sm is a reimplementation of Nintendo's service manager. It allows Atmosphère to add or remove process handle limits, add new services, or intercept service calls. This allows high-level intercepting of Horizon OS functionality.
## Atmosphère Extensions
There are a few ways in which the Stratosphere implementation of sm differs intentionally from the stock sm.
### IPC: MITM Commands
The Stratosphere implementation of sm adds a few additional commands to the [`sm:`](https://reswitched.github.io/SwIPC/ifaces.html#nn::sm::detail::IUserInterface) port session.
This command alters the registration for the named service, in order to allow services to intercept communication between client processes and their intended services. It is used by [fs_mitm](fs_mitm.md).
It takes the name of the service to install an MITM for, and returns two handles. The first is a port handle, similar to those returned from the [RegisterService](https://reswitched.github.io/SwIPC/ifaces.html#nn::sm::detail::IUserInterface(2)) command. The second is the server side of a session, called the query session. This session will used by sm to determine whether or not a new session should be intercepted, and to inform the MITM service of the identity of new processes.
The query session is expected to implement the following interface.
```
interface MitmQueryService {
[65000] ShouldMitm(u64 pid) -> u64 should_mitm;
[65001] AssociatePidTid(u64 pid, u64 tid);
}
```
The `ShouldMitm` command is invoked whenever a process attempts to make a new connection to the MITM'd service. It should return `0` if the process's connection should not be intercepted. Any other value will cause the process's connection to be intercepted. If the command returns an error code, the process's connection will not be intercepted.
The `AssociatePidTid` command is invoked on all MITM query sessions whenever a new process is created, in order to inform those services of the identity of a newly created process before it attempts to connect to any services.
If the process that installed the MITM attempts to connect to the service, it will always connect to the original service.
If the service has not yet been registered, the request will be deferred until the service is registered in the same manner as IUserInterface::GetService.
This command requires that the session be initialized, returning error code 0x415 if it is not.
#### AtmosphereAssociatePidTidForMitm
This command is used internally by the Stratosphere implementation of the [loader](loader.md) sysmodule, when a new process is created. It will call the `AssociatePidTid` command on every registered MITM query session.
If the given process ID refers to a kernel internal process, error code 0x1015 is returned. This command requires that the session be initialized, returning error code 0x415 if it is not.
Retrieves `sizeof(SmServiceRecord)` for a service. The current format of `SmServiceRecord` structure follows.
```
struct SmServiceRecord {
uint64_t service_name;
uint64_t owner_pid;
uint64_t max_sessions;
uint64_t mitm_pid;
uint64_t mitm_waiting_ack_pid;
bool is_light;
bool mitm_waiting_ack;
};
```
#### AtmosphereGetServiceRecord
Retrieves a service registration record for a service.
#### AtmosphereListServiceRecords
Provides a list of service registrations records.
The command will return an array of `SmServiceRecord`s, skipping `offset` records. The number of records returned is indicated by `count`.
If `count` is less than the size of the buffer divided by `sizeof(SmServiceRecord)` (the buffer was not completely filled), the end of the service registration list has been reached. Otherwise, client code
should increment `offset` by `count` and call again. Client code should retrieve a record size using `AtmosphereGetServiceRecordSize`, and either make sure that the size of a record matches what it expects,
or should make sure to use the correct size as the stride while iterating over the array of returned records. Example pseudocode is shown below.
When a service is registered, the sysmodule registering it must specify a limit on the number of sessions that are allowed to be active for that service at a time. This is used to ensure that services like `fs-pr`, `fs-ldr`, and `ldr:pm` can only be connected to once, adding an additional layer of safety over the regular service verification to ensure that those services are only connected to by the highly priveleged process they are intended to be used by.
By default, the Stratosphere implementation of PM will raise any session limits to at least 8, meaning that for services like `fs-pr` and those mentioned above, up to 8 processes will be able to connect to those sessions, leaving 7 sessions for homebrew to use.
In system firmware versions before 3.0.1, if a process did not call the [Initialize](https://reswitched.github.io/SwIPC/ifaces.html#nn::sm::detail::IUserInterface(0)) command on its `sm:` session, normally used to inform sm of the process's identity, sm would assume that the process was a kernel internal process and skip any service registration or access checks. The Stratosphere implementation of sm does not implement this vulnerability, and initialization is required on all firmware versions.
