Re: [Qemu-devel] [PATCH] linux-user: Let user specify random seed

[Eric Blake <eblake@redhat.com>]

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On 10/08/2014 06:13 AM, Magnus Reftel wrote:
> This patch introduces the -seed command line option and the
> QEMU_RAND_SEED environment variable for setting the random seed, which
> is used for the AT_RANDOM ELF aux entry.
> 
> Signed-off-by: Magnus Reftel <reftel@spotify.com>
> ---
>  linux-user/elfload.c |  1 -
>  linux-user/main.c    | 21 +++++++++++++++++++++
>  2 files changed, 21 insertions(+), 1 deletion(-)
> 

> +++ b/linux-user/main.c
> @@ -46,6 +46,8 @@ unsigned long mmap_min_addr;
>  #if defined(CONFIG_USE_GUEST_BASE)
>  unsigned long guest_base;
>  int have_guest_base;
> +static bool have_rand_seed = false;

static variables are automatically 0-initialized without needing an
explicit initializer.

> +static int rand_seed;
>  #if (TARGET_LONG_BITS == 32) && (HOST_LONG_BITS == 64)
>  /*
>   * When running 32-on-64 we should make sure we can fit all of the possible
> @@ -3546,6 +3548,12 @@ static void handle_arg_pagesize(const char *arg)
>      }
>  }
>  
> +static void handle_arg_randseed(const char *arg)
> +{
> +    have_rand_seed = true;
> +    rand_seed = atoi(arg);
> +}

atoi() is trash when compared to strtol() - it doesn't diagnose
overflow, trailing garbage, or empty input.


> @@ -3926,6 +3936,17 @@ int main(int argc, char **argv, char **envp)
>          do_strace = 1;
>      }
>  
> +    if (getenv("QEMU_RAND_SEED")) {
> +        have_rand_seed = true;
> +        rand_seed = atoi(getenv("QEMU_RAND_SEED"));
> +    }

why not call handle_arg_randseed(getenv("QEMU_RAND_SEED")) here?

> +
> +    if (have_rand_seed) {
> +        srand(rand_seed);
> +    } else {
> +        srand((int)time(NULL));

The cast is pointless.  This is C.

> +    }

Do you even need have_rand_seed?  Why not just pre-initialize
rand_seed=time(NULL) and then overwrite rand_seed if the environment
variable is present?

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org


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