USB: usbmon: end ugly tricks with DMA peeking

This patch fixes crashes when usbmon attempts to access GART aperture.
The old code attempted to take a bus address and convert it into a
virtual address, which clearly was impossible on systems with actual
IOMMUs. Let us not persist in this foolishness, and use transfer_buffer
in all cases instead.

I think downsides are negligible. The ones I see are:
- A driver may pass an address of one buffer down as transfer_buffer,
and entirely different entity mapped for DMA, resulting in misleading
output of usbmon. Note, however, that PIO based controllers would
do transfer the same data that usbmon sees here.
- Out of tree drivers may crash usbmon if they store garbage in
transfer_buffer. I inspected the in-tree drivers, and clarified
the documentation in comments.
- Drivers that use get_user_pages will not be possible to monitor.
I only found one driver with this problem (drivers/staging/rspiusb).
- Same happens with with usb_storage transferring from highmem, but
it works fine on 64-bit systems, so I think it's not a concern.
At least we don't crash anymore.

Why didn't we do this in 2.6.10? That's because back in those days
it was popular not to fill in transfer_buffer, so almost all
traffic would be invisible (e.g. all of HID was like that).
But now, the tree is almost 100% PIO friendly, so we can do the
right thing at last.

Signed-off-by: Pete Zaitcev <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/usb/mon/Makefile

···
 # Makefile for USB monitor
 #
 
-usbmon-objs	:= mon_main.o mon_stat.o mon_text.o mon_bin.o mon_dma.o
+usbmon-objs	:= mon_main.o mon_stat.o mon_text.o mon_bin.o
 
 obj-$(CONFIG_USB_MON)	+= usbmon.o

drivers/usb/mon/mon_bin.c

···
 
 /*
  * This is a "chunked memcpy". It does not manipulate any counters.
- * But it returns the new offset for repeated application.
  */
-unsigned int mon_copy_to_buff(const struct mon_reader_bin *this,
+static void mon_copy_to_buff(const struct mon_reader_bin *this,
     unsigned int off, const unsigned char *from, unsigned int length)
 {
 	unsigned int step_len;
···
 		from += step_len;
 		length -= step_len;
 	}
-	return off;
 }
 
 /*
···
     unsigned int offset, struct urb *urb, unsigned int length)
 {
 
-	if (urb->dev->bus->uses_dma &&
-	    (urb->transfer_flags & URB_NO_TRANSFER_DMA_MAP)) {
-		mon_dmapeek_vec(rp, offset, urb->transfer_dma, length);
-		return 0;
-	}
-
 	if (urb->transfer_buffer == NULL)
 		return 'Z';
-
 	mon_copy_to_buff(rp, offset, urb->transfer_buffer, length);
 	return 0;
 }
···
 	spin_lock_init(&rp->b_lock);
 	init_waitqueue_head(&rp->b_wait);
 	mutex_init(&rp->fetch_lock);
-
 	rp->b_size = BUFF_DFL;
 
 	size = sizeof(struct mon_pgmap) * (rp->b_size/CHUNK_SIZE);

drivers/usb/mon/mon_dma.c

···
-/*
- * The USB Monitor, inspired by Dave Harding's USBMon.
- *
- * mon_dma.c: Library which snoops on DMA areas.
- *
- * Copyright (C) 2005 Pete Zaitcev ([e-mail])
- */
-#include <linux/kernel.h>
-#include <linux/list.h>
-#include <linux/highmem.h>
-#include <asm/page.h>
-
-#include <linux/usb.h>	/* Only needed for declarations in usb_mon.h */
-#include "usb_mon.h"
-
-/*
- * PC-compatibles, are, fortunately, sufficiently cache-coherent for this.
- */
-#if defined(__i386__) || defined(__x86_64__) /* CONFIG_ARCH_I386 doesn't exit */
-#define MON_HAS_UNMAP 1
-
-#define phys_to_page(phys)	pfn_to_page((phys) >> PAGE_SHIFT)
-
-char mon_dmapeek(unsigned char *dst, dma_addr_t dma_addr, int len)
-{
-	struct page *pg;
-	unsigned long flags;
-	unsigned char *map;
-	unsigned char *ptr;
-
-	/*
-	 * On i386, a DMA handle is the "physical" address of a page.
-	 * In other words, the bus address is equal to physical address.
-	 * There is no IOMMU.
-	 */
-	pg = phys_to_page(dma_addr);
-
-	/*
-	 * We are called from hardware IRQs in case of callbacks.
-	 * But we can be called from softirq or process context in case
-	 * of submissions. In such case, we need to protect KM_IRQ0.
-	 */
-	local_irq_save(flags);
-	map = kmap_atomic(pg, KM_IRQ0);
-	ptr = map + (dma_addr & (PAGE_SIZE-1));
-	memcpy(dst, ptr, len);
-	kunmap_atomic(map, KM_IRQ0);
-	local_irq_restore(flags);
-	return 0;
-}
-
-void mon_dmapeek_vec(const struct mon_reader_bin *rp,
-    unsigned int offset, dma_addr_t dma_addr, unsigned int length)
-{
-	unsigned long flags;
-	unsigned int step_len;
-	struct page *pg;
-	unsigned char *map;
-	unsigned long page_off, page_len;
-
-	local_irq_save(flags);
-	while (length) {
-		/* compute number of bytes we are going to copy in this page */
-		step_len = length;
-		page_off = dma_addr & (PAGE_SIZE-1);
-		page_len = PAGE_SIZE - page_off;
-		if (page_len < step_len)
-			step_len = page_len;
-
-		/* copy data and advance pointers */
-		pg = phys_to_page(dma_addr);
-		map = kmap_atomic(pg, KM_IRQ0);
-		offset = mon_copy_to_buff(rp, offset, map + page_off, step_len);
-		kunmap_atomic(map, KM_IRQ0);
-		dma_addr += step_len;
-		length -= step_len;
-	}
-	local_irq_restore(flags);
-}
-
-#endif /* __i386__ */
-
-#ifndef MON_HAS_UNMAP
-char mon_dmapeek(unsigned char *dst, dma_addr_t dma_addr, int len)
-{
-	return 'D';
-}
-
-void mon_dmapeek_vec(const struct mon_reader_bin *rp,
-    unsigned int offset, dma_addr_t dma_addr, unsigned int length)
-{
-	;
-}
-
-#endif /* MON_HAS_UNMAP */

