linux-kernel - Re: [PATCH] serial/8250: fix uninitialized warnings

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Date:	Mon, 10 Nov 2008 14:30:39 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
Cc:	kosaki.motohiro@...fujitsu.com, linux-serial@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] serial/8250: fix uninitialized warnings

On Tue, 4 Nov 2008 17:41:42 +0000
Alan Cox <alan@...rguk.ukuu.org.uk> wrote:

> On Wed, 5 Nov 2008 02:26:00 +0900 (JST)
> KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com> wrote:
> 
> > 
> > fix following warnings.
> > 
> > drivers/serial/8250.c: In function ___serial8250_shutdown___:
> > drivers/serial/8250.c:1612: warnings: ___i___ may be used uninitialized in this function
> 
> NAK
> 
> This is an incorrect compiler warning. It's also one that current gcc
> does not emit warnings for.
> 

That's a regression in current gcc, surely?

static void serial_unlink_irq_chain(struct uart_8250_port *up)
{
	struct irq_info *i;
	struct hlist_node *n;
	struct hlist_head *h;

	mutex_lock(&hash_mutex);

	h = &irq_lists[up->port.irq % NR_IRQ_HASH];

	hlist_for_each(n, h) {
		i = hlist_entry(n, struct irq_info, node);
		if (i->irq == up->port.irq)
			break;
	}

	BUG_ON(n == NULL);
	BUG_ON(i->head == NULL);


#define hlist_for_each(pos, head) \
	for (pos = (head)->first; pos && ({ prefetch(pos->next); 1; }); \
	     pos = pos->next)

hlist_for_each() can execute that loop zero times, in which case
serial_unlink_irq_chain will dereference an uninitialised variable.

Presumably the list shouldn't be empty at this stage, but this
is not particularly robust behaviour if that should happen..
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