linux-kernel - Re: [PATCH v2 4/4] copy_signal cleanup: clean tty_audit

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 for Android: free password hash cracker in your pocket

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

Date:	Sun, 6 Dec 2009 15:49:34 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Miloslav Trmac <mitr@...hat.com>
Cc:	linux-kernel@...r.kernel.org, Greg Kroah-Hartman <gregkh@...e.de>,
	Al Viro <viro@...iv.linux.org.uk>,
	James Morris <jmorris@...ei.org>,
	Alan Cox <alan@...ux.intel.com>, Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <peterz@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Balbir Singh <balbir@...ux.vnet.ibm.com>,
	Alexey Dobriyan <adobriyan@...il.com>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	Renaud Lottiaux <renaud.lottiaux@...labs.com>,
	Louis Rilling <louis.rilling@...labs.com>,
	David Howells <dhowells@...hat.com>,
	Stanislaw Gruszka <sgruszka@...hat.com>,
	Veaceslav Falico <vfalico@...hat.com>
Subject: Re: [PATCH v2 4/4] copy_signal cleanup: clean tty_audit_fork()

On 12/05, Miloslav Trmac wrote:
>
> ----- "Oleg Nesterov" <oleg@...hat.com> wrote:
> > On 12/04, Veaceslav Falico wrote:
> > > diff --git a/drivers/char/tty_audit.c b/drivers/char/tty_audit.c
> > > index ac16fbe..283a15b 100644
> > > --- a/drivers/char/tty_audit.c
> > > +++ b/drivers/char/tty_audit.c
> > > @@ -148,7 +148,6 @@ void tty_audit_fork(struct signal_struct *sig)
> > >  	spin_lock_irq(&current->sighand->siglock);
> > >  	sig->audit_tty = current->signal->audit_tty;
> > >  	spin_unlock_irq(&current->sighand->siglock);
> > > -	sig->tty_audit_buf = NULL;
> > >  }
> >
> > Off-topic question to this who understands this code.
> >
> > But afaics we can also remove ->siglock from this helper and make
> > it really trivial for being inline. ->siglock buys nothing, we just
> > read a boolean. In fact, after the quick grep I do not understand
> > how ->siglock is connected to ->audit_tty. OK, it protects
> > tty_audit_buf,
> > but why we always take ->siglock to access ->audit_tty ?
> AFAIK there is no explicit documentation of the atomicity semantics
> expected by the Linux kernel (both from the hardware and from the compiler),
> so every access to the boolean is protected by a lock, to be on the safe side.

Not sure I understand, but the kernel relies on fact it is always safe
to load/store a word.

What atomicity semantics do you mean and how ->siglock can help? Sure,
we can race with AUDIT_TTY_SET, but this can happen with or without
this lock. This "race" is unavoidable and harmless.

I believe every spin_lock(siglock) around ->audit_tty is bogus.

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/