linux-kernel - [PATCH v2] scsi: avoid a double-fetch and a redundant copy

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Date:   Wed,  9 Jan 2019 01:25:57 -0600
From:   Kangjie Lu <kjlu@....edu>
To:     kjlu@....edu
Cc:     pakki001@....edu, Doug Gilbert <dgilbert@...erlog.com>,
        "James E.J. Bottomley" <jejb@...ux.vnet.ibm.com>,
        "Martin K. Petersen" <martin.petersen@...cle.com>,
        linux-scsi@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [PATCH v2] scsi: avoid a double-fetch and a redundant copy

What we need is only "pack_id", so do not create a heap object or copy
the whole object in. The fix efficiently copies "pack_id" only and
also avoids double-fetch.

Signed-off-by: Kangjie Lu <kjlu@....edu>
---
 drivers/scsi/sg.c | 12 ++----------
 1 file changed, 2 insertions(+), 10 deletions(-)

diff --git a/drivers/scsi/sg.c b/drivers/scsi/sg.c
index c6ad00703c5b..13662c41058a 100644
--- a/drivers/scsi/sg.c
+++ b/drivers/scsi/sg.c
@@ -446,16 +446,8 @@ sg_read(struct file *filp, char __user *buf, size_t count, loff_t * ppos)
 		}
 		if (old_hdr->reply_len < 0) {
 			if (count >= SZ_SG_IO_HDR) {
-				sg_io_hdr_t *new_hdr;
-				new_hdr = kmalloc(SZ_SG_IO_HDR, GFP_KERNEL);
-				if (!new_hdr) {
-					retval = -ENOMEM;
-					goto free_old_hdr;
-				}
-				retval =__copy_from_user
-				    (new_hdr, buf, SZ_SG_IO_HDR);
-				req_pack_id = new_hdr->pack_id;
-				kfree(new_hdr);
+				retval = get_user(req_pack_id,
+						&((sg_io_hdr_t *)buf)->pack_id);
 				if (retval) {
 					retval = -EFAULT;
 					goto free_old_hdr;
-- 
2.17.1