[FFmpeg-devel] [PATCH] random_seed: Limit the time taken by get_generic_seed

[Michael Niedermayer]

Hi Martin

On Wed, Jan 29, 2025 at 11:53:53AM +0200, Martin Storsjö wrote:
> On a Zen 5, on Ubuntu 24.04 (with CLOCKS_PER_SEC 1000000), the
> value of clock() in this loop increments by 0 most of the time,
> and when it does increment, it usually increments by 1 compared
> to the previous round.
> 
> Due to the "last_t + 2*last_td + (CLOCKS_PER_SEC > 1000) >= t"
> expression, we only manage to take one step forward in this loop
> (incrementing i) if clock() increments by 2, while it incremented
> by 0 in the previous iteration (last_td).
> 
> As we do mutate the buffer state even on loop iterations where we
> don't increment i, limit the number of times we consecutively can
> do this.
> 
> This is similar to the change done in
> c4152fc42e480c41efb7f761b1bbe5f0bc43d5bc, to speed it up on
> systems with very small CLOCKS_PER_SEC. However in this case,
> CLOCKS_PER_SEC is still very large, but the machine is fast enough
> to hit every clock increment repeatedly.
> 
> This makes sure that fate-random-seed actually terminates within a
> reasonable time on such a system (where it previously could hang,
> running for many minutes).
> ---
>  libavutil/random_seed.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/libavutil/random_seed.c b/libavutil/random_seed.c
> index 8a4e4f1fc0..8f969060a0 100644
> --- a/libavutil/random_seed.c
> +++ b/libavutil/random_seed.c
> @@ -83,6 +83,7 @@ static uint32_t get_generic_seed(void)
>      static uint32_t buffer[512] = { 0 };
>      unsigned char digest[20];
>      uint64_t last_i = i;
> +    int cur_iters = 0;
>  
>      av_assert0(sizeof(tmp) >= av_sha_size);
>  
> @@ -98,11 +99,13 @@ static uint32_t get_generic_seed(void)
>  
>      for (;;) {
>          clock_t t = clock();
> -        if (last_t + 2*last_td + (CLOCKS_PER_SEC > 1000) >= t) {
> +        if (last_t + 2*last_td + (CLOCKS_PER_SEC > 1000) >= t && cur_iters < 128) {
>              last_td = t - last_t;
>              buffer[i & 511] = 1664525*buffer[i & 511] + 1013904223 + (last_td % 3294638521U);
> +            cur_iters++;
>          } else {
>              last_td = t - last_t;
> +            cur_iters = 0;

Iam concerned this could negatively impact entropy
The "else" should be run when a interrupt/task switch occured.
If that doesnt occur in 128 iterations that doesnt gurantee the entropy
has increased.

If there are only 0 and 1, ideally we should look at the distribution and
go to the else when the pattern differs from the past / has some signs of
randomness

thx

[...]
-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

Awnsering whenever a program halts or runs forever is
On a turing machine, in general impossible (turings halting problem).
On any real computer, always possible as a real computer has a finite number
of states N, and will either halt in less than N cycles or never halt.
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