/* Based on code from <https://git.zx2c4.com/seedrng/about/>. */

#include <linux/random.h>
#include <sys/random.h>
#include <sys/ioctl.h>
#include <sys/file.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <fcntl.h>
#include <poll.h>
#include <unistd.h>
#include <time.h>
#include <errno.h>
#include <endian.h>
#include <stdbool.h>
#include <stdint.h>
#include <string.h>
#include <stdio.h>
#include <stdlib.h>

#ifndef LOCALSTATEDIR
#define LOCALSTATEDIR "/var/lib"
#endif

#define SEED_DIR LOCALSTATEDIR "/seedrng"
#define CREDITABLE_SEED "seed.credit"
#define NON_CREDITABLE_SEED "seed.no-credit"

enum blake2s_lengths {
	BLAKE2S_BLOCK_LEN = 64,
	BLAKE2S_HASH_LEN = 32,
	BLAKE2S_KEY_LEN = 32
};

enum seedrng_lengths {
	MAX_SEED_LEN = 512,
	MIN_SEED_LEN = BLAKE2S_HASH_LEN
};

struct blake2s_state {
	uint32_t h[8];
	uint32_t t[2];
	uint32_t f[2];
	uint8_t buf[BLAKE2S_BLOCK_LEN];
	unsigned int buflen;
	unsigned int outlen;
};

#define le32_to_cpup(a) le32toh(*(a))
#define cpu_to_le32(a) htole32(a)
#ifndef ARRAY_SIZE
#define ARRAY_SIZE(x) (sizeof(x) / sizeof((x)[0]))
#endif
#ifndef DIV_ROUND_UP
#define DIV_ROUND_UP(n, d) (((n) + (d) - 1) / (d))
#endif

static inline void cpu_to_le32_array(uint32_t *buf, unsigned int words)
{
	while (words--) {
		*buf = cpu_to_le32(*buf);
		++buf;
	}
}

static inline void le32_to_cpu_array(uint32_t *buf, unsigned int words)
{
	while (words--) {
		*buf = le32_to_cpup(buf);
		++buf;
	}
}

static inline uint32_t ror32(uint32_t word, unsigned int shift)
{
	return (word >> (shift & 31)) | (word << ((-shift) & 31));
}

static const uint32_t blake2s_iv[8] = {
	0x6A09E667UL, 0xBB67AE85UL, 0x3C6EF372UL, 0xA54FF53AUL,
	0x510E527FUL, 0x9B05688CUL, 0x1F83D9ABUL, 0x5BE0CD19UL
};

static const uint8_t blake2s_sigma[10][16] = {
	{ 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15 },
	{ 14, 10, 4, 8, 9, 15, 13, 6, 1, 12, 0, 2, 11, 7, 5, 3 },
	{ 11, 8, 12, 0, 5, 2, 15, 13, 10, 14, 3, 6, 7, 1, 9, 4 },
	{ 7, 9, 3, 1, 13, 12, 11, 14, 2, 6, 5, 10, 4, 0, 15, 8 },
	{ 9, 0, 5, 7, 2, 4, 10, 15, 14, 1, 11, 12, 6, 8, 3, 13 },
	{ 2, 12, 6, 10, 0, 11, 8, 3, 4, 13, 7, 5, 15, 14, 1, 9 },
	{ 12, 5, 1, 15, 14, 13, 4, 10, 0, 7, 6, 3, 9, 2, 8, 11 },
	{ 13, 11, 7, 14, 12, 1, 3, 9, 5, 0, 15, 4, 8, 6, 2, 10 },
	{ 6, 15, 14, 9, 11, 3, 0, 8, 12, 2, 13, 7, 1, 4, 10, 5 },
	{ 10, 2, 8, 4, 7, 6, 1, 5, 15, 11, 9, 14, 3, 12, 13, 0 },
};

static void blake2s_set_lastblock(struct blake2s_state *state)
{
	state->f[0] = -1;
}

static void blake2s_increment_counter(struct blake2s_state *state, const uint32_t inc)
{
	state->t[0] += inc;
	state->t[1] += (state->t[0] < inc);
}

static void blake2s_init_param(struct blake2s_state *state, const uint32_t param)
{
	int i;

	memset(state, 0, sizeof(*state));
	for (i = 0; i < 8; ++i)
		state->h[i] = blake2s_iv[i];
	state->h[0] ^= param;
}

