linux-kernel - Re: [PATCH 1/3, RFC] misc char dev BKL pushdown

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Message-ID: <9968.1211325938@vena.lwn.net>
Date:	Tue, 20 May 2008 17:25:38 -0600
From:	corbet@....net (Jonathan Corbet)
To:	"Mike Frysinger" <vapier.adi@...il.com>
Cc:	"Arnd Bergmann" <arnd@...db.de>, "Wu, Bryan" <Bryan.Wu@...log.com>,
	"Linus Torvalds" <torvalds@...ux-foundation.org>,
	"Ingo Molnar" <mingo@...e.hu>,
	"Andrew Morton" <akpm@...ux-foundation.org>,
	"Peter Zijlstra" <a.p.zijlstra@...llo.nl>,
	"Thomas Gleixner" <tglx@...utronix.de>,
	"Alan Cox" <alan@...rguk.ukuu.org.uk>,
	"Alexander Viro" <viro@....linux.org.uk>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3, RFC] misc char dev BKL pushdown 

Mike Frysinger <vapier.adi@...il.com> wrote:

> please drop the coreb.c changes from your patch

At a minimum, I would hope such a request would say something like "I've
looked at the driver's locking and am convinced that the BKL is not
needed."  Have you done that?  There is a certain leap of faith involved
in removing that protection from a driver.

I decided to take a quick look...

- You use spin_lock_irq(&coreb_lock) in a number of places, but you do
  not take the lock in the interrupt handler.  You also do not take the
  lock in coreb_write() or coreb_read(), so those can race with the
  interrupt handler, with ioctl(), and with each other.

- coreb_write() and coreb_read() do interruptible waits, but do not
  check to see whether they were interrupted.  They will, in fact,
  continue in their I/O loops after a signal.

- In both functions you have:

	unsigned long p = *ppos;

	if (p + count > coreb_size)
		return -EFAULT;

  that calculation can overflow.

- You also do this:

  static ssize_t coreb_write(struct file *file, const char *buf, size_t count,
	 		     loff_t * ppos)
  /* ... */
  		set_dma_start_addr(CH_MEM_STREAM2_SRC, (unsigned long)buf);

  In other words, the DMA is done directly to/from a user-space
  address.  Maybe that's safe on Blackfin, I don't know...

- I have no idea why some of your functions are using d_inode->i_mutex.

- In coreb_ioctl():

		spin_lock_irq(&coreb_lock);
		if (coreb_status & COREB_IS_RUNNING) {
			retval = -EBUSY;
			break;
		}

  this will exit the function with the spinlock still held and
  interrupts disabled.

	case CMD_COREB_RESET:
		printk(KERN_INFO "Resetting Core B\n");
		bfin_write_SICB_SYSCR(bfin_read_SICB_SYSCR() | 0x0080);
		break;

  You do not acquire the lock here, so this can race against other
  ioctl() calls.  And ioctl() can race against read() and write().

Registration and such seem reasonable, so I can't come up with a
scenario where loss of BKL protection will create trouble.  Given the
other problems there, though, I'll confess to being a bit nervous about
it.

jon
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