[stgt] [PATCH] COMPARE_AND_WRITE: If the compare fails we should not continue to write the data

[Ronnie Sahlberg]

Fix a bug in COMPARE-AND-WRITE. If the compare fails, we have to abort with a sense code immediately and we should NOT continue to over-write the backing store with the mismatching data.

IF the data is not matching, we also should compute at which offset the first mismatch in the buffer occurs.
This is to be reported in the INFORMATION field (bytes 3-7) in the sense buffer in a later patch.

Signed-off-by: Ronnie Sahlberg <ronniesahlberg at gmail.com>
---
 usr/bs_rdwr.c |   25 +++++++++++++++++++++++--
 1 files changed, 23 insertions(+), 2 deletions(-)

diff --git a/usr/bs_rdwr.c b/usr/bs_rdwr.c
index a79dd02..c7d72e6 100644
--- a/usr/bs_rdwr.c
+++ b/usr/bs_rdwr.c
@@ -65,6 +65,7 @@ static void bs_rdwr_request(struct scsi_cmd *cmd)
 	int result = SAM_STAT_GOOD;
 	uint8_t key;
 	uint16_t asc;
+	uint32_t info = 0;
 	char *tmpbuf;
 	size_t blocksize;
 	uint64_t offset = cmd->offset;
@@ -117,12 +118,32 @@ static void bs_rdwr_request(struct scsi_cmd *cmd)
 
 		ret = pread64(fd, tmpbuf, length, offset);
 
-		if (ret != length)
+		if (ret != length) {
 			set_medium_error(&result, &key, &asc);
-		else if (memcmp(scsi_get_out_buffer(cmd), tmpbuf, length)) {
+			free(tmpbuf);
+			break;
+		}
+
+		if (memcmp(scsi_get_out_buffer(cmd), tmpbuf, length)) {
+			uint32_t pos = 0;
+			char *spos = scsi_get_out_buffer(cmd);
+			char *dpos = tmpbuf;
+
+			/* data differed, this is assumed to be 'rare' so use a
+			   much more expensive byte-by-byte comparasion to find
+			   out at which offset the data differs.
+			 */
+			while (pos < length) {
+				if (*spos++ != *dpos++)
+					break;
+				pos++;
+			}
+			info = pos;
 			result = SAM_STAT_CHECK_CONDITION;
 			key = MISCOMPARE;
 			asc = ASC_MISCOMPARE_DURING_VERIFY_OPERATION;
+			free(tmpbuf);
+			break;
 		}
 
 		if (cmd->scb[1] & 0x10)
-- 
1.7.3.1

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