Mkdir_all: Handling Dangling Symlinks - A Deep Dive

Hey everyone! Let's dive into a fascinating discussion about how mkdir_all should handle dangling symlinks. This topic has come up in the context of the cyphar and libpathrs projects, and it touches on some pretty intricate aspects of file system operations. So, grab your favorite beverage, and let's explore the challenges and potential solutions!

Understanding the Dilemma of Dangling Symlinks

First off, what are we even talking about? A dangling symlink is a symbolic link that points to a target that doesn't exist. In most file system operations, these are treated as errors. Think of it like a broken signpost – it points somewhere, but there's nothing there. Typically, operations reject dangling symlinks, treating them much like any other missing parent directory (ENOENT), a regular ENOENT error, or even an EEXIST error in the case of mknod.

However, Root::mkdir_all, which is used to create a directory and all its parent directories as needed, presents a unique challenge. In principle, it could be designed to handle dangling symlinks differently. Instead of simply erroring out, it could potentially follow the dangling symlink's target and attempt to create the directories there, along with any trailing parts of the original path. This is where things get interesting, and a bit complex. The core question here is: Should mkdir_all attempt to resolve and create directories through dangling symlinks? It sounds simple, but there are significant trade-offs to consider, as well as some potential efficiency impacts that can quickly add up.

Concerns and Challenges of Supporting Dangling Symlinks

While the idea of mkdir_all intelligently handling dangling symlinks might seem appealing, there are several serious concerns that need to be addressed. These concerns range from implementation complexity to potential race conditions and confusing error messages. Let's break them down:

1. Nested Lookups and Symlink Stacks

Imagine a scenario where you have a chain of dangling symlinks, each pointing to another non-existent target. To handle this, mkdir_all would need to perform nested lookups, essentially chasing the chain of symlinks until it reaches a valid target or hits a limit. This is similar to the symlink stack emulation used in openat2, which is designed to handle complex symlink resolutions. If we decide to support dangling symlinks in mkdir_all, we'd likely need a similar mechanism. Implementing this adds significant complexity to the code, and the big question is whether it's worth the added overhead.

One of the biggest concerns is the impact on performance. Introducing nested lookups could make openat2 lookups less efficient, and that's a critical consideration. Furthermore, we'd need to set an iteration limit to prevent infinite loops in case of circular symlink chains. This means limiting the number of hops the function can take from one symlink to another. We also have to be extra careful when piecing together the trailing bits of the original path after resolving symlinks to correctly get return values from resolve_partial, which can get confusing quickly.

2. Race Conditions

Another significant concern is the possibility of race conditions. In the context of file systems, a race condition occurs when the outcome of an operation depends on the unpredictable timing of events. Specifically, there's a risk that a symlink could be swapped or modified after openat2 fails to resolve it initially. While this might seem like a rare scenario, it could lead to unexpected behavior and potentially introduce security vulnerabilities. Imagine the symlink being changed to point to somewhere sensitive after the check has been done, resulting in unintended access or creation of content in that location.

It's crucial to carefully model these potential risks and understand the implications before implementing support for dangling symlinks. We need to determine how likely these races are to occur, and what the worst-case scenarios might be. Although it is suspected that this is "fine", it still warrants the proper care and attention needed to model possible risks thoroughly.

3. Lookups from Non-Root Directories

Historically, the lookups we've performed have always started from the root directory. This simplifies the logic and ensures consistency. However, when dealing with dangling symlinks, we might need to start a lookup from a non-root directory, specifically the target of the dangling symlink. This poses a challenge for existing resolver mechanisms, particularly openat2.

For openat2, this will likely require kernel patches to allow specifying a separate current working directory (cwd) from the root directory. This is especially relevant for the O_PATH resolver, where the symlink stack might need to be dropped. The implications of this change are significant, as it affects the fundamental way lookups are performed. This represents a major deviation from the existing architecture and will necessitate careful design considerations and kernel level integration.

4. Handling ".." in Non-Existent Portions

A particularly tricky case arises when a symlink contains the .. (parent directory) component in the non-existent portion of the path. For example, a symlink might point to path/to/../nonexistent_dir. In this scenario, mkdir_all would likely return an error, but the error message might be confusing to users. It might indicate an issue with the .. component, even though that component only exists within the dangling symlink's target path. This discrepancy between the user's mental model and the actual error message can lead to frustration and debugging difficulties. The error could lead to misleading feedback, suggesting an issue with parent directory traversals when the real problem lies in the symlink resolution logic.

Alternative Approaches and Considerations

Given these challenges, it's important to consider alternative approaches and weigh the trade-offs carefully. One option is to simply reject dangling symlinks outright, as is the current behavior. This approach is straightforward and avoids the complexities of nested lookups, race conditions, and non-root directory lookups. However, it might limit the flexibility of mkdir_all in certain scenarios.

Another approach could involve a more limited form of dangling symlink support. For instance, mkdir_all could be configured to follow only a single level of dangling symlink, or only to follow dangling symlinks that point to absolute paths. These restrictions could mitigate some of the risks and complexities while still providing some added functionality. The exact choice depends on specific use cases, but the core tradeoff lies in balancing usability and complexity.

Balancing Usability and Complexity

The crux of this discussion comes down to striking the right balance between usability and complexity. Adding support for dangling symlinks in mkdir_all could make the function more versatile and user-friendly in certain situations. However, it also introduces significant complexity, potential performance overhead, and the risk of subtle bugs and race conditions. Any change must justify the extra effort required for implementation and future maintenance.

It's crucial to carefully consider the common use cases for mkdir_all and whether the added complexity of handling dangling symlinks is justified by the potential benefits. In situations where the presence of dangling symlinks is rare, or where they can be easily avoided, the added complexity might not be worth the effort. Conversely, if there are specific scenarios where handling dangling symlinks would significantly simplify workflows or improve usability, then it might be worth exploring the more complex implementation.

Conclusion: A Path Forward

In conclusion, the question of whether mkdir_all should support dangling symlinks is a nuanced one, with no easy answer. While there are potential benefits to be gained, there are also significant challenges and risks to consider. The decision ultimately depends on the specific requirements and priorities of the projects that use mkdir_all, namely cyphar and libpathrs.

Before making a final decision, it's crucial to thoroughly evaluate the trade-offs, consider alternative approaches, and carefully model the potential risks. We need to think about the long-term implications of each approach, not just in terms of implementation complexity, but also in terms of performance, security, and maintainability. By carefully weighing these factors, we can arrive at a solution that best serves the needs of the community. The ongoing discussion ensures that all angles are considered, resulting in a solution that meets user requirements without introducing undue complexity or risk.

What do you guys think? Should mkdir_all support dangling symlinks? Let's continue the discussion in the comments below!