linux-kernel - Re: [PATCH] vt: Rework the console spawning variables.

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:	Mon, 11 Sep 2006 00:33:42 +0400
From:	Oleg Nesterov <oleg@...sign.ru>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	Andrew Morton <akpm@...l.org>, linux-kernel@...r.kernel.org,
	Linux Containers <containers@...ts.osdl.org>,
	Alan Cox <alan@...rguk.ukuu.org.uk>
Subject: Re: [PATCH] vt: Rework the console spawning variables.

On 09/10, Eric W. Biederman wrote:
>
> Oleg Nesterov <oleg@...sign.ru> writes:
> 
> > On 09/09, Eric W. Biederman wrote:
> >> 
> >> This patch does several things.
> >> - The variables used are moved into a structure and declared in vt_kern.h
> >> - A spinlock is added so we don't have SMP races updating the values.
> >> - Instead of raw pid_t value a struct_pid is used to guard against
> >>   pid wrap around issues, if the daemon to spawn a new console dies.
> >
> > I am not arguing against this patch, but it's a pity we can't use 'struct pid'
> > lockless. What dou you think about this:
> 
> Actually with xchg I can use a reference counted struct pid lockless.
>
> ...
>
> Perhaps:
> void update_pid(struct pid **ref, struct pid *new)
> {
>         struct pid *old;
>         get_pid(new);
>         old = xchg(ref, new);
>         put_pid(old);
> }

This can't work. This put_pid() can actually free the memory, while
'old' is still in use (lockless).

> rcu is definitely not the solution in these cases as the struct pid
> is stored for a long time so we need the reference count.

Surely we need the reference count, I don't understand you.
Look at put_pid_rcu().

That said,

> In the general case you have more then one variable you want to keep
> in sync and you need the lock for that.

Yes.

> But since I can write it as a moderately clear one liner in the
> case that matters I don't much care.

Ok.

Oleg.

-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/