Return helpful error when attaching to a process is impossible

service/debugger/debugger.go

@@ -46,7 +46,7 @@ func New(config *Config) (*Debugger, error) {
 		log.Printf("attaching to pid %d", d.config.AttachPid)
 		p, err := proc.Attach(d.config.AttachPid)
 		if err != nil {
-			return nil, fmt.Errorf("could not attach to pid %d: %s", d.config.AttachPid, err)
+			return nil, attachErrorMessage(d.config.AttachPid, err)
 		}
 		d.process = p
 	} else {

service/debugger/debugger_darwin.go

@@ -0,0 +1,10 @@
+package debugger
+
+import (
+	"fmt"
+)
+
+func attachErrorMessage(pid int, err error) error {
+	//TODO: mention certificates?
+	return fmt.Errorf("could not attach to pid %d: %s", pid, err)
+}

service/debugger/debugger_linux.go

@@ -0,0 +1,30 @@
+package debugger
+
+import (
+	"fmt"
+	"io/ioutil"
+	"os"
+	"syscall"
+)
+
+func attachErrorMessage(pid int, err error) error {
+	fallbackerr := fmt.Errorf("could not attach to pid %d: %s", pid, err)
+	if serr, ok := err.(syscall.Errno); ok {
+		switch serr {
+		case syscall.EPERM:
+			bs, err := ioutil.ReadFile("/proc/sys/kernel/yama/ptrace_scope")
+			if err == nil && len(bs) >= 1 && bs[0] != '0' {
+				// Yama documentation: https://www.kernel.org/doc/Documentation/security/Yama.txt
+				return fmt.Errorf("Could not attach to pid %d: set /proc/sys/kernel/yama/ptrace_scope to 0", pid)
+			}
+			fi, err := os.Stat(fmt.Sprintf("/proc/%d", pid))
+			if err != nil {
+				return fallbackerr
+			}
+			if fi.Sys().(*syscall.Stat_t).Uid != uint32(os.Getuid()) {
+				return fmt.Errorf("Could not attach to pid %d: current user does not own the process", pid)
+			}
+		}
+	}
+	return fallbackerr
+}