On Wed, May 25, 2016 at 12:14 AM, Qu Wenruo <quwenruo@xxxxxxxxxxxxxx> wrote:
> David has reported some quite chaos usage of pseudo random numbers.
> Like using static srand seed, or even calling rand() without setting
> seed correctly.
>
> The new pseudo random API will initialize the random seed on its first
> calling and use uniformly distributed pseudo random number generator as
> backend.
>
> Signed-off-by: Qu Wenruo <quwenruo@xxxxxxxxxxxxxx>
> ---
> utils.c | 36 ++++++++++++++++++++++++++++++++++++
> utils.h | 36 ++++++++++++++++++++++++++++++++++++
> 2 files changed, 72 insertions(+)
>
> diff --git a/utils.c b/utils.c
> index 7761165..c0e860a 100644
> --- a/utils.c
> +++ b/utils.c
> @@ -54,9 +54,11 @@
> #define BLKDISCARD _IO(0x12,119)
> #endif
>
> +static int seed_initlized = 0;
> static int btrfs_scan_done = 0;
>
> static char argv0_buf[ARGV0_BUF_SIZE] = "btrfs";
> +static unsigned short seeds[3];
>
> const char *get_argv0_buf(void)
> {
> @@ -4051,3 +4053,37 @@ out:
>
> return ret;
> }
> +
> +void rand_seed(u64 seed)
> +{
> + int i;
> + /* only use the last 48 bits */
> + for (i = 0; i < 3; i++) {
> + seeds[i] = (unsigned short)(seed ^ (unsigned short)(-1));
> + seed >>= 16;
> + }
> + seed_initlized = 1;
> +}
> +
> +u32 rand_u32(void)
> +{
> + struct timeval tv;
> +
> + if (seed_initlized)
> + return nrand48(seeds);
> +
Missing a (u32) cast.
> + /*
> + * It's possible to use /dev/random, but we don't need that true
> + * random number nor want to wait for entropy,
Sounds like a good candidate for /dev/urandom, then?
> + * since we're only using random API to do corruption to test.
> + * Time and pid/ppid based seed would be good enough, and won't
> + * cause sleep for entropy pool.
> + */
> + gettimeofday(&tv, 0);
> + seeds[0] = getpid() ^ (tv.tv_sec & 0xFFFF);
> + seeds[1] = getppid() ^ (tv.tv_usec & 0xFFFF);
> + seeds[2] = (tv.tv_sec ^ tv.tv_usec) >> 16;
> + seed_initlized = 1;
> +
> + return (u32)nrand48(seeds);
> +}
> diff --git a/utils.h b/utils.h
> index ebe6d61..0977262 100644
> --- a/utils.h
> +++ b/utils.h
> @@ -362,4 +362,40 @@ static inline int error_on(int condition, const char *fmt, ...)
> return 1;
> }
>
> +/* pseudo random number generator wrappers */
> +u32 rand_u32(void);
> +
> +static inline int rand_int(void)
> +{
> + return (int)(rand_u32());
> +}
> +
> +static inline u64 rand_u64(void)
> +{
> + u64 ret = 0;
> +
> + ret += rand_u32();
> + ret <<= 32;
> + ret += rand_u32();
> + return ret;
> +}
> +
> +static inline u16 rand_u16(void)
> +{
> + return (u16)(rand_u32());
> +}
> +
> +static inline u8 rand_u8(void)
> +{
> + return (u8)(rand_u32());
> +}
> +
> +/* Return random number in range [0, limit) */
> +static inline unsigned int rand_range(unsigned int up)
> +{
> + return (unsigned int)(rand_u32() % up);
> +}
Wouldn't distribution be more uniform if you took the full 48 bits
from nrand64(), and only downcast after the mod?
> +
> +/* Also allow setting seeds manually */
> +void rand_seed(u64 seed);
> #endif
> --
> 2.8.2
>
>
>
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