Date: Sat, 10 Mar 2012 16:12:20 -0800 From: Linus Torvalds <torvalds@...ux-foundation.org> To: Djalal Harouni <tixxdz@...ndz.org> Cc: linux-kernel@...r.kernel.org, kernel-hardening@...ts.openwall.com, Andrew Morton <akpm@...ux-foundation.org>, Al Viro <viro@...iv.linux.org.uk>, Alexey Dobriyan <adobriyan@...il.com>, "Eric W. Biederman" <ebiederm@...ssion.com>, Vasiliy Kulikov <segoon@...nwall.com>, Kees Cook <keescook@...omium.org>, Solar Designer <solar@...nwall.com>, WANG Cong <xiyou.wangcong@...il.com>, James Morris <james.l.morris@...cle.com>, Oleg Nesterov <oleg@...hat.com>, linux-security-module@...r.kernel.org, linux-fsdevel@...r.kernel.org, Alan Cox <alan@...rguk.ukuu.org.uk>, Greg KH <gregkh@...uxfoundation.org>, Ingo Molnar <mingo@...e.hu>, Stephen Wilson <wilsons@...rt.ca>, "Jason A. Donenfeld" <Jason@...c4.com> Subject: Re: [PATCH 1/9] exec: add a global execve counter On Sat, Mar 10, 2012 at 3:25 PM, Djalal Harouni <tixxdz@...ndz.org> wrote: > > Given that consideration this patch introduces two counters: > A global atomic execve counter that will be incremented on every > do_execve_common() call, and an atomic exec_id member for the task_struct. This seems horribly expensive on most 32-bit architectures, including very much x86-32. That atomic64_inc_return() is not cheap. It's possible that it's basically an impossible operation to do atomically on certain platforms, causing it to use some random spinlock instead. I wonder if we couldn't make something much cheaper, since we don't actually care about it being globally incrementing, we just care about it being globally unique. IOW, it could easily be a 56-bit per-cpu counter along with the CPU number in the high bits or something like that. Avoiding the whole atomicity issue, and thus avoiding the main reason those things are really expensive. IOW, something like cpu = get_cpu(); .. increment percpu 64-bit counter .. id = counter * MAX_CPUS + cpu; put_cpu(cpu); or equivalent would seem to be a potentially much cheaper approach. Linus
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