pkg/host: improve KMEMLEAK detection

EBUSY is returned when KMEMLEAK is in fact turned off,
not when scanning is already turned off.
That can't be recovered.
So instead check that we can write to the file in the check
function and give useful diagnostics.

pkg/host/host_linux.go

@@ -437,9 +437,17 @@ func checkLeakChecking() string {
 	if reason := checkDebugFS(); reason != "" {
 		return reason
 	}
-	if err := osutil.IsAccessible("/sys/kernel/debug/kmemleak"); err != nil {
+	fd, err := syscall.Open("/sys/kernel/debug/kmemleak", syscall.O_RDWR, 0)
+	if err != nil {
 		return "CONFIG_DEBUG_KMEMLEAK is not enabled"
 	}
+	defer syscall.Close(fd)
+	if _, err := syscall.Write(fd, []byte("scan=off")); err != nil {
+		if err == syscall.EBUSY {
+			return "KMEMLEAK disabled: increase CONFIG_DEBUG_KMEMLEAK_EARLY_LOG_SIZE or unset CONFIG_DEBUG_KMEMLEAK_DEFAULT_OFF"
+		}
+		return fmt.Sprintf("/sys/kernel/debug/kmemleak write failed: %v", err)
+	}
 	return ""
 }
 
@@ -449,12 +457,6 @@ func setupLeakChecking() error {
 		return fmt.Errorf("failed to open /sys/kernel/debug/kmemleak: %v", err)
 	}
 	defer syscall.Close(fd)
-	if _, err := syscall.Write(fd, []byte("scan=off")); err != nil {
-		// kmemleak returns EBUSY when kmemleak is already turned off.
-		if err != syscall.EBUSY {
-			return fmt.Errorf("write(kmemleak, scan=off) failed: %v", err)
-		}
-	}
 	// Flush boot leaks.
 	if _, err := syscall.Write(fd, []byte("scan")); err != nil {
 		return fmt.Errorf("write(kmemleak, scan) failed: %v", err)