From: "Jason A. Donenfeld" <Jason@zx2c4.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: x86@kernel.org, Jann Horn <jannh@google.com>,
Natalie Silvanovich <natashenka@google.com>,
Seth Jenkins <sethjenkins@google.com>,
Kees Cook <keescook@chromium.org>,
linux-kernel@vger.kernel.org
Subject: Re: x86/mm: Randomize per-cpu entry area
Date: Fri, 7 Oct 2022 08:44:52 -0600 [thread overview]
Message-ID: <Y0A7ZAjtDcq2PJaX@zx2c4.com> (raw)
In-Reply-To: <Yz/mfJ1gjgshF19t@hirez.programming.kicks-ass.net>
On Fri, Oct 07, 2022 at 10:42:36AM +0200, Peter Zijlstra wrote:
> + max_cea = (CPU_ENTRY_AREA_MAP_SIZE - PAGE_SIZE) / CPU_ENTRY_AREA_SIZE;
> +
> + /* O(sodding terrible) */
> + for_each_possible_cpu(i) {
> + unsigned int cea;
> +
> +again:
> + cea = prandom_u32_max(max_cea);
Just FYI, max_cea is 2274876 (at least in my test rig), which means the
values returned from prandom_u32_max() won't be uniformly distributed.
Right now the kernel doesn't have a function that does rejection
sampling to get uniform distribution, but I could add one if you need.
Alternatively, maybe you don't actually *need* this to be perfectly
distributed -- which seems to be a common perspective -- and so this is
good enough. Your call, but just wanted to make you aware.
Jason
next prev parent reply other threads:[~2022-10-07 14:45 UTC|newest]
Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-10-07 8:42 x86/mm: Randomize per-cpu entry area Peter Zijlstra
2022-10-07 14:44 ` Jason A. Donenfeld [this message]
2022-10-07 15:14 ` Peter Zijlstra
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