Properties.Get() SIGSEGV(segment violation)

[leiless]

Hi, godbus developers, I have a minimal reproducible code to represent the segment fault.

I believe there certainly some kinda misuse of the propsSpec?
And I don't know how to present the Configuration result according to https://developer.gnome.org/NetworkManager/stable/gdbus-org.freedesktop.NetworkManager.DnsManager.html

package main

import (
	"fmt"
	"github.com/godbus/dbus/v5"
	"github.com/godbus/dbus/v5/prop"
)

func main() {
	conn, err := dbus.ConnectSystemBus()
	if err != nil {
		panic(fmt.Errorf("cannot connect to system d-bus"))
	}
	defer conn.Close()

	propsSpec := map[string]map[string]*prop.Prop{
		"org.freedesktop.NetworkManager.DnsManager": {
			"Configuration": {
				nil, // Q: misuse?
				false,
				prop.EmitTrue,
				func(change *prop.Change) *dbus.Error {
					panic(fmt.Errorf("should never be called: %v", change))
				},
			},
		},
	}

	properties, err := prop.Export(conn, "/org/freedesktop/NetworkManager/DnsManager", propsSpec)
	if err != nil {
		panic(fmt.Errorf("cannot export: %w", err))
	}
	fmt.Printf("%+v\n", properties)

	v, err := properties.Get("org.freedesktop.NetworkManager.DnsManager", "Configuration")
	if err != nil {
		panic(fmt.Errorf("cannot get from properties: %w", err))
	}
	fmt.Printf("%+v\n", v)
}

Run result:

$ go run main.go         
&{m:map[org.freedesktop.NetworkManager.DnsManager:map[Configuration:0xc0000aa000]] mut:{w:{state:0 sema:0} writerSem:0 readerSem:0 readerCount:0 readerWait:0} conn:0xc000136000 path:/org/freedesktop/NetworkManager/DnsManager}

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x98 pc=0x54cd9a]

goroutine 1 [running]:
github.com/godbus/dbus/v5.getSignature(0x0, 0x0, 0x1, 0xe2)
	/home/lei/go/pkg/mod/github.com/godbus/dbus/v5@v5.0.4/sig.go:51 +0x3a
github.com/godbus/dbus/v5.SignatureOf(0xc000143e98, 0x1, 0x1, 0xd, 0xc0000ac088)
	/home/lei/go/pkg/mod/github.com/godbus/dbus/v5@v5.0.4/sig.go:37 +0xa5
github.com/godbus/dbus/v5.MakeVariant(...)
	/home/lei/go/pkg/mod/github.com/godbus/dbus/v5@v5.0.4/variant.go:20
github.com/godbus/dbus/v5/prop.(*Properties).Get(0xc0000ae000, 0x5d19bb, 0x29, 0x5ca792, 0xd, 0x0, 0x0, 0x0, 0x0, 0x0)
	/home/lei/go/pkg/mod/github.com/godbus/dbus/v5@v5.0.4/prop/prop.go:202 +0x15d
main.main()
	/tmp/dbus-demo-go/main.go:35 +0x334
exit status 2

[leiless]

$ busctl get-property org.freedesktop.NetworkManager /org/freedesktop/NetworkManager/DnsManager org.freedesktop.NetworkManager.DnsManager Configuration
aa{sv} 1 4 "nameservers" as 1 "192.168.200.1" "interface" s "eno1" "priority" i 100 "vpn" b false

[esendjer]

@leiless Hello!
May I ask, did you manage to solve this issue? I've just faced a pretty similar error in an attempt to get a property for /org/freedesktop/login1/session/self and it looks like you are the sole person with the same issue that I've found.

[leiless]

May I ask, did you manage to solve this issue?

Not quite yet, have no idea what's going on here, I switch to not to use godbus to solve my problem.

[esendjer]

Hey!
I've managed to get around the issue in another way. Perhaps, it might be helpful for you or someone else.
Workable example:

package main

import (
	"fmt"
	"os"

	"github.com/godbus/dbus/v5"
)

const (
	IFACEGETPROP  = "org.freedesktop.DBus.Properties.Get"
	NMNODE        = "org.freedesktop.NetworkManager"
	NMOBJPATH     = "/org/freedesktop/NetworkManager/DnsManager"
	NMIFACECONF   = "org.freedesktop.NetworkManager.DnsManager"
	NMPROPNAME    = "Configuration"
)

func main() {
	conn, err := dbus.ConnectSystemBus()
	if err != nil {
		fmt.Fprintln(os.Stderr, "Failed to connect to system bus:", err)
		os.Exit(1)
	}
	defer conn.Close()

	nmObj := conn.Object(NMNODE, NMOBJPATH)

	nmConfig := nmObj.Call(IFACEGETPROP, 0, NMIFACECONF, NMPROPNAME).Body[0]
	fmt.Printf("NM Config: %+v\n", nmConfig)
}

Output:

$ go run main.go
NM Config: [{"interface": <"eth0">, "nameservers": <["8.8.8.8", "1.1.1.1"]>, "priority": <100>, "vpn": <false>}]

[guelfey]

@leiless Are you just trying to read this property from NetworkManager, or are you trying to implement a NetworkManager-compatible interface (i.e. "fake" this property to someone else trying to read it)? Because the first part of your example is accomplishing the latter, but the second part does the former. In case of only reading, @esendjer's code is enough.

The prop package is just a helper if you want to implement the properties interface to that other D-Bus apps can get or set them. It's not used for dealing with properties from other apps, you want https://pkg.go.dev/github.com/godbus/dbus?utm_source=godoc#Object.GetProperty for that. (For the record, if you wanted to export such a Configuration type as mentioned in your link, you'd need to put a []map[string]dbus.Variant as Value in propsSpec).