Are maps passed by value or by reference in Go?

No. Maps are reference by default.

    package main

    import "fmt"

    func mapToAnotherFunction(m map[string]int) {
        m["hello"] = 3
        m["world"] = 4
        m["new_word"] = 5
    }

    // func mapToAnotherFunctionAsRef(m *map[string]int) {
    // m["hello"] = 30
    // m["world"] = 40
    // m["2ndFunction"] = 5
    // }

    func main() {
        m := make(map[string]int)
        m["hello"] = 1
        m["world"] = 2

        // Initial State
        for key, val := range m {
            fmt.Println(key, "=>", val)
        }

        fmt.Println("-----------------------")

        mapToAnotherFunction(m)
        // After Passing to the function as a pointer
        for key, val := range m {
            fmt.Println(key, "=>", val)
        }

        // Try Un Commenting This Line
        fmt.Println("-----------------------")

        // mapToAnotherFunctionAsRef(&m)
        // // After Passing to the function as a pointer
        // for key, val := range m {
        //  fmt.Println(key, "=>", val)
        // }

        // Outputs
        // hello => 1
        // world => 2
        // -----------------------
        // hello => 3
        // world => 4
        // new_word => 5
        // -----------------------

    }

From Golang Blog-

Map types are reference types, like pointers or slices, and so the value of m above is nil; it doesn't point to an initialized map. A nil map behaves like an empty map when reading, but attempts to write to a nil map will cause a runtime panic; don't do that. To initialize a map, use the built in make function:

// Ex of make function
m = make(map[string]int)

Code Snippet Link Play with it.

Here are some parts from If a map isn’t a reference variable, what is it? by Dave Cheney:

A map value is a pointer to a runtime.hmap structure.

and conclusion:

Conclusion

Maps, like channels, but unlike slices, are just pointers to runtime types. As you saw above, a map is just a pointer to a runtime.hmap structure.

Maps have the same pointer semantics as any other pointer value in a Go program. There is no magic save the rewriting of map syntax by the compiler into calls to functions in runtime/hmap.go.

And an interesting bit about history/explanation of map syntax:

If maps are pointers, shouldn’t they be *map[key]value?

It’s a good question that if maps are pointer values, why does the expression make(map[int]int) return a value with the type map[int]int. Shouldn’t it return a *map[int]int? Ian Taylor answered this recently in a golang-nuts thread¹.

In the very early days what we call maps now were written as pointers, so you wrote *map[int]int. We moved away from that when we realized that no one ever wrote map without writing *map.

Arguably renaming the type from *map[int]int to map[int]int, while confusing because the type does not look like a pointer, was less confusing than a pointer shaped value which cannot be dereferenced.

In this thread you will find your answer :

Golang: Accessing a map using its reference

You don't need to use a pointer with a map.

Map types are reference types, like pointers or slices[1]

If you needed to change the Session you could use a pointer:

map[string]*Session

https://blog.golang.org/go-maps-in-action