Homebrew ABI

Entrypoint

Entrypoint is at binary_ptr+0. At this offset, there is a branch instruction that jumps past the NRO0 header. This is for (.text) file format polyglot compatibility with NSO0.

Every application should clean itself up before returning to loader.

This includes tasks like:

• Not leaking handles
• Resetting MemoryState
• No leftover threads running in the background

Original LR given to entrypoint should be returned to, X0 contains an error code. If original LR is NULL, svcExitProcess should be used.

Entrypoint Arguments

NSO0: the system executes binary_ptr+0 with X0=NULL, X1=main_thread_handle.

NRO0: the homebrew loader puts X0=loader_config_ptr, X1=0xFFFFFFFFFFFFFFFF.

When X0 is non-zero and X1 is not 0xFFFFFFFFFFFFFFFF, the application must handle userland exceptions (with the same entrypoint args described there). This must be done without corrupting registers for this (x8..x29).

The loader should forward exceptions to the mapped NRO by just jumping to the NRO entrypoint, without the above register corruption. When the NRO is not mapped the loader can run: svcReturnFromException(0xf801); while(1);

Loader Config

Loader config allows overriding functionality to workaround limitations in a limited homebrew environment.

It is an array of tuples:

struct LoaderConfig {
  LoaderConfigEntry Entries[];
};

Each entry is key-value pair:

struct LoaderConfigEntry {
  u32 Key;
  u32 Flags;
  u64 Value[2];
};

enum LoaderConfigFlags {
  IsMandatory = BIT(0),
};

Loader Config Keys

A loader key can be marked as mandatory or not in its Flags field.

If a key is marked as mandatory it means that the application cannot safely ignore it.

Thus if said key is not recognized by the application, it should exit with result_code=346 | ((100 + key) << 9);, as the default behaviour may be unsafe.

If a key that is required is not found (for example with an outdated loader), use result_code=346 | ((200 + key) << 9);.

If there is some error encountered while validating an entry's values, use result_code = 346 | ((300 + key) << 9);.

• 0: #EndOfList: Must be present

• 1: #MainThreadHandle: Must be present

• 2: #NextLoadPath

• 3: #OverrideHeap: If present, must not be ignored

• 4: #OverrideService

• 5: #Argv

• 6: #SyscallAvailableHint

• 7: #AppletType: Must be present

• 8: #AppletWorkaround: If present, must not be ignored

• 9: #Reserved9

• 10: #ProcessHandle

• 11: #LastLoadResult

• 12: #AllocPages

• 13: #LockRegion: If present, must not be ignored

• 14: #RandomSeed

• 15: #UserIdStorage

• 16: #HosVersion

EndOfList

EndOfList is the final entry in the LoaderConfig. It also provides optional human readable information about the Homebrew loader.

• Key: 0
• Value[0]: Pointer to loader information string, or zero if not present.
• Value[1]: Size in bytes of the information string, or zero if not present.

MainThreadHandle

This is the handle to the thread that is executing the entrypoint. Required for mutex to function.

• Key: 1
• Value[0]: Handle to the main thread.
• Value[1]: Reserved/should be zero.
• DefaultBehavior: Use main thread handle from entry function arguments.

NextLoadPath

Homebrew menu uses this pointer to write the path of next NRO to load, before returning back to Homebrew loader.

• Key: 2
• Value[0]: Pointer to buffer of size 512. NRO file path, should start with "sdmc:/".
• Value[1]: Pointer to buffer of size 2048. Argv string to boot NRO with.

OverrideHeap

If the NRO loader has reserved some space in the heap for itself, the application must not manipulate the heap. If an #AllocPages key is present, the region this points to should be accepted by free_pages.

• Key: 3
• Value[0]: Base address of heap. Must be MemoryType 4, 5, or 9 with all reference counts being zero.
• Value[1]: Size of heap.
• DefaultBehavior: Allocates heap using svcSetHeapSize instead.

OverrideService

The NRO loader should be able to steal handles from more priliveged processes. In this case, homebrew should use this handle instead of the normal one.

Homebrew should allow at least 32 service overrides.

Note: Overridden service handles should not be converted to domains or closed. If they are domain, they should be converted to sessions before passed to the homebrew application.

Note: For services that authenticate with pid, the app should not attempt re-authentication with an overridden handle.

Note: There needs be special handling for the file system service. If the original fsp-srv handle doesn't allow MountSdcard, it shall fallback to the stolen one, etc etc.

• Key: 4
• Value[0]: Name of service, same format as for sm.
• Value[1]: Lower 32-bits: Service handle
• DefaultBehavior: Fetches service from "sm:" named port.

Argv

The NRO loader should be able to send argv.

• Key: 5
• Value[0]: Reserved/should be zero.
• Value[1]: Argv string pointer.
• DefaultBehavior: Setting (argc == 1, argv[0] == "", argv[1] == NULL), or argv parsed in NSO0 fashion.

