Code reading auxiliary vector present in executable binary.

aux.c

#define _GNU_SOURCE

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


static unsigned long *getauxv(void) {
	char **env = environ;
	while (*env++) {
		/* 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_NUM(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_STR(key, desc) { key, #key, handle_print, desc, "%-16s %2lu %-15s  // %s\n" }

static const struct aux_handler aux_handlers[] = {
	AUX_HANDLER(AT_IGNORE,  NULL, NULL, NULL),
	AUX_HANDLER(AT_RANDOM,  handle_random, "16 random bytes", NULL),
	AUX_NUM(AT_EXECFD,            "file descriptor of program"),
	AUX_HEX(AT_PHDR,              "program headers for program"),
	AUX_NUM(AT_PHENT,             "size of program header entry"),
	AUX_NUM(AT_PHNUM,             "number of program headers"),
	AUX_NUM(AT_PAGESZ,            "system page size"),
	AUX_HEX(AT_BASE,              "base address of interpreter"),
	AUX_HEX(AT_FLAGS,             "flags"),
	AUX_HEX(AT_ENTRY,             "entry point of program"),
	AUX_NUM(AT_NOTELF,            "program is not ELF"),
	AUX_NUM(AT_UID,               "real uid"),
	AUX_NUM(AT_EUID,              "effective uid"),
	AUX_NUM(AT_GID,               "real gid"),
	AUX_NUM(AT_EGID,              "effective gid"),
	AUX_STR(AT_PLATFORM,          "CPU identity for optimizations"),
	AUX_HEX(AT_HWCAP,             "hints at CPU capabilities"),
	AUX_HEX(AT_HWCAP2,            "further hints at CPU capabilities"),
	AUX_NUM(AT_CLKTCK,            "frequency at which times() increments"),
	AUX_NUM(AT_SECURE,            "secure mode boolean"),
	AUX_STR(AT_BASE_PLATFORM,     "string identifying real platform"),
	AUX_STR(AT_EXECFN,            "filename of program"),
#ifdef AT_DCACHEBSIZE
	AUX_NUM(AT_DCACHEBSIZE,       "data cache block size"),
#endif
#ifdef AT_ICACHEBSIZE
	AUX_NUM(AT_ICACHEBSIZE,       "instruction cache block size"),
#endif
#ifdef AT_UCACHEBSIZE
	AUX_NUM(AT_UCACHEBSIZE,       "unified cache block size"),
#endif
#ifdef AT_L1D_CACHEGEOMETRY
	AUX_HEX(AT_L1D_CACHEGEOMETRY, "L1 data cache geometry"),
#endif
#ifdef AT_L1D_CACHESIZE
	AUX_NUM(AT_L1D_CACHESIZE,     "L1 data cache size"),
#endif
#ifdef AT_L1I_CACHEGEOMETRY
	AUX_HEX(AT_L1I_CACHEGEOMETRY, "L1 inst. cache geometry"),
#endif
#ifdef AT_L1I_CACHESIZE
	AUX_NUM(AT_L1I_CACHESIZE,     "L1 inst. cache size"),
#endif
#ifdef AT_L2_CACHEGEOMETRY
	AUX_HEX(AT_L2_CACHEGEOMETRY,  "L2 cache geometry"),
#endif
#ifdef AT_L2_CACHESIZE
	AUX_NUM(AT_L2_CACHESIZE,      "L2 cache size"),
#endif
#ifdef AT_L3_CACHEGEOMETRY
	AUX_HEX(AT_L3_CACHEGEOMETRY,  "L3 cache geometry"),
#endif
#ifdef AT_L3_CACHESIZE
	AUX_NUM(AT_L3_CACHESIZE,      "L3 cache size"),
#endif
#ifdef AT_FPUCW
	AUX_NUM(AT_FPUCW,             "used FPU control word"),
#endif
#ifdef AT_SYSINFO
	AUX_HEX(AT_SYSINFO,           "entry point to syscall in the vDSO"),
#endif
#ifdef AT_SYSINFO_EHDR
	AUX_HEX(AT_SYSINFO_EHDR,      "address of vDSO page"),
#endif
	{ AT_NULL, "(unknown)", handle_print,
	  "unknown field", "%-16s %2lu %-15lu\n" }
};

#undef AUX_HANDLER
#undef AUX_NUM
#undef AUX_STR


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

	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;
}