static void blake2s_init(struct blake2s_state *state, const size_t outlen)
{
	blake2s_init_param(state, 0x01010000 | outlen);
	state->outlen = outlen;
}

static void blake2s_compress(struct blake2s_state *state, const uint8_t *block, size_t nblocks, const uint32_t inc)
{
	uint32_t m[16];
	uint32_t v[16];
	int i;

	while (nblocks > 0) {
		blake2s_increment_counter(state, inc);
		memcpy(m, block, BLAKE2S_BLOCK_LEN);
		le32_to_cpu_array(m, ARRAY_SIZE(m));
		memcpy(v, state->h, 32);
		v[ 8] = blake2s_iv[0];
		v[ 9] = blake2s_iv[1];
		v[10] = blake2s_iv[2];
		v[11] = blake2s_iv[3];
		v[12] = blake2s_iv[4] ^ state->t[0];
		v[13] = blake2s_iv[5] ^ state->t[1];
		v[14] = blake2s_iv[6] ^ state->f[0];
		v[15] = blake2s_iv[7] ^ state->f[1];

#define G(r, i, a, b, c, d) do { \
	a += b + m[blake2s_sigma[r][2 * i + 0]]; \
	d = ror32(d ^ a, 16); \
	c += d; \
	b = ror32(b ^ c, 12); \
	a += b + m[blake2s_sigma[r][2 * i + 1]]; \
	d = ror32(d ^ a, 8); \
	c += d; \
	b = ror32(b ^ c, 7); \
} while (0)

#define ROUND(r) do { \
	G(r, 0, v[0], v[ 4], v[ 8], v[12]); \
	G(r, 1, v[1], v[ 5], v[ 9], v[13]); \
	G(r, 2, v[2], v[ 6], v[10], v[14]); \
	G(r, 3, v[3], v[ 7], v[11], v[15]); \
	G(r, 4, v[0], v[ 5], v[10], v[15]); \
	G(r, 5, v[1], v[ 6], v[11], v[12]); \
	G(r, 6, v[2], v[ 7], v[ 8], v[13]); \
	G(r, 7, v[3], v[ 4], v[ 9], v[14]); \
} while (0)
		ROUND(0);
		ROUND(1);
		ROUND(2);
		ROUND(3);
		ROUND(4);
		ROUND(5);
		ROUND(6);
		ROUND(7);
		ROUND(8);
		ROUND(9);

#undef G
#undef ROUND

		for (i = 0; i < 8; ++i)
			state->h[i] ^= v[i] ^ v[i + 8];

		block += BLAKE2S_BLOCK_LEN;
		--nblocks;
	}
}

static void blake2s_update(struct blake2s_state *state, const void *inp, size_t inlen)
{
	const size_t fill = BLAKE2S_BLOCK_LEN - state->buflen;
	const uint8_t *in = inp;

	if (!inlen)
		return;
	if (inlen > fill) {
		memcpy(state->buf + state->buflen, in, fill);
		blake2s_compress(state, state->buf, 1, BLAKE2S_BLOCK_LEN);
		state->buflen = 0;
		in += fill;
		inlen -= fill;
	}
	if (inlen > BLAKE2S_BLOCK_LEN) {
		const size_t nblocks = DIV_ROUND_UP(inlen, BLAKE2S_BLOCK_LEN);
		blake2s_compress(state, in, nblocks - 1, BLAKE2S_BLOCK_LEN);
		in += BLAKE2S_BLOCK_LEN * (nblocks - 1);
		inlen -= BLAKE2S_BLOCK_LEN * (nblocks - 1);
	}
	memcpy(state->buf + state->buflen, in, inlen);
	state->buflen += inlen;
}

static void blake2s_final(struct blake2s_state *state, uint8_t *out)
{
	blake2s_set_lastblock(state);
	memset(state->buf + state->buflen, 0, BLAKE2S_BLOCK_LEN - state->buflen);
	blake2s_compress(state, state->buf, 1, state->buflen);
	cpu_to_le32_array(state->h, ARRAY_SIZE(state->h));
	memcpy(out, state->h, state->outlen);
}

static ssize_t getrandom_full(void *buf, size_t count, unsigned int flags)
{
	ssize_t ret, total = 0;
	uint8_t *p = buf;