SyscallAvailableHint

Homebrew doesn't know if any privileged syscalls are available.

This entry allows loader to give hints about having access to rare syscalls (such as JIT).

• Key: 6
• Value[0]: 64-bit mask for the 0-0x3F SVC range. n:th bit set means SVC is accessible.
• Value[1]: 64-bit mask for the 0x40-0x7F SVC range.
• DefaultBehavior: If NSO0, assume kernelhax thus all rare syscalls are allowed. If NRO0, assume all rare syscalls are inaccessible.

AppletType

Specifies the AM AppletType, used for selecting which Open*Proxy command to use.

• Key: 7
• Value[0]: AppletType
• Value[1]: AppletFlags

AppletFlags ApplicationOverride: Use AppletType_Application instead of AppletType_SystemApplication, only when AppletType is SystemApplication. This should only be handled when AppletType_Application is properly supported where applet may have been previously initialized in the context of the current process, for AppletType_Application. This avoids breaking backwards-compatibility with apps which don't support AppletType_Application as previously described.

enum LoaderConfigAppletType {
  LoaderConfigAppletType_Application = 0,
  LoaderConfigAppletType_SystemApplet = 1,
  LoaderConfigAppletType_LibraryApplet = 2,
  LoaderConfigAppletType_OverlayApplet = 3,
  LoaderConfigAppletType_SystemApplication = 4,
};

enum LoaderConfigAppletFlags {
  LoaderConfigAppletFlags_ApplicationOverride = BIT(0),
};

AppletWorkaround

This flag means that AM services is broken, and must not be used.

• Key: 8
• Value[0]: AppletResourceUserId
• Value[1]: Reserved/should be zero.

Reserved9

This key has been deleted/reserved for future use.

ProcessHandle

Handle to self process.

• Key: 10
• Value[0]: Process handle.
• Value[1]: Reserved/should be zero.

LastLoadResult

This is used to feed the last NRO error code back to the Homebrew Menu so that an error dialog can be displayed.

• Key: 11
• Value[0]: Result.
• Value[1]: Reserved/should be zero.

AllocPages

This key is used to provide function pointers for a memory allocator. If an #OverrideHeap key is present and this key is recognized, free_pages may be called with the address of the overridden heap as an argument. There may not be more than one #AllocPages key.

If this key is recognized, applications should not attempt to use heap regions without first obtaining them through alloc_pages. If applications should not manage the heap themselves, an #OverrideHeap key should be passed in case this key is not recognized. If an #OverrideHeap key is not present, applications may attempt to manage the heap themselves. Any regions of memory within the heap that would be unsafe for an application to overwrite (such as the application's own stack) must be specified by a #LockRegion key.

void *alloc_pages(size_t min, size_t max, size_t *actual):

This function shall allocate a page-aligned region of memory with a page-aligned size between min and max bytes (inclusive). If a failure is encountered, it shall return NULL. If actual is not NULL, the value it points to shall be overwritten with the actual size of the allocated region. Regions of memory allocated by this function may be passed to free_pages to mark them as eligible to be allocated again.

Regions of memory returned from this function must be read/write type-5 (HEAP) pages with no memory attributes set.

bool free_pages(void *pages):

This function shall mark a region of memory either previously returned by alloc_pages or passed by the #OverrideHeap key as being eligible for future allocations. If any failure is encountered, a value of TRUE shall be returned. Otherwise, a value of FALSE is returned.

• Key: 12
• Value[0]: void *(*alloc_pages)(size_t min, size_t max, size_t *actual)
• Value[1]: bool (*free_pages)(void *pages)

LockRegion

This key is used to hint to the application that a certain region of the heap is unsafe to overwrite. There is no limit on the number of #LockRegion keys that may be passed. If an application is unable to process the number of regions that are locked, it must exit before overwriting any heap memory. It is an error for this key to be present in the same configuration as an #OverrideHeap or #AllocPages key.

• Key: 13
• Value[0]: Pointer to region to lock
• Value[1]: Size of region to lock

RandomSeed

This key is used to provide an extra entropy source to the application (in addition to the seed provided by the kernel), allowing it to generate different pseudo-random numbers every time it's launched from within the same process.

• Key: 14
• Value[0]: Random data.
• Value[1]: More random data.

UserIdStorage

This key is used to provide persistent storage space for the preselected user id when launching homebrew under an application that has an account selection screen.

• Key: 15
• Value[0]: Pointer to a buffer containing enough space (16 bytes) to store a user id.
• Value[1]: Reserved/should be zero.

HosVersion

This key provides the currently running version of Horizon OS.

• Key: 16
• Value[0]: HOS version, formatted using libnx's MAKEHOSVERSION macro.
• Value[1]: Reserved/should be zero.