drivers/usb/mon/mon_main.c

···
 	}
 	// MOD_INC_USE_COUNT(which_module?);
 
-
 	mutex_lock(&usb_bus_list_lock);
 	list_for_each_entry (ubus, &usb_bus_list, bus_list) {
 		mon_bus_init(ubus);

drivers/usb/mon/mon_text.c

···
 			return '>';
 	}
 
-	/*
-	 * The check to see if it's safe to poke at data has an enormous
-	 * number of corner cases, but it seems that the following is
-	 * more or less safe.
-	 *
-	 * We do not even try to look at transfer_buffer, because it can
-	 * contain non-NULL garbage in case the upper level promised to
-	 * set DMA for the HCD.
-	 */
-	if (urb->dev->bus->uses_dma &&
-	    (urb->transfer_flags & URB_NO_TRANSFER_DMA_MAP)) {
-		return mon_dmapeek(ep->data, urb->transfer_dma, len);
-	}
-
 	if (urb->transfer_buffer == NULL)
 		return 'Z';	/* '0' would be not as pretty. */
 

drivers/usb/mon/usb_mon.h

···
 void mon_bin_exit(void);
 
 /*
- * DMA interface.
- *
- * XXX The vectored side needs a serious re-thinking. Abstracting vectors,
- * like in Paolo's original patch, produces a double pkmap. We need an idea.
-*/
-extern char mon_dmapeek(unsigned char *dst, dma_addr_t dma_addr, int len);
-
-struct mon_reader_bin;
-extern void mon_dmapeek_vec(const struct mon_reader_bin *rp,
-    unsigned int offset, dma_addr_t dma_addr, unsigned int len);
-extern unsigned int mon_copy_to_buff(const struct mon_reader_bin *rp,
-    unsigned int offset, const unsigned char *from, unsigned int len);
-
-/*
  */
 extern struct mutex mon_lock;
 

include/linux/usb.h

···
  * @transfer_flags: A variety of flags may be used to affect how URB
  *	submission, unlinking, or operation are handled.  Different
  *	kinds of URB can use different flags.
- * @transfer_buffer:  This identifies the buffer to (or from) which
- * 	the I/O request will be performed (unless URB_NO_TRANSFER_DMA_MAP
- *	is set).  This buffer must be suitable for DMA; allocate it with
+ * @transfer_buffer:  This identifies the buffer to (or from) which the I/O
+ *	request will be performed unless URB_NO_TRANSFER_DMA_MAP is set
+ *	(however, do not leave garbage in transfer_buffer even then).
+ *	This buffer must be suitable for DMA; allocate it with
  *	kmalloc() or equivalent.  For transfers to "in" endpoints, contents
  *	of this buffer will be modified.  This buffer is used for the data
  *	stage of control transfers.
···
  * allocate a DMA buffer with usb_buffer_alloc() or call usb_buffer_map().
  * When these transfer flags are provided, host controller drivers will
  * attempt to use the dma addresses found in the transfer_dma and/or
- * setup_dma fields rather than determining a dma address themselves.  (Note
- * that transfer_buffer and setup_packet must still be set because not all
- * host controllers use DMA, nor do virtual root hubs).
+ * setup_dma fields rather than determining a dma address themselves.
+ *
+ * Note that transfer_buffer must still be set if the controller
+ * does not support DMA (as indicated by bus.uses_dma) and when talking
+ * to root hub. If you have to trasfer between highmem zone and the device
+ * on such controller, create a bounce buffer or bail out with an error.
+ * If transfer_buffer cannot be set (is in highmem) and the controller is DMA
+ * capable, assign NULL to it, so that usbmon knows not to use the value.
+ * The setup_packet must always be set, so it cannot be located in highmem.
  *
  * Initialization:
  *