e1e227307
Denys Vlasenko
2009-06-19
Subject
mkswap: improve randomness of UUID generation set version/variant bits
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util-linux/mkswap.c+55 -4
| @@ -53,14 +53,65 @@ static void mkswap_selinux_setcontext(int fd, const char *path) | |
| #if ENABLE_DESKTOP | |
| static void mkswap_generate_uuid(uint8_t *buf) | |
| { | |
| + | pid_t pid; |
| unsigned i; | |
| char uuid_string[32]; | |
| - | /* rand() is guaranteed to generate at least [0, 2^15) range, |
| + | /* http://www.ietf.org/rfc/rfc4122.txt |
| + | * 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 |
| + | * +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| + | * | time_low | |
| + | * +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| + | * | time_mid | time_hi_and_version | |
| + | * +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| + | * |clk_seq__and_variant | node (0-1) | |
| + | * +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| + | * | node (2-5) | |
| + | * +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| + | * IOW, uuid has this layout: |
| + | * uint32_t time_low (big endian) |
| + | * uint16_t time_mid (big endian) |
| + | * uint16_t time_hi_and_version (big endian) |
| + | * version is a 4-bit field: |
| + | * 1 Time-based version |
| + | * 2 DCE Security version, with embedded POSIX UIDs |
| + | * 3 Name-based version (MD5) |
| + | * 4 Randomly generated version |
| + | * 5 Name-based version (SHA-1) |
| + | * uint16_t clk_seq_and_variant (big endian) |
| + | * variant is a 3-bit field: |
| + | * 0xx Reserved, NCS backward compatibility |
| + | * 10x The variant specified in rfc4122 |
| + | * 110 Reserved, Microsoft backward compatibility |
| + | * 111 Reserved for future definition |
| + | * uint8_t node[6] |
| + | * |
| + | * For version 4, these bits are set/cleared: |
| + | * time_hi_and_version & 0x0fff | 0x4000 |
| + | * clk_seq_and_variant & 0x3fff | 0x8000 |
| + | */ |
| + | |
| + | i = open("/dev/urandom", O_RDONLY); |
| + | if (i >= 0) { |
| + | read(i, buf, 16); |
| + | close(i); |
| + | } |
| + | /* Paranoia. /dev/urandom may be missing. |
| + | * rand() is guaranteed to generate at least [0, 2^15) range, |
| * but lowest bits in some libc are not so "random". */ | |
| - | srand((unsigned)monotonic_us() + getpid()); |
| - | for (i = 0; i < 16; i++) |
| - | buf[i] = rand() >> 5; |
| + | srand(monotonic_us()); |
| + | pid = getpid(); |
| + | while (1) { |
| + | for (i = 0; i < 16; i++) |
| + | buf[i] ^= rand() >> 5; |
| + | if (pid == 0) |
| + | break; |
| + | srand(pid); |
| + | pid = 0; |
| + | } |
| + | |
| + | buf[4 + 2 ] = (buf[4 + 2 ] & 0x0f) | 0x40; /* time_hi_and_version */ |
| + | buf[4 + 2 + 2] = (buf[4 + 2 + 2] & 0x3f) | 0x80; /* clk_seq_and_variant */ |
| bin2hex(uuid_string, (void*) buf, 16); | |
| /* f.e. UUID=dfd9c173-be52-4d27-99a5-c34c6c2ff55f */ | |