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):
SegmentHeader
| Offset | Size | Description |
|---|---|---|
| 0x0 | 4 | file offset of data |
| 0x4 | 4 | memory offset loaded to |
| 0x8 | 4 | size of data copied to memory offset (i.e. size after decompression) |
| 0xC | 4 | alignment used on memory size / size of .bss in the case of .data segment |
.rodata-relative extent
| Offset | Size | Description |
|---|---|---|
| 0x0 | 4 | offset (relative to .rodata) |
| 0x4 | 4 | size of region |
NSO Header
| Offset | Size | Description |
|---|---|---|
| 0x0 | 4 | Magic "NSO0" |
| 0x4 | 4 | |
| 0x8 | 4 | |
| 0xC | 4 | Always 0x3f? |
| 0x10 | 0x10 * 3 | SegmentHeader for each segment |
| 0x40 | 0x20 | Value of "build id" from ELF's GNU .note section. Contains variable sized digest, up to 32bytes. |
| 0x60 | 0x4 * 3 | file size of each segment (i.e. LZ4-compressed size) |
| 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.
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 | zero padding |
| 0x04 | 4 | offset to magic. 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. |