of memory in one process is immediately visible and impactful to the other. This is why your expectation of a crash after free(stack) in the parent when CLONE_VM is used is a valid one.

If the parent frees the stack memory, and the child is actively using that same memory region as its stack, the child will encounter issues. This is because the memory manager in the parent has essentially marked that region as no longer allocated to the parent. If the child attempts to access it, it might face segmentation faults or other memory access errors.

Your Hypothesis and the Nuance

Your suspicion that “when I call free, it’s only freed by the internal allocator but the memory is still mapped to my process and thus using that memory is still valid” touches upon a subtle but important point.

When you call free(stack) in the parent, you are indeed telling the parent’s C library’s memory allocator (e.g., ptmalloc) that the memory block pointed to by stack is now available for reallocation. The allocator might return this memory to the operating system’s heap management or mark it internally as free.

However, the core issue with CLONE_VM is not just about the allocator’s internal bookkeeping. It’s about the memory mapping and the validity of the physical pages. If the child’s stack pointer is pointing into the stack buffer, and the parent calls free() on that buffer, the underlying physical memory pages could become invalid or be reused by the system if not handled carefully.

The fact that your program might still be working correctly even with CLONE_VM and the subsequent free(stack) in the parent suggests a few possibilities, or a potential race condition:

  1. Stack Usage Pattern: The child’s execution of my_func might not be writing to the exact memory locations that the parent’s free() operation invalidates before the child has completed its crucial stack operations. This is a risky assumption.
  2. Timing and Race Conditions: The free() call in the parent might be happening after the child has already completed its initial stack setup and perhaps has already triggered COW on the necessary stack pages (if COW is still somehow involved at a lower level for the initial stack pointer setup, which is unlikely with CLONE_VM). However, with CLONE_VM, the sharing is so pervasive that it’s unlikely COW would be a mitigating factor.
  3. Allocator Behavior: The specific behavior of the malloc/free implementation and the underlying memory management by the kernel for that specific memory region might not immediately unmap the memory from the process’s address space upon free(). The memory might remain mapped but marked as unallocated by the allocator. If the child still holds valid page table entries pointing to these pages and the kernel hasn’t yet reclaimed or remapped them, it might appear to work. This is highly dependent on the exact kernel version and the precise sequence of events.

However, relying on such behavior is extremely precarious. The contract of CLONE_VM is that memory is shared. Freeing memory in the parent should logically invalidate it for the child as well, because they are operating on the same memory.

Why CLONE_VM Might Seem to Work (and why it’s dangerous)

Your specific code, void * stack = malloc(stack_size); clone(my_func, stack + stack_size, CLONE_FILES | CLONE_VM); free(stack);, is fascinating because of the CLONE_VM.

If the child process is created with CLONE_VM, it shares the exact same memory pages as the parent. When you call free(stack) in the parent, you are returning the memory block pointed to by stack to the parent’s memory manager. The parent’s memory manager now considers this memory “free” and available for reuse.

If the child process’s stack pointer (rsp on x86_64) is within the range of memory that was just freed by the parent, and the parent’s memory manager has indeed deallocated or marked those pages as unallocated, then any subsequent attempt by the child to write to its stack (which is this very same memory) will likely result in a segmentation fault.

The fact that it might appear to work suggests that the child process, upon being created with CLONE_VM, still has valid pointers to the memory region. Even if the parent’s malloc implementation has marked the block as free, the underlying physical pages might not be immediately unmapped by the kernel. The kernel’s page tables for the child still point to those pages.

However, the moment the child attempts to use the stack in a way that requires the parent’s memory manager to be aware of its “free” status (e.g., if the child’s stack allocation within that buffer triggers some implicit memory management or if the parent’s free implementation involves interactions with the kernel that affect the mapping), you could see errors.

Consider this: free() in C doesn’t always instantaneously tell the kernel to unmap memory. It tells the allocator that the memory is available. The allocator might coalesce it with other free blocks, and only when the system needs memory might it truly unmap pages. If the child writes to a part of the stack that the parent’s allocator has “freed” but the kernel hasn’t yet unmapped, it could appear to work. This is a very fragile state and depends heavily on the exact timing and the specifics of the memory allocator and kernel behavior.

