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

[Marton Balint <cus@passwd.hu>]

On Fri, 7 Jul 2023, Anton Khirnov wrote:

> It is a better interface for /dev/u?random on Linux, which avoids the
> issues associated with opening files.


getrandom() actually have the same problem as read(). It can read less 
than requested. So you should use it in a loop in that case or if it 
returns EINTR.

Regards,
Marton


> ---
> configure               |  2 ++
> libavutil/random_seed.c | 15 +++++++++++++++
> 2 files changed, 17 insertions(+)
>
> diff --git a/configure b/configure
> index d6e78297fe..a4b09577cf 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 2980e565e0..9a3a5aa133 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 +152,17 @@ 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);
> +        err = read < 0    ? AVERROR(errno)  :
> +              read != len ? AVERROR_UNKNOWN : 0;
> +    }
> +#else
>     err = read_random(buf, len, "/dev/urandom");
> +#endif
>     if (!err)
>         return err;
>
> -- 
> 2.40.1
>
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