ARM Binaries

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ARM binaries are contained inside .wad files in update partitions. The .wad format and how to decrypt it is described in WAD_Files.

ELF format

IOS modules, at least, use bare ELF files. The files seem to be compiled with GCC 3.4.3, and they are EABI compliant.

ELFLOADER format

The ELFLOADER ARM binary format is used for the "bootup" files, including the IOS kernel (or the entirety of the IOS in earlier versions which are monolithic) and BOOT2. Once decrypted, the data has the following format:

Start End Length Description
0x000 0x004 0x004 Header size = 0x0010
0x004 0x008 0x004 Offset to ELF file after header
0x008 0x00C 0x004 Size of ELF file
0x00C 0x010 0x004 0x00 padding / unused
0x010 variable variable ELF file stub loader binary
variable variable variable ELF file

BOOT2 elf loader

The BOOT2 elf stub loader sets up a stack, calculates its own address, and switches to THUMB mode. Then it does the following:

 if( ! (*((u32 *)0xD800060) & 0x20) ) {
   *((u32 *)0xD800060) |= 0x20;
 }

0xD800000 seems to be the start of the (a?) hardware register space.

After this, it loads the ELF file, and then zeroes out the memory area where the ELF file resides. Then it goes back to ARM mode and vectors to 0xFFFF0000 (the entrypoint of the ARM / vector table). The entire BOOT2 code seems to be position-independent: it can be loaded at any address and will still work, as long as it doesn't overlap with the destination of the ELF load. The entire BOOT2 file cleartext is loaded and then the loader is called, so the loader can calculate the offset of the header simply by subtracting 0x10 from the PC at its entrypoint.

Extract ELF file

The following program extracts the ELF from the ARM binary. Normal ELF tools can handle the generated output.

#include <stdio.h>
#include <stdint.h>
#include <malloc.h>
#include <netinet/in.h>

/** Header for Wii ARM binaries. */
typedef struct {
	/** Size of this header. */
	uint32_t headerSize;
	/** Offset to ELF file. */
	uint32_t offset;
	/** Size of ELF file. */
	uint32_t size;
	/** Padded with zeroes. */
	uint32_t resevered;
} arm_binary_header_t;

int main(int argc, char *argv[])
{
	const char *inFilename;
	const char *outFilename;
	FILE *fin = NULL;
	FILE *fout = NULL;
	arm_binary_header_t header;
	char *buffer = NULL;

	if (argc != 3) {
		fprintf(stderr, "%s: [Wii ARM Binary] [ELF output file]\n\n", argv[0]);
		fprintf(stderr, "Extract ELF from Wii ARM binary.\n");
		fprintf(stderr, "Error: Parameter wrong.\n");
		return -1;
	}
	inFilename = argv[1];
	outFilename = argv[2];

	fin = fopen(inFilename, "rb");
	if (fin == NULL) {
		fprintf(stderr, "Error: Failed to open input file \"%s\".\n", inFilename);
		return -2;
	}
	if (fread(&header, sizeof(header), 1, fin) != 1) {
		fclose(fin);
		fprintf(stderr, "Error: Input file \"%s\" is too small.\n", inFilename);
		return -3;
	}
	header.headerSize = ntohl(header.headerSize);
	header.offset = ntohl(header.offset);
	header.size = ntohl(header.size);

	if (header.headerSize != sizeof(arm_binary_header_t)) {
		fclose(fin);
		fprintf(stderr, "Error: Input file \"%s\" is not a Wii ARM binary.\n", inFilename);
		return -4;
	}
	if (fseek(fin, header.offset, SEEK_CUR) != 0) {
		fclose(fin);
		fprintf(stderr, "Error: Input file \"%s\" is too small (seek to %d failed).\n", inFilename, header.offset);
		return -5;
	}
	buffer = malloc(header.size);
	if (buffer == NULL) {
		fclose(fin);
		fprintf(stderr, "Error: Out of memory.\n");
		return -6;
	}
	if (fread(buffer, header.size, 1, fin) != 1) {
		fclose(fin);
		fprintf(stderr, "Error: Input file \"%s\" is too small.\n", inFilename);
		return -7;
	}
	fclose(fin);

	fout = fopen(outFilename, "wb");
	if (fout == NULL) {
		fclose(fin);
		fprintf(stderr, "Error: Failed to open output file \"%s\".\n", outFilename);
		return -8;
	}
	if (fwrite(buffer, header.size, 1, fout) != 1) {
		fclose(fin);
		fprintf(stderr, "Error: Output file \"%s\" write error (disc full?).\n", outFilename);
		return -9;
	}
	fclose(fout);
	free(buffer);
	return 0;
}

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