scsi_debug: fix do_device_access() with wrap around range

do_device_access() is a function that abstracts copying SG list from/to
ramdisk storage (fake_storep).

It must deal with the ranges exceeding actual fake_storep size, because
such ranges are valid if virtual_gb is set greater than zero, and they
should be treated as fake_storep is repeatedly mirrored up to virtual
size.

Unfortunately, it can't deal with the range which wraps around the end of
fake_storep.  A wrap around range is copied by two
sg_copy_{from,to}_buffer() calls, but sg_copy_{from,to}_buffer() can't
copy from/to in the middle of SG list, therefore the second call can't
copy correctly.

This fixes it by using sg_pcopy_{from,to}_buffer() that can copy from/to
the middle of SG list.

This also simplifies the assignment of sdb->resid in
fill_from_dev_buffer().  Because fill_from_dev_buffer() is now only called
once per command execution cycle.  So it is not necessary to take care to
decrease sdb->resid if fill_from_dev_buffer() is called more than once.

Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: "James E.J. Bottomley" <JBottomley@parallels.com>
Cc: Douglas Gilbert <dgilbert@interlog.com>
Cc: Herbert Xu <herbert@gondor.apana.org.au>
Cc: Horia Geanta <horia.geanta@freescale.com>
Cc: Imre Deak <imre.deak@intel.com>
Cc: Tejun Heo <tj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/scsi/scsi_debug.c

@@ -439,10 +439,7 @@ static int fill_from_dev_buffer(struct scsi_cmnd *scp, unsigned char *arr,
 
 	act_len = sg_copy_from_buffer(sdb->table.sgl, sdb->table.nents,
 				      arr, arr_len);
-	if (sdb->resid)
-		sdb->resid -= act_len;
-	else
-		sdb->resid = scsi_bufflen(scp) - act_len;
+	sdb->resid = scsi_bufflen(scp) - act_len;
 
 	return 0;
 }
@@ -1693,24 +1690,48 @@ static int check_device_access_params(struct sdebug_dev_info *devi,
 	return 0;
 }
 
+/* Returns number of bytes copied or -1 if error. */
 static int do_device_access(struct scsi_cmnd *scmd,
 			    struct sdebug_dev_info *devi,
 			    unsigned long long lba, unsigned int num, int write)
 {
 	int ret;
 	unsigned long long block, rest = 0;
-	int (*func)(struct scsi_cmnd *, unsigned char *, int);
+	struct scsi_data_buffer *sdb;
+	enum dma_data_direction dir;
+	size_t (*func)(struct scatterlist *, unsigned int, void *, size_t,
+		       off_t);
+
+	if (write) {
+		sdb = scsi_out(scmd);
+		dir = DMA_TO_DEVICE;
+		func = sg_pcopy_to_buffer;
+	} else {
+		sdb = scsi_in(scmd);
+		dir = DMA_FROM_DEVICE;
+		func = sg_pcopy_from_buffer;
+	}
 
-	func = write ? fetch_to_dev_buffer : fill_from_dev_buffer;
+	if (!sdb->length)
+		return 0;
+	if (!(scsi_bidi_cmnd(scmd) || scmd->sc_data_direction == dir))
+		return -1;
 
 	block = do_div(lba, sdebug_store_sectors);
 	if (block + num > sdebug_store_sectors)
 		rest = block + num - sdebug_store_sectors;
 
-	ret = func(scmd, fake_storep + (block * scsi_debug_sector_size),
-		   (num - rest) * scsi_debug_sector_size);
-	if (!ret && rest)
-		ret = func(scmd, fake_storep, rest * scsi_debug_sector_size);
+	ret = func(sdb->table.sgl, sdb->table.nents,
+		   fake_storep + (block * scsi_debug_sector_size),
+		   (num - rest) * scsi_debug_sector_size, 0);
+	if (ret != (num - rest) * scsi_debug_sector_size)
+		return ret;
+
+	if (rest) {
+		ret += func(sdb->table.sgl, sdb->table.nents,
+			    fake_storep, rest * scsi_debug_sector_size,
+			    (num - rest) * scsi_debug_sector_size);
+	}
 
 	return ret;
 }
@@ -1849,7 +1870,12 @@ static int resp_read(struct scsi_cmnd *SCpnt, unsigned long long lba,
 	read_lock_irqsave(&atomic_rw, iflags);
 	ret = do_device_access(SCpnt, devip, lba, num, 0);
 	read_unlock_irqrestore(&atomic_rw, iflags);
-	return ret;
+	if (ret == -1)
+		return DID_ERROR << 16;
+
+	scsi_in(SCpnt)->resid = scsi_bufflen(SCpnt) - ret;
+
+	return 0;
 }
 
 void dump_sector(unsigned char *buf, int len)