Subject: Re: [PATCH 1/6] maintenance: add get_random_minute()

From: Junio C Hamano <[email-masked]>
Date: Tue, 08 Aug 2023 09:34:21 -0700

Taylor Blau <[email-masked]> writes:

> I think that's an acceptable price to pay here, since we can drop the
> code to remember whether or not srand() has been called or not. Here's a
> patch that we could take in that direction:
> ...
>   - If a different function also calls `srand()`, the PRNG may start
>     generating repeated values (if that caller also initialized the PRNG
>     with `getpid()`).

Hmph, I didn't think about that one.  I do not think it can be a
viable attack vector to attack _this_ code, but the other function
might be.  But if the other function is worth attacking and
attackable, it ought to be using crypto-secure one and not srand(),
so this argument may not give us a good justification X-<.

Provided if code simplification is a good enough rationale, the
patch looks sensible, but I find its use of u64 a bit questionable
(though not wrong).  I would have expected that the type of "rand"
would be the same as backoff_ms and wait_ms, two variables involved
in the same expression.

Thanks.

> diff --git a/lockfile.c b/lockfile.c
> index 1d5ed01682..6587d407f4 100644
> --- a/lockfile.c
> +++ b/lockfile.c
> @@ -107,22 +107,17 @@ static int lock_file_timeout(struct lock_file *lk, const char *path,
>  	int n = 1;
>  	int multiplier = 1;
>  	long remaining_ms = 0;
> -	static int random_initialized = 0;
>
>  	if (timeout_ms == 0)
>  		return lock_file(lk, path, flags, mode);
>
> -	if (!random_initialized) {
> -		srand((unsigned int)getpid());
> -		random_initialized = 1;
> -	}
> -
>  	if (timeout_ms > 0)
>  		remaining_ms = timeout_ms;
>
>  	while (1) {
>  		long backoff_ms, wait_ms;
>  		int fd;
> +		uint64_t rand;
>
>  		fd = lock_file(lk, path, flags, mode);
>
> @@ -135,7 +130,10 @@ static int lock_file_timeout(struct lock_file *lk, const char *path,
>
>  		backoff_ms = multiplier * INITIAL_BACKOFF_MS;
>  		/* back off for between 0.75*backoff_ms and 1.25*backoff_ms */
> -		wait_ms = (750 + rand() % 500) * backoff_ms / 1000;
> +		if (csprng_bytes(&rand, sizeof(uint64_t)) < 0)
> +			return error_errno(_("unable to get random bytes for"
> +					     "lockfile backoff"));
> +		wait_ms = (750 + rand % 500) * backoff_ms / 1000;
>  		sleep_millisec(wait_ms);
>  		remaining_ms -= wait_ms;
>
> --
> 2.42.0.rc0.26.g802d811bac.dirty
>
> --- >8 ---
>
> Thanks,
> Taylor