find_dtbs: scan a binary file for devicetree binary magic bytes, and allow extracting the DTBs

find_dtbs.c

// SPDX-License-Identifier: GPL-2.0-only
// Copyright (C) 2018-2024 Andre Przywara <osp@apritzel.eu>

#include <stdio.h>
#include <stdint.h>
#include <stdlib.h>
#include <stdbool.h>
#include <getopt.h>
#include <arpa/inet.h>		// for htonl

struct fdt_header {
	uint32_t magic;
	uint32_t totalsize;
	uint32_t off_dt_struct;
	uint32_t off_dt_strings;
	uint32_t off_mem_rsvmap;
	uint32_t version;
	uint32_t last_comp_version;
	uint32_t boot_cpuid_phys;
	uint32_t size_dt_strings;
	uint32_t size_dt_struct;
};

#define BUFSIZE 65536

int main(int argc, char **argv)
{
	FILE *inf = stdin;
	size_t ret;
	off_t fpos = 0;
	struct fdt_header header;
	int nr = 1, extract = 0;
	bool verbose = false;
	uint32_t size, version;
	int option;
	void *buf = NULL;

	while ((option = getopt(argc, argv, "ve:h")) != -1) {
		switch (option) {
		case 'v':
			verbose = true;
			break;
		case 'e':
			extract = atoi(optarg);
			break;
		case 'h':
			fprintf(stdout, "usage: %s [-hv] [-e <nr>] [<file>]\n",
				argv[0]);
			return 0;
		}
	}
	
	if (optind < argc) {
		inf = fopen(argv[optind], "rb");
		if (!inf) {
			perror(argv[optind]);
			return -2;
		}
	}

	if (extract)
		buf = malloc(BUFSIZE);

	while (!feof(inf)) {
		ret = fread(&header, 4, 1, inf);
		if (ret < 1)
			break;
		if (ntohl(header.magic) != 0xd00dfeed) {
			fpos += 4;
			continue;
		}
		ret = fread(&header.totalsize, 4, 9, inf);
		version = ntohl(header.version);
		size = ntohl(header.totalsize);
		if (ntohl(header.last_comp_version) != 16 || version != 17) {
			if (verbose && !extract)
				fprintf(stdout, "DTB signature found at 0x%llx, version: %d, size: %d bytes\n",
					(unsigned long long)fpos, version,
					size);
			fpos += (ret + 1) * 4;
			continue;
		}
		if (!extract)
			fprintf(stdout, "%2d: found DTB v17 at 0x%llx, %d bytes\n",
				nr, (unsigned long long)fpos, size);

		fpos += (ret + 1) * 4;
		if (nr == extract) {
			ret = fwrite(&header, 4, 10, stdout);
			size -= 40;
			while (size > 0) {
				int toread = size;

				if (toread > BUFSIZE)
					toread = BUFSIZE;
				ret = fread(buf, 1, toread, inf);
				fpos += ret;
				if (ferror(inf)) {
					perror("reading input file");
					return 3;
				}
				if (ret > 0)
					fwrite(buf, 1, ret, stdout);

				size -= ret;
			}
			break;
		}
		nr++;
	}

	if (extract)
		free(buf);

	fclose(inf);

	return 0;
}