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mkswap: improve randomness of UUID generation set version/variant bits

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function                                             old     new   delta
mkswap_main                                          317     410     +93

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
This commit is contained in:
Denys Vlasenko 2009-06-19 09:49:01 +02:00
parent bf4aeed129
commit e1e2273076
1 changed files with 55 additions and 4 deletions
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57
util-linux/mkswap.c
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@ -53,14 +53,65 @@ static void mkswap_selinux_setcontext(int fd, const char *path)
#if ENABLE_DESKTOP #if ENABLE_DESKTOP
static void mkswap_generate_uuid(uint8_t *buf) static void mkswap_generate_uuid(uint8_t *buf)
{ {
pid_t pid;
unsigned i; unsigned i;
char uuid_string[32]; 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". */ * but lowest bits in some libc are not so "random". */
srand((unsigned)monotonic_us() + getpid()); srand(monotonic_us());
pid = getpid();
while (1) {
for (i = 0; i < 16; i++) for (i = 0; i < 16; i++)
buf[i] = rand() >> 5; 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); bin2hex(uuid_string, (void*) buf, 16);
/* f.e. UUID=dfd9c173-be52-4d27-99a5-c34c6c2ff55f */ /* f.e. UUID=dfd9c173-be52-4d27-99a5-c34c6c2ff55f */