Standard library masks away bits in permissions struct

[tmerr]

Behavior

On linux, mkdir mydir, chmod 2777 mydir then run:

fn main() {
  let path = "mydir";
  let perms = std::fs::metadata(path).unwrap().permissions();
  std::fs::set_permissions(path, perms).unwrap();
}

This changes the permissions of mydir to 777. Is this expected?

The cause

Metadata.permissions() masks away everything other than the lower 3 triads when it calls out to this code:

rust/src/libstd/sys/unix/fs.rs

Lines 95 to 97 in d2d5069

	pub fn perm(&self) -> FilePermissions {
	FilePermissions { mode: (self.stat.st_mode as mode_t) & 0o777 }
	}

Then std::fs::set_permissions sends these mode bits into into chmod which considers four triads, not three. Possibly useful references: chmod and mode_t

Should we change Metadata.permissions to preserve that triad, maybe by masking with 0o7777 instead of 0o777?

[tmerr]

I guess the solution depends on whether Unix's std::fs::Permissions is supposed to care about 9 bits or 12 (this is not an implementation detail since it's exposed to the programmer via Permissions.mode() of PermissionsExt). @alexcrichton any thoughts on this?

[sfackler]

It seems to me like we should extend the mask to the full 12 bits.

[alexcrichton]

Seems reasonable to me to avoid masking where it makes sense!