Re: [FFmpeg-devel] [PATCH] lavu/random_seed: use getrandom() when available

[James Almer <jamrial@gmail.com>]

On 7/9/2023 7:06 AM, Anton Khirnov wrote:
> It is a better interface for /dev/u?random on Linux, which avoids the
> issues associated with opening files.
> ---
>   configure               |  2 ++
>   libavutil/random_seed.c | 16 ++++++++++++++++
>   2 files changed, 18 insertions(+)
> 
> diff --git a/configure b/configure
> index 0ab0761011..f8c809e491 100755
> --- a/configure
> +++ b/configure
> @@ -2310,6 +2310,7 @@ SYSTEM_FUNCS="
>       getauxval
>       getenv
>       gethrtime
> +    getrandom
>       getopt
>       GetModuleHandle
>       GetProcessAffinityMask
> @@ -6387,6 +6388,7 @@ check_func  fcntl
>   check_func  fork
>   check_func  gethrtime
>   check_func  getopt
> +check_func_headers "sys/random.h" getrandom
>   check_func  getrusage
>   check_func  gettimeofday
>   check_func  isatty
> diff --git a/libavutil/random_seed.c b/libavutil/random_seed.c
> index f167b172f2..92bf9ef8e1 100644
> --- a/libavutil/random_seed.c
> +++ b/libavutil/random_seed.c
> @@ -35,6 +35,9 @@
>   #elif CONFIG_OPENSSL
>   #include <openssl/rand.h>
>   #endif
> +#if HAVE_GETRANDOM
> +#include <sys/random.h>
> +#endif
>   #include <fcntl.h>
>   #include <math.h>
>   #include <time.h>
> @@ -51,6 +54,7 @@
>   #define TEST 0
>   #endif
>   
> +#if !HAVE_GETRANDOM
>   static int read_random(uint8_t *dst, size_t len, const char *file)
>   {
>   #if HAVE_UNISTD_H
> @@ -70,6 +74,7 @@ static int read_random(uint8_t *dst, size_t len, const char *file)
>       return AVERROR(ENOSYS);
>   #endif
>   }
> +#endif
>   
>   static uint32_t get_generic_seed(void)
>   {
> @@ -147,7 +152,18 @@ int av_random_bytes(uint8_t* buf, size_t len)
>       return 0;
>   #endif
>   
> +    // getrandom() is a better interface for /dev/(u)random on Linux,
> +    // so it makes no sense to try both
> +#if HAVE_GETRANDOM
> +    {
> +        ssize_t read = getrandom(buf, len, GRND_NONBLOCK);
> +        if (read < 0)
> +            return errno == EAGAIN ? AVERROR_EXTERNAL : AVERROR(errno);
> +        return read != len ? AVERROR_UNKNOWN : 0;

This is returning no matter what the result, when it should only return 
on success (the !err check below), or fallback to the other 
implementations otherwise.

> +    }
> +#else
>       err = read_random(buf, len, "/dev/urandom");
> +#endif
>       if (!err)
>           return err;
>   
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