[Mesa-dev] [PATCH V2 2/3] util/rand_xor: add function to seed rand

[Nicolai Hähnle]

On 22.03.2017 00:59, Timothy Arceri wrote:
> V2: pass the seed to the seed function so that we can isolate
>     its uses. Stop leaking fd when urandom couldn't be read.
> ---
>  src/gallium/drivers/radeon/r600_test_dma.c |  3 +-
>  src/util/rand_xor.c                        | 57 ++++++++++++++++++++++++++++++
>  src/util/rand_xor.h                        |  4 +++
>  3 files changed, 62 insertions(+), 2 deletions(-)
>
> diff --git a/src/gallium/drivers/radeon/r600_test_dma.c b/src/gallium/drivers/radeon/r600_test_dma.c
> index 8b4149a..3c23b09 100644
> --- a/src/gallium/drivers/radeon/r600_test_dma.c
> +++ b/src/gallium/drivers/radeon/r600_test_dma.c
> @@ -176,22 +176,21 @@ void r600_test_dma(struct r600_common_screen *rscreen)
>
>  	max_levels = screen->get_param(screen, PIPE_CAP_MAX_TEXTURE_2D_LEVELS);
>  	max_tex_side = 1 << (max_levels - 1);
>
>  	/* Max 128 MB allowed for both textures. */
>  	max_alloc_size = 128 * 1024 * 1024;
>
>  	/* the seed for random test parameters */
>  	srand(0x9b47d95b);
>  	/* the seed for random pixel data */
> -	seed_xorshift128plus[0] = 0x3bffb83978e24f88;
> -	seed_xorshift128plus[1] = 0x9238d5d56c71cd35;
> +	s_rand_xorshift128plus(seed_xorshift128plus, false);
>
>  	iterations = 1000000000; /* just kill it when you are bored */
>  	num_partial_copies = 30;
>
>  	/* These parameters are randomly generated per test:
>  	 * - whether to do one whole-surface copy or N partial copies per test
>  	 * - which tiling modes to use (LINEAR_ALIGNED, 1D, 2D)
>  	 * - which texture dimensions to use
>  	 * - whether to use VRAM (all tiling modes) and GTT (staging, linear
>  	 *   only) allocations
> diff --git a/src/util/rand_xor.c b/src/util/rand_xor.c
> index 07b4c22..ee506a2 100644
> --- a/src/util/rand_xor.c
> +++ b/src/util/rand_xor.c
> @@ -1,20 +1,77 @@
> +/*
> + * Copyright 2017 Timothy Arceri
> + *
> + * Permission is hereby granted, free of charge, to any person obtaining a
> + * copy of this software and associated documentation files (the "Software"),
> + * to deal in the Software without restriction, including without limitation
> + * the rights to use, copy, modify, merge, publish, distribute, sublicense,
> + * and/or sell copies of the Software, and to permit persons to whom the
> + * Software is furnished to do so, subject to the following conditions:
> + *
> + * The above copyright notice and this permission notice (including the next
> + * paragraph) shall be included in all copies or substantial portions of the
> + * Software.
> + *
> + * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
> + * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
> + * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.  IN NO EVENT SHALL
> + * THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
> + * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
> + * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
> + * SOFTWARE.
> + *
> + */
> +
> +#if defined(__linux__)
> +#include <sys/file.h>
> +#include <unistd.h>
> +#endif
> +
>  #include "rand_xor.h"
>
>  /* Super fast random number generator.
>   *
>   * This rand_xorshift128plus function by Sebastiano Vigna belongs
>   * to the public domain.
>   */
>  uint64_t
>  rand_xorshift128plus(uint64_t *seed)
>  {
>     uint64_t *s = seed;
>
>     uint64_t s1 = s[0];
>     const uint64_t s0 = s[1];
>     s[0] = s0;
>     s1 ^= s1 << 23;
>     s[1] = s1 ^ s0 ^ (s1 >> 18) ^ (s0 >> 5);
>
>     return s[1] + s0;
>  }
> +
> +void
> +s_rand_xorshift128plus(uint64_t *seed, bool randomised_seed)
> +{
> +#if defined(__linux__)
> +   if (!randomised_seed)
> +      goto fixed_seed;
> +
> +   int fd = open("/dev/urandom", O_RDONLY);
> +   if (fd < 0)
> +      goto fixed_seed;
> +
> +   size_t seed_size = sizeof(uint64_t) * 2;
> +   if (read(fd, seed, seed_size) != seed_size) {
> +      close(fd);
> +      goto fixed_seed;
> +   }
> +
> +   close(fd);
> +   return;
> +
> +fixed_seed:
> +#endif
> +
> +   /* Fallback to a fixed seed */
> +   seed[0] = 0x3bffb83978e24f88;
> +   seed[1] = 0x9238d5d56c71cd35;
> +}

Actually, on second thought, could you add a fallback that just uses the 
time, so that non-__linux__ isn't completely screwed by this?

xorshift128+ has the property that all seeds are good, except the 
all-zeros one. So something like

     seed[0] = 0x3bffb83978e24f88;
     seed[1] = something derived from current time;

     /* Ensure that time-based entropy is spread across the
      * initial seed bits.
      */
     for (int i = 0; i < 8; ++i)
         rand_xorshift128plus(seed);

will be sufficient.

Thanks,
Nicolai


> diff --git a/src/util/rand_xor.h b/src/util/rand_xor.h
> index d5144e9..532d549 100644
> --- a/src/util/rand_xor.h
> +++ b/src/util/rand_xor.h
> @@ -19,15 +19,19 @@
>   * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
>   * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
>   * SOFTWARE.
>   *
>   */
>
>  #ifndef RAND_XOR_H
>  #define RAND_XOR_H
>
>  #include <stdint.h>
> +#include <stdbool.h>
>
>  uint64_t
>  rand_xorshift128plus(uint64_t *seed);
>
> +void
> +s_rand_xorshift128plus(uint64_t *seed, bool randomised_seed);
> +
>  #endif /* RAND_XOR_H */
>