windows: fix --register-service when executed from within binary directory

Go 1.15.7 contained a security fix for CVE-2021-3115, which allowed arbitrary
code to be executed at build time when using cgo on Windows.

This issue was not limited to the go command itself, and could also affect binaries
that use `os.Command`, `os.LookPath`, etc.

From the related blogpost (https://blog.golang.org/path-security):

> Are your own programs affected?
>
> If you use exec.LookPath or exec.Command in your own programs, you only need to
> be concerned if you (or your users) run your program in a directory with untrusted
> contents. If so, then a subprocess could be started using an executable from dot
> instead of from a system directory. (Again, using an executable from dot happens
> always on Windows and only with uncommon PATH settings on Unix.)
>
> If you are concerned, then we’ve published the more restricted variant of os/exec
> as golang.org/x/sys/execabs. You can use it in your program by simply replacing

At time of the go1.15 release, the Go team considered changing the behavior of
`os.LookPath()` and `exec.LookPath()` to be a breaking change, and made the
behavior "opt-in" by providing the `golang.org/x/sys/execabs` package as a
replacement.

However, for the go1.19 release, this changed, and the default behavior of
`os.LookPath()` and `exec.LookPath()` was changed. From the release notes:
https://go.dev/doc/go1.19#os-exec-path

> Command and LookPath no longer allow results from a PATH search to be found
> relative to the current directory. This removes a common source of security
> problems but may also break existing programs that depend on using, say,
> exec.Command("prog") to run a binary named prog (or, on Windows, prog.exe)
> in the current directory. See the os/exec package documentation for information
> about how best to update such programs.
>
> On Windows, Command and LookPath now respect the NoDefaultCurrentDirectoryInExePath
> environment variable, making it possible to disable the default implicit search
> of “.” in PATH lookups on Windows systems.

A result of this change was that registering the daemon as a Windows service
no longer worked when done from within the directory of the binary itself:

    C:\> cd "Program Files\Docker\Docker\resources"
    C:\Program Files\Docker\Docker\resources> dockerd --register-service
    exec: "dockerd": cannot run executable found relative to current directory

Note that using an absolute path would work around the issue:

    C:\Program Files\Docker\Docker>resources\dockerd.exe --register-service

This patch changes `registerService()` to use `os.Executable()`, instead of
depending on `os.Args[0]` and `exec.LookPath()` for resolving the absolute
path of the binary.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>
(cherry picked from commit 3e8fda0a70)
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

cmd/dockerd/service_windows.go

@@ -7,7 +7,6 @@ import (
 	"io"
 	"log"
 	"os"
-	"os/exec"
 	"path/filepath"
 	"time"
 
@@ -145,16 +144,8 @@ func (h *etwHook) Fire(e *logrus.Entry) error {
 	return windows.ReportEvent(h.log.Handle, etype, 0, eid, 0, count, 0, &ss[0], nil)
 }
 
-func getServicePath() (string, error) {
-	p, err := exec.LookPath(os.Args[0])
-	if err != nil {
-		return "", err
-	}
-	return filepath.Abs(p)
-}
-
 func registerService() error {
-	p, err := getServicePath()
+	p, err := os.Executable()
 	if err != nil {
 		return err
 	}