Figure out final solution for volume-relative windows paths

[lilyball]

As extensively documented in #11579, volume-relative paths on Windows are a relic of the past. We either need to remove support for it, or we need to figure out how ls, etc running in cmd.exe are emulating volume-relative cwds and teach getcwd() about that.

If we remove support for volume-relative paths, we should probably not make it a condition, because that's pretty crappy. We may want to just treat it as a path without a prefix.

[lilyball]

Volume-relative paths make a lot of things more complicated. And notably, it means os::make_absolute(foo).is_absolute() can no longer be guaranteed to return true.

[mitsuhiko]

I want to bump this a bit because I think this is security related.

Currently the WindowsPath::is_absolute() behavior is downright dangerous. The generic path behavior for is_absolute() defines this as such:

Returns whether self represents an absolute path. An absolute path is defined as one that, when joined to another path, will yield back the same absolute path.

Different working directories on different drives is a concept long gone. The only software that still has this is cmd.exe and it emulates it. (http://blogs.msdn.com/b/oldnewthing/archive/2010/10/11/10073890.aspx)

The reason the current behavior is dangerous because you can end up trusting is_absolute() to the current behavior to implement path manipulation algorithms where you expect the end result to not escape a certain folder when used in a certain context. (Obviously that by itself does not help you but you might make wrong conclusions from it).

[steveklabnik]

@aturon @alexcrichton how did this fall out with path reform?

[retep998]

This issue isn't done yet, because we still need to implement a way to turn volume relative paths into true absolute paths. This means getting the current working directory for that drive and joining the paths.

While cmd.exe has a variable current directory for each drive, Win32 has only a single variable current directory. Unfortunately Win32 still has immutable current directories set at process startup for each drive which it respects when interpreting volume relative paths.

At first I thought we were doomed because GetCurrentDirectory only returns that single current directory and not the drive specific current directories. Furthermore if we don't want to be limited by MAX_PATH we can't call GetFullPathName with the full path. As it turns out, however, we can simply call GetFullPathName with just the prefix like C: and it'll give us the correct current directory on which to join the rest of the path.

[aturon]

@retep998 Would you be interested in taking this project on?

[retep998]

@aturon Sure. Just point me at what functions need doing and I'll do them.

[huonw]

Has anything happened here? (@aturon?)

[retep998]

There was a PR to turn relative paths and absolute paths into verbatim paths. #27916
There were lots of concerns in it regarding various edge cases.
I think what should happen at this point is someone should develop such a function in an outside crate first and then it can be pulled into std once people seem satisfied by how it handles various edge cases.

[aturon]

I agree with @retep998. I'm going to close this issue, and we can address it via an eventual addition to std as @retep998 is suggesting.