Code reading auxiliary vector present in executable binary.

aux.c

#include <stdio.h>
#include <stdlib.h>
#include <linux/auxvec.h>


static unsigned long *getauxv(char **env) {
	while (*env++ != NULL) {
		/* nop */
	}
	return (void*)env;
}


struct aux_handler {
	unsigned long key;
	const char *name;
	void (*handler)(const struct aux_handler *h, unsigned long *aux);
	const char *desc, *arg;
};

static void handle_print(const struct aux_handler *h, unsigned long *aux) {
	printf(h->arg, h->name, aux[0], aux[1], h->desc);
}

static void handle_random(const struct aux_handler *h, unsigned long *aux) {
	const unsigned char *ch = (void *)aux[1], *end = ch + 16;
	printf("%-16s %2lu ", h->name, aux[0]);
	for (; ch != end; ++ch) {
		printf("%02x", (unsigned)*ch);
	}
	printf("  // %s\n", h->desc);
}

#define AUX_HANDLER(key, handler, desc, arg) { key, #key, handler, desc, arg }
#define AUX_NUMBER(key, desc) { key, #key, handle_print, desc, "%-16s %2lu %-15lu  // %s\n" }
#define AUX_HEX(key, desc) { key, #key, handle_print, desc, "%-16s %2lu 0x%-13lx  // %s\n" }
#define AUX_STRING(key, desc) { key, #key, handle_print, desc, "%-16s %2lu \"%s\"  // %s\n" }

static const struct aux_handler aux_handlers[] = {
	AUX_HANDLER(AT_IGNORE,	NULL, NULL, NULL),
	AUX_NUMBER(AT_EXECFD,	"file descriptor of program"),
	AUX_NUMBER(AT_PHDR,	"program headers for program"),
	AUX_NUMBER(AT_PHENT,	"size of program header entry"),
	AUX_NUMBER(AT_PHNUM,	"number of program headers"),
	AUX_NUMBER(AT_PAGESZ,	"system page size"),
	AUX_NUMBER(AT_BASE,	"base address of interpreter"),
	AUX_NUMBER(AT_FLAGS,	"flags"),
	AUX_NUMBER(AT_ENTRY,	"entry point of program"),
	AUX_NUMBER(AT_NOTELF,	"program is not ELF"),
	AUX_NUMBER(AT_UID,	"real uid"),
	AUX_NUMBER(AT_EUID,	"effective uid"),
	AUX_NUMBER(AT_GID,	"real gid"),
	AUX_NUMBER(AT_EGID,	"effective gid"),
	AUX_STRING(AT_PLATFORM,	"string identifying CPU for optimizations"),
	AUX_HEX(AT_HWCAP,	"arch dependent hints at CPU capabilities"),
	AUX_NUMBER(AT_CLKTCK,	"frequency at which times() increments"),
	AUX_NUMBER(AT_SECURE,	"secure mode boolean"),
	AUX_STRING(AT_BASE_PLATFORM,	"string identifying real platform, may differ from AT_PLATFORM."),
	AUX_HANDLER(AT_RANDOM,	handle_random, "16 random bytes", NULL),
	AUX_STRING(AT_EXECFN,	"filename of program"),
	{ AT_NULL, "(unknown)", handle_print, "unknown field", "%-16s %2lu %-15lu\n" }
};

#undef AUX_HANDLER
#undef AUX_NUMBER
#undef AUX_STRING


int main(int argc, char **argv, char **env) {
	unsigned long *auxv = getauxv(env), key;
	const struct aux_handler *h;

	argc = argc; /* make compiler happy */
	argv = argv;

	while (*auxv != AT_NULL) {
		h = aux_handlers;
		key = *auxv;
		while (h->key != key && h->key != AT_NULL) {
			++h;
		}
		if (h->handler) {
			h->handler(h, auxv);
		}
		auxv += 2;
	}
	return 0;
}