It is crucial to understand that using CLONE_VM means the child is sharing memory. Any operation that modifies or deallocates memory in the parent is reflected in the child.

The Correct Approach with CLONE_VM

If you intend to use CLONE_VM and want to provide a custom stack, you should not free the memory in the parent after clone(). The memory you allocate for the stack in the parent is now also part of the child’s shared virtual memory space. If the child needs that memory for its stack, and the parent deallocates it, that memory region becomes invalid for both.

If you need a smaller stack with CLONE_VM and want to manage its lifetime explicitly, you would typically:

  1. Allocate the stack in the parent.
  2. Call clone() with CLONE_VM and the custom stack.
  3. Do not free() the stack memory in the parent while the child process is still expected to be alive and potentially using that stack.
  4. The memory for the stack will be reclaimed by the system when the child process terminates and its resources are cleaned up, or when the parent process terminates.

Why Your Original Approach (Without CLONE_VM) is Safer

Your initial strategy of using clone() without CLONE_VM and then free()ing the allocated stack memory in the parent is generally the more robust and predictable approach when you want to manage the custom stack’s lifecycle independently of the parent’s main memory. The COW mechanism ensures that the child gets its own independent copy of the stack pages it needs, so the parent’s deallocation of the original buffer doesn’t corrupt the child’s working stack.

Alternative Stack Management Strategies

Given the complexities, let’s briefly consider alternative perspectives and best practices:

Using CLONE_CHILD_CLEARTID and CLONE_CHILD_SETTID

If your goal is to manage the child’s stack and ensure proper cleanup, you might consider flags like CLONE_CHILD_CLEARTID and CLONE_CHILD_SETTID. These are often used in conjunction with CLONE_THREAD to manage thread cleanup. While not directly related to stack deallocation, they highlight the kernel’s mechanisms for resource management in child processes.

The memfd_create and mmap Approach

For more advanced scenarios, or if you’re concerned about the exact lifecycle of memory regions, you could explore using memfd_create to create an anonymous file in memory, mmap it, and then pass the file descriptor to the child. This provides a more granular way to manage memory pages. However, for simply providing a custom stack, malloc followed by clone (without CLONE_VM) is the standard and most straightforward method.

The unshare() System Call

While setns() is not allowed due to multithreading, unshare() can be used to create a new namespace for the calling process or a child process without necessarily forking. However, this also comes with its own set of complexities, especially regarding thread safety and how it interacts with existing namespaces.

Summary and Best Practice Recommendation

Let’s consolidate our findings:

  • clone() without CLONE_VM: When you malloc a stack, pass it to clone() without CLONE_VM, and then free() the original malloc’d memory in the parent, this is generally safe. The child utilizes copy-on-write for its stack, creating independent copies of the memory pages it accesses. The parent’s free() operation deallocates the original buffer, which does not affect the child’s COW’d pages.
  • clone() with CLONE_VM: When CLONE_VM is used, the parent and child share the exact same virtual memory space. Any free() operation in the parent that deallocates memory used by the child’s stack will likely lead to segmentation faults or undefined behavior in the child. Your observation that it “works” might be due to timing or specific allocator behaviors, but it is an unreliable and dangerous practice.

Recommendation: For your use case, where you need to provide a custom-sized stack to a clone()d child and then reclaim that memory in the parent, stick to the approach without the CLONE_VM flag. This leverages COW effectively and allows for safe memory management of the allocated stack buffer in the parent.

The behavior you’ve observed with CLONE_VM and free() is a testament to the intricate, and sometimes surprising, ways memory management can operate at the system level. However, for predictable and robust code, adhering to the fundamental principles of shared versus copied memory is paramount. Your initial understanding of the non-CLONE_VM case was sound, and it remains the recommended path for managing custom stacks with clone() when the parent needs to reclaim the memory.