	do {
		ret = getrandom(p, count, flags);
		if (ret < 0 && errno == EINTR)
			continue;
		else if (ret < 0)
			return ret;
		total += ret;
		p += ret;
		count -= ret;
	} while (count);
	return total;
}

static ssize_t read_full(int fd, void *buf, size_t count)
{
	ssize_t ret, total = 0;
	uint8_t *p = buf;

	do {
		ret = read(fd, p, count);
		if (ret < 0 && errno == EINTR)
			continue;
		else if (ret < 0)
			return ret;
		else if (ret == 0)
			break;
		total += ret;
		p += ret;
		count -= ret;
	} while (count);
	return total;
}

static ssize_t write_full(int fd, const void *buf, size_t count)
{
	ssize_t ret, total = 0;
	const uint8_t *p = buf;

	do {
		ret = write(fd, p, count);
		if (ret < 0 && errno == EINTR)
			continue;
		else if (ret < 0)
			return ret;
		total += ret;
		p += ret;
		count -= ret;
	} while (count);
	return total;
}

static size_t determine_optimal_seed_len(void)
{
	size_t ret = 0;
	char poolsize_str[11] = { 0 };
	int fd = open("/proc/sys/kernel/random/poolsize", O_RDONLY);

	if (fd < 0 || read_full(fd, poolsize_str, sizeof(poolsize_str) - 1) < 0) {
		perror("Unable to determine pool size, falling back to 256 bits");
		ret = MIN_SEED_LEN;
	} else
		ret = DIV_ROUND_UP(strtoul(poolsize_str, NULL, 10), 8);
	if (fd >= 0)
		close(fd);
	if (ret < MIN_SEED_LEN)
		ret = MIN_SEED_LEN;
	else if (ret > MAX_SEED_LEN)
		ret = MAX_SEED_LEN;
	return ret;
}

static int read_new_seed(uint8_t *seed, size_t len, bool *is_creditable)
{
	ssize_t ret;
	int urandom_fd;

	*is_creditable = false;
	ret = getrandom_full(seed, len, GRND_NONBLOCK);
	if (ret == (ssize_t)len) {
		*is_creditable = true;
		return 0;
	} else if (ret < 0 && errno == ENOSYS) {
		struct pollfd random_fd = {
			.fd = open("/dev/random", O_RDONLY),
			.events = POLLIN
		};
		if (random_fd.fd < 0)
			return -errno;
		*is_creditable = poll(&random_fd, 1, 0) == 1;
		close(random_fd.fd);
	} else if (getrandom_full(seed, len, GRND_INSECURE) == (ssize_t)len)
		return 0;
	urandom_fd = open("/dev/urandom", O_RDONLY);
	if (urandom_fd < 0)
		return -1;
	ret = read_full(urandom_fd, seed, len);
	if (ret == (ssize_t)len)
		ret = 0;
	else
		ret = -errno ? -errno : -EIO;
	close(urandom_fd);
	errno = -ret;
	return ret ? -1 : 0;
}

static int seed_rng(uint8_t *seed, size_t len, bool credit)
{
	struct {
		int entropy_count;
		int buf_size;
		uint8_t buffer[MAX_SEED_LEN];
	} req = {
		.entropy_count = credit ? len * 8 : 0,
		.buf_size = len
	};
	int random_fd, ret;

	if (len > sizeof(req.buffer)) {
		errno = EFBIG;
		return -1;
	}
	memcpy(req.buffer, seed, len);

	random_fd = open("/dev/urandom", O_RDONLY);
	if (random_fd < 0)
		return -1;
	ret = ioctl(random_fd, RNDADDENTROPY, &req);
	if (ret)
		ret = -errno ? -errno : -EIO;
	close(random_fd);
	errno = -ret;
	return ret ? -1 : 0;
}

static int seed_from_file_if_exists(const char *filename, int dfd, bool credit, struct blake2s_state *hash)
{
	uint8_t seed[MAX_SEED_LEN];
	ssize_t seed_len;
	int fd = -1, ret = 0;

	fd = openat(dfd, filename, O_RDONLY);
	if (fd < 0 && errno == ENOENT)
		return 0;
	else if (fd < 0) {
		ret = -errno;
		perror("Unable to open seed file");
		goto out;
	}
	seed_len = read_full(fd, seed, sizeof(seed));
	if (seed_len < 0) {
		ret = -errno;
		perror("Unable to read seed file");
		goto out;
	}
	if ((unlinkat(dfd, filename, 0) < 0 || fsync(dfd) < 0) && seed_len) {
		ret = -errno;
		perror("Unable to remove seed after reading, so not seeding");
		goto out;
	}
	if (!seed_len)
		goto out;

