NSO is the main executable format.
It starts with the "NSO" header and mainly describes .text, .rodata, and .data segments (like a short-form of ELF program headers):
NSO Header
| Offset | Size | Description |
|---|---|---|
| 0x0 | 4 | Magic "NSO0" |
| 0x4 | 4 | |
| 0x8 | 4 | |
| 0xC | 4 | Always 0x3f? |
| 0x10 | 0x10 * 3 | SegmentHeader[3] |
| 0x40 | 0x20 | Value of "build id" from ELF's GNU .note section. Contains variable sized digest, up to 32bytes. |
| 0x60 | 0x4 * 3 | CompressedSize[3] |
| 0x6c | 0x24 | Padding |
| 0x90 | 8 | .rodata-relative extents of .dynstr |
| 0x98 | 8 | .rodata-relative extents of .dynsym |
| 0xA0 | 0x20 * 3 | SHA256 hashes over the decompressed sections using the above byte-sizes: .text, .rodata, and .data. |
| 0x100 | Compressed sections |
Most data in Switch binaries are standard ELF structures, however some are custom. For example, the MOD header is essentially a replacement for a PT_DYNAMIC program header.
SegmentHeader
| Offset | Size | Description |
|---|---|---|
| 0x0 | 4 | FileOffset |
| 0x4 | 4 | MemoryOffset |
| 0x8 | 4 | DecompressedSize |
| 0xC | 4 | UnkOffset/UnkSize/BssSize |
.rodata-relative extent
| Offset | Size | Description |
|---|---|---|
| 0x0 | 4 | RegionRoDataOffset |
| 0x4 | 4 | RegionSize |
MOD
All offsets are signed 32bit values relative to the magic field. The 32bits at image base + 4 must point to the magic field. The MOD structure is designed such that it can be placed at image base and point to itself. The 2 fields preceding the magic field get copied around with the structure, even if it is relocated to somewhere besides the image base. If MOD is not located at image base, the value at offset 4 must still point to the MOD magic. In the case of .text being at image base, this implies that the first instruction can only be an unconditional branch over the offset literal.
| Offset | Size | Description |
|---|---|---|
| 0x00 | 4 | ZeroPadding |
| 0x04 | 4 | MagicOffset. Always 8 (so it works when MOD is at image_base + 0). |
| 0x08 | 4 | Magic "MOD0" |
| 0x0C | 4 | .dynamic offset |
| 0x10 | 4 | .bss start offset |
| 0x14 | 4 | .bss end offset |
| 0x18 | 4 | .eh_frame_hdr start offset |
| 0x1C | 4 | .eh_frame_hdr end offset |
| 0x20 | 4 | offset to runtime-generated module object. typically equal to .bss base. |
Arguments
It's unknown how Loader determines where to store the arguments. Official processes use argdata_addr = {page-aligned _end}. svcQueryMemory is used by official sw to verify that argdata_addr is mapped RW. Afterwards, official sw aligns the argdata_addr to 4-bytes. Structure located at argdata_addr:
| Offset | Size | Description |
|---|---|---|
| 0x0 | 0x4 | This is the total allocated space relative to argdata_addr, used for calculating the max size of the argv ptr array. |
| 0x4 | 0x4 | This is the total_bytesize of the actual argdata string. |
| 0x20 | See above | Actual argdata string. |
argv_ptrarray written by official processes is at (actual_argdata_string+(actual_argdata_size*2)) + 0x9 & ~0x7.