[PATCH] scsi: scsi-lib.c: increase cmd_size by sizeof(struct scatterlist)

[Michael Schmitz <schmitzmic@gmail.com>]

In scsi_mq_setup_tags(), cmd_size is calculated based on zero size
for the scatter-gather list in case the low level driver uses SG_NONE
in its host template.

cmd_size is passed on to the block layer for calculation of the request
size, and we've seen NULL pointer dereference errors from the block
layer in drivers where SG_NONE is used and a mq IO scheduler is active,
apparently as a consequence of this (see commit 68ab2d76e4be for the
cxflash driver, and a recent patch by Finn Thain converting the three
m68k NFR5380 drivers to avoid setting SG_NONE).

Try to avoid these errors by accounting for at least one sg list entry
when caculating cmd_size, regardless of whether the low level driver
set a zero sg_tablesize.

Tested on 030 m68k with the atari_scsi driver - setting sg_tablesize to
SG_NONE no longer results in a crash when loading this driver.

Signed-off-by: Michael Schmitz <schmitzmic@gmail.com>
CC: Finn Thain <fthain@telegraphics.com.au>
---
 drivers/scsi/scsi_lib.c |    3 ++-
 1 files changed, 2 insertions(+), 1 deletions(-)

diff --git a/drivers/scsi/scsi_lib.c b/drivers/scsi/scsi_lib.c
index d47d637..d38b0e1 100644
--- a/drivers/scsi/scsi_lib.c
+++ b/drivers/scsi/scsi_lib.c
@@ -1867,7 +1867,8 @@ int scsi_mq_setup_tags(struct Scsi_Host *shost)
 {
 	unsigned int cmd_size, sgl_size;
 
-	sgl_size = scsi_mq_inline_sgl_size(shost);
+	sgl_size = max_t(unsigned int, sizeof(struct scatterlist),
+				scsi_mq_inline_sgl_size(shost));
 	cmd_size = sizeof(struct scsi_cmnd) + shost->hostt->cmd_size + sgl_size;
 	if (scsi_host_get_prot(shost))
 		cmd_size += sizeof(struct scsi_data_buffer) +
-- 
1.7.0.4