fix(proxy-identity): use execve call on Linux to start the proxy (#14307)

A recent change (#14126) altered the semantics of running the proxy from the identity wrapper. This has introduced [regression in behavior on Linux systems](#14289). This PR reverts the logic on Unix to use `execve`. This way we use `os/exec` only on Windows systems. 

Further investigation into Windows behavior will be carried out but for now we aim to revert back to the behavior we had on Unix prior to #14126

Signed-off-by: Zahari Dichev <[email protected]>

Author: zaharidichev

proxy-identity/main.go

@@ -8,9 +8,7 @@ import (
 	"errors"
 	"fmt"
 	"os"
-	"os/exec"
 	"path/filepath"
-	"runtime"
 
 	"github.com/linkerd/linkerd2/pkg/tls"
 	log "github.com/sirupsen/logrus"
@@ -143,20 +141,3 @@ func generateAndStoreCSR(p, id string, key *ecdsa.PrivateKey) ([]byte, error) {
 
 	return csrb, nil
 }
-
-func runProxy() {
-	path := "/usr/lib/linkerd/linkerd2-proxy"
-	if runtime.GOOS == "windows" {
-		path = "C:\\linkerd\\linkerd2-proxy.exe"
-	}
-
-	cmd := exec.Command(path)
-	cmd.Env = os.Environ()
-	cmd.Stdin = os.Stdin
-	cmd.Stdout = os.Stdout
-	cmd.Stderr = os.Stderr
-	err := cmd.Run()
-	if err != nil {
-		log.Fatalf("Failed to run proxy: %s", err)
-	}
-}

proxy-identity/run_proxy_unix.go

@@ -0,0 +1,17 @@
+package main
+
+import (
+	"os"
+	"syscall"
+
+	log "github.com/sirupsen/logrus"
+)
+
+func runProxy() {
+	// The input arguments are static.
+	//nolint:gosec
+	err := syscall.Exec("/usr/lib/linkerd/linkerd2-proxy", []string{}, os.Environ())
+	if err != nil {
+		log.Fatalf("Failed to run proxy: %s", err)
+	}
+}

proxy-identity/run_proxy_windows.go

@@ -0,0 +1,21 @@
+package main
+
+import (
+	"os"
+	"os/exec"
+
+	log "github.com/sirupsen/logrus"
+)
+
+func runProxy() {
+	path := "C:\\linkerd\\linkerd2-proxy.exe"
+	cmd := exec.Command(path)
+	cmd.Env = os.Environ()
+	cmd.Stdin = os.Stdin
+	cmd.Stdout = os.Stdout
+	cmd.Stderr = os.Stderr
+	err := cmd.Run()
+	if err != nil {
+		log.Fatalf("Failed to run proxy: %s", err)
+	}
+}