[FFmpeg-devel] [PATCH] Make av_get_random_seed not block when waiting for more entropy

[Måns Rullgård]

Martin Storsj? <martin at martin.st> writes:

> Hi,
>
> I recently ran into a situation where av_get_random_seed() blocks for long 
> times if a machine is out of entropy. Since most (all?) uses of 
> av_get_random_seed() within ffmpeg don't really need full cryptographic 
> strength, using /dev/urandom instead of blocking while waiting for data 
> from /dev/random - the attached patch does this.
>
> // Martin
>
> From d793a93fd6f8504e9a454c109ca992fb59b2c104 Mon Sep 17 00:00:00 2001
> From: Martin Storsjo <martin at martin.st>
> Date: Tue, 29 Jun 2010 15:21:35 +0300
> Subject: [PATCH] Make av_get_random_seed not block when waiting for more entropy
>
> The /dev/random device currently used for gathering entropy may block
> for (very) long times if the kernel is out of entropy.
> ---
>  libavutil/random_seed.c |    1 -
>  1 files changed, 0 insertions(+), 1 deletions(-)
>
> diff --git a/libavutil/random_seed.c b/libavutil/random_seed.c
> index 00d6317..7f82a4f 100644
> --- a/libavutil/random_seed.c
> +++ b/libavutil/random_seed.c
> @@ -29,7 +29,6 @@ uint32_t av_get_random_seed(void)
>      uint32_t seed;
>      int fd;
>  
> -    if ((fd = open("/dev/random", O_RDONLY)) == -1)
>          fd = open("/dev/urandom", O_RDONLY);
>      if (fd != -1){
>          int err = read(fd, &seed, 4);

Neither /dev/random nor /dev/urandom are standard devices and may or
may not be available on a given system.  Attempting both is probably a
good idea.  Since we don't need cryptographically strong random
numbers, giving priority to /dev/urandom makes sense.

-- 
M?ns Rullg?rd
mans at mansr.com