unix: allow to pass a NULL pointer to the pledge syscall

Added PledgePromises and PledgeExecpromises functions
Added the pledgeAvailable helper function to check for pledge(2)
availability

Change-Id: Ib40e7eb481f70e4c02219db0ad445dfaa92aba2b
Reviewed-on: https://go-review.googlesource.com/c/142898
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

unix/openbsd_pledge.go

@@ -30,15 +30,9 @@ func Pledge(promises, execpromises string) error {
 		return err
 	}
 
-	// If OpenBSD <= 5.9, pledge is not available.
-	if (maj == 5 && min != 9) || maj < 5 {
-		return fmt.Errorf("pledge syscall is not available on OpenBSD %d.%d", maj, min)
-	}
-
-	// If OpenBSD <= 6.2 and execpromises is not empty
-	// return an error - execpromises is not available before 6.3
-	if (maj < 6 || (maj == 6 && min <= 2)) && execpromises != "" {
-		return fmt.Errorf("cannot use execpromises on OpenBSD %d.%d", maj, min)
+	err = pledgeAvailable(maj, min, execpromises)
+	if err != nil {
+		return err
 	}
 
 	pptr, err := syscall.BytePtrFromString(promises)
@@ -67,6 +61,70 @@ func Pledge(promises, execpromises string) error {
 	return nil
 }
 
+// PledgePromises implements the pledge syscall.
+//
+// This changes the promises and leaves the execpromises untouched.
+//
+// For more information see pledge(2).
+func PledgePromises(promises string) error {
+	maj, min, err := majmin()
+	if err != nil {
+		return err
+	}
+
+	err = pledgeAvailable(maj, min, "")
+	if err != nil {
+		return err
+	}
+
+	// This variable holds the execpromises and is always nil.
+	var expr unsafe.Pointer
+
+	pptr, err := syscall.BytePtrFromString(promises)
+	if err != nil {
+		return err
+	}
+
+	_, _, e := syscall.Syscall(SYS_PLEDGE, uintptr(unsafe.Pointer(pptr)), uintptr(expr), 0)
+	if e != 0 {
+		return e
+	}
+
+	return nil
+}
+
+// PledgeExecpromises implements the pledge syscall.
+//
+// This changes the execpromises and leaves the promises untouched.
+//
+// For more information see pledge(2).
+func PledgeExecpromises(execpromises string) error {
+	maj, min, err := majmin()
+	if err != nil {
+		return err
+	}
+
+	err = pledgeAvailable(maj, min, execpromises)
+	if err != nil {
+		return err
+	}
+
+	// This variable holds the promises and is always nil.
+	var pptr unsafe.Pointer
+
+	exptr, err := syscall.BytePtrFromString(execpromises)
+	if err != nil {
+		return err
+	}
+
+	_, _, e := syscall.Syscall(SYS_PLEDGE, uintptr(pptr), uintptr(unsafe.Pointer(exptr)), 0)
+	if e != 0 {
+		return e
+	}
+
+	return nil
+}
+
 // majmin returns major and minor version number for an OpenBSD system.
 func majmin() (major int, minor int, err error) {
 	var v Utsname
@@ -89,3 +147,20 @@ func majmin() (major int, minor int, err error) {
 
 	return
 }
+
+// pledgeAvailable checks for availability of the pledge(2) syscall
+// based on the running OpenBSD version.
+func pledgeAvailable(maj, min int, execpromises string) error {
+	// If OpenBSD <= 5.9, pledge is not available.
+	if (maj == 5 && min != 9) || maj < 5 {
+		return fmt.Errorf("pledge syscall is not available on OpenBSD %d.%d", maj, min)
+	}
+
+	// If OpenBSD <= 6.2 and execpromises is not empty,
+	// return an error - execpromises is not available before 6.3
+	if (maj < 6 || (maj == 6 && min <= 2)) && execpromises != "" {
+		return fmt.Errorf("cannot use execpromises on OpenBSD %d.%d", maj, min)
+	}
+
+	return nil
+}