chown+chgrp+chmod: Fix handling of preserve root flag and error messages

[BenWiederhake]

This makes the GNU preserve-root.log test green.

In particular:

• We used to have the wrong logic to detect whether or not to issue a warning. The root (punt not intended) cause was that Path::is_dir() completely ignores whether a given argument has a trailing slash or not, but that is the criterion that GNU (and many other tools) apparently uses to decide whether the path points "at" or "into" a directory. (See below for demonstration.)
• chown and chgrp also didn't correctly output an indication why the warning was being given
• And finally, chmod simply forgot to output its own name in one place.

Additional notes:

• This makes the uucore function resolve_relative_path superfluous. This function is by necessity ill-defined: Depending on the context, .. is either the logical parent directory, sometimes the physical parent directory. This function can only work for the latter case, in which case Path::canonicalize is often a better approach.
• While rewriting, I noticed and documented an unavoidable TOCTOU bug. (If root runs chown -R -L and the attacker can quickly inject a directory and replace it with a symlink to /, then --preserve-root does not trigger. However, in this scenario the attacker could probably just as well create a handful of symlinks, one for each directory in /, for the same effect. So this is not a vulnerability, just an unavoidable bug.)

Path::is_dir demo

fn main() {
    for name in "/tmp /tmp/ /xxx /xxx/".split(" ") {
        let buf: &std::path::Path = name.as_ref();
        println!("{:?}.is_dir() = {}", buf.as_os_str(), buf.is_dir());
    }
}

produces

"/tmp".is_dir() = true
"/tmp/".is_dir() = true
"/xxx".is_dir() = false
"/xxx/".is_dir() = false

[github-actions]

GNU testsuite comparison:

Congrats! The gnu test tests/chown/preserve-root is no longer failing!
Congrats! The gnu test tests/mv/backup-dir is no longer failing!

[BenWiederhake]

Changes since last push:

• Uglified the code away from OsStr::ends_with(), because that is Rust 1.74.0, and MSRV is Rust 1.70.0.
• I don't understand why mv/backup-dir is doing better now. I touched none of these tools O.o

[github-actions]

GNU testsuite comparison:

Congrats! The gnu test tests/chown/preserve-root is no longer failing!
Congrats! The gnu test tests/mv/backup-dir is no longer failing!
Skipping an intermittent issue tests/tail/inotify-dir-recreate (passes in this run but fails in the 'main' branch)

[tertsdiepraam]

Alright, please check that I understand correctly. Here's what I got from this:

We need to know whether symlinks will be dereferenced on the path to know how to check for root. Dereferencing happens if either the setting for dereferencing is passed or if the / is at the end, so that we need to enter the directory and to do that need to dereference.

Does that make sense? If so, could you add that last part in a comment somewhere. It took me a while to understand :)

I also wonder if these path checks are the right way to do it. Maybe there's some other way to check this? It all feels inherently finnicky.

[BenWiederhake]

Changes since last push:

• rebased on current main
• Converted the big introduction in fn is_root to a docstring
• In it, mentioned that this is specific to chown and chgrp
• Simplified the to_str/match/print handling in case the check triggers
• Renamed the argument to would_traverse_symlink. All as suggested by @tertsdiepraam

[BenWiederhake]

Nope, wait, I got something wrong with the quoting.

[BenWiederhake]

Changes since last push:

• Fixed the double-quoting issue (I used "blah '{}' blah", pathbuf.quote(), which results in the quotes appearing twice. Doink!)

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/tail/inotify-dir-recreate (fails in this run but passes in the 'main' branch)
Congrats! The gnu test tests/chown/preserve-root is no longer failing!

[sylvestre]

well done :)