	blake2s_update(hash, &seed_len, sizeof(seed_len));
	blake2s_update(hash, seed, seed_len);

	printf("Seeding %zd bits %s crediting\n", seed_len * 8, credit ? "and" : "without");
	if (seed_rng(seed, seed_len, credit) < 0) {
		ret = -errno;
		perror("Unable to seed");
	}

out:
	if (fd >= 0)
		close(fd);
	errno = -ret;
	return ret ? -1 : 0;
}

static bool skip_credit(void)
{
	const char *skip = getenv("SEEDRNG_SKIP_CREDIT");
	return skip && (!strcmp(skip, "1") || !strcasecmp(skip, "true") ||
			!strcasecmp(skip, "yes") || !strcasecmp(skip, "y"));
}

int main(int argc __attribute__((unused)), char *argv[] __attribute__((unused)))
{
	static const char seedrng_prefix[] = "SeedRNG v1 Old+New Prefix";
	static const char seedrng_failure[] = "SeedRNG v1 No New Seed Failure";
	int fd = -1, dfd = -1, program_ret = 0;
	uint8_t new_seed[MAX_SEED_LEN];
	size_t new_seed_len;
	bool new_seed_creditable;
	struct timespec realtime = { 0 }, boottime = { 0 };
	struct blake2s_state hash;

	umask(0077);
	if (getuid()) {
		errno = EACCES;
		perror("This program requires root");
		return 1;
	}

	blake2s_init(&hash, BLAKE2S_HASH_LEN);
	blake2s_update(&hash, seedrng_prefix, strlen(seedrng_prefix));
	clock_gettime(CLOCK_REALTIME, &realtime);
	clock_gettime(CLOCK_BOOTTIME, &boottime);
	blake2s_update(&hash, &realtime, sizeof(realtime));
	blake2s_update(&hash, &boottime, sizeof(boottime));

	if (mkdir(SEED_DIR, 0700) < 0 && errno != EEXIST) {
		perror("Unable to create seed directory");
		return 1;
	}

	dfd = open(SEED_DIR, O_DIRECTORY | O_RDONLY);
	if (dfd < 0 || flock(dfd, LOCK_EX) < 0) {
		perror("Unable to lock seed directory");
		program_ret = 1;
		goto out;
	}

	if (seed_from_file_if_exists(NON_CREDITABLE_SEED, dfd, false, &hash) < 0)
		program_ret |= 1 << 1;
	if (seed_from_file_if_exists(CREDITABLE_SEED, dfd, !skip_credit(), &hash) < 0)
		program_ret |= 1 << 2;

	new_seed_len = determine_optimal_seed_len();
	if (read_new_seed(new_seed, new_seed_len, &new_seed_creditable) < 0) {
		perror("Unable to read new seed");
		new_seed_len = BLAKE2S_HASH_LEN;
		strncpy((char *)new_seed, seedrng_failure, new_seed_len);
		program_ret |= 1 << 3;
	}
	blake2s_update(&hash, &new_seed_len, sizeof(new_seed_len));
	blake2s_update(&hash, new_seed, new_seed_len);
	blake2s_final(&hash, new_seed + new_seed_len - BLAKE2S_HASH_LEN);

	printf("Saving %zu bits of %s seed for next boot\n", new_seed_len * 8, new_seed_creditable ? "creditable" : "non-creditable");
	fd = openat(dfd, NON_CREDITABLE_SEED, O_WRONLY | O_CREAT | O_TRUNC, 0400);
	if (fd < 0) {
		perror("Unable to open seed file for writing");
		program_ret |= 1 << 4;
		goto out;
	}
	if (write_full(fd, new_seed, new_seed_len) != (ssize_t)new_seed_len || fsync(fd) < 0) {
		perror("Unable to write seed file");
		program_ret |= 1 << 5;
		goto out;
	}
	if (new_seed_creditable && renameat(dfd, NON_CREDITABLE_SEED, dfd, CREDITABLE_SEED) < 0) {
		perror("Unable to make new seed creditable");
		program_ret |= 1 << 6;
	}
out:
	if (fd >= 0)
		close(fd);
	if (dfd >= 0)
		close(dfd);
	return program_ret;
}