unix: Getwd: ensure returned path is absolute

Since Linux kernel 2.6.36, the pathname returned by the getcwd() system
call can be prefixed with the string "(unreachable)" in some cases [1].

Getcwd should return an absolute path, and doing otherwise is a
conformance issue; it also can be dangerous, since the path returned
can be an existing relative path.

Fix by returning ENOENT in case the path is not absolute. This is
essentially the same as what glibc does (since [2]).

[1] https://man7.org/linux/man-pages/man2/getcwd.2.html#BUGS
[2] https://sourceware.org/git/?p=glibc.git;a=commit;h=52a713fdd0a30e1bd79818e2e3c4ab44ddca1a94

Change-Id: I6c48e7ec9b53be348138e3d974d3f11401c7bea6
Reviewed-on: https://go-review.googlesource.com/c/sys/+/387194
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
Trust: Tobias Klauser <tobias.klauser@gmail.com>

unix/syscall_linux.go

@@ -250,6 +250,13 @@ func Getwd() (wd string, err error) {
 	if n < 1 || n > len(buf) || buf[n-1] != 0 {
 		return "", EINVAL
 	}
+	// In some cases, Linux can return a path that starts with the
+	// "(unreachable)" prefix, which can potentially be a valid relative
+	// path. To work around that, return ENOENT if path is not absolute.
+	if buf[0] != '/' {
+		return "", ENOENT
+	}
+
 	return string(buf[0 : n-1]), nil
 }