NSO
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 | Flags, bit 0-2: section is compressed, bit 3-5: check section hash when loading |
| 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
Loader maps memory and writes the arguments to {end of rwdata section specified by last SegmentHeader}. Official processes use argdata_addr = {page-aligned _end}. svcQueryMemory is used by official sw to verify that argdata_addr is mapped RW, since this memory is only mapped when arguments are specified via that command. 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. Normally 0x9000? |
| 0x4 | 0x4 | This is the total_bytesize of the actual argdata string. |
| 0x8 | 0x18 | Unused by official sw. |
| 0x20 | See above | Actual argdata string. |
- The copy of the args used with the argv array is written by official processes to actual_argdata_string+actual_argdata_size.
- argv_ptrarray written by official processes is at (args_copy+actual_argdata_size) + 0x9 & ~0x7.