Fix copy-on-write corruption

BRANCH

@@ -0,0 +1,91 @@
+What does this fix?  It includes the fixes
+mailed to the nbd mailing list as the README suggests I do
+
+
+Here is the mail
+
+Hi,
+
+I have a simple test case where nbd corrupts the date read back if
+copyonwrite=true, and another one if sparse_cow=true.
+
+I also have patches that I believe fix this.
+
+
+For the first case, it's enough to read and write more than one
+sequential block.  The second and subsequent blocks read will read into
+the wrong offset of the buffer, and copy invalid data to the client.
+
+I use a 4096 block size in this example, but I've used others.  I did
+that to match the filesystem, but for the test I don't even need a
+filesystem.
+
+The test:
+
+export OFFSET=0
+export COUNT=3 # anything >= 1
+dd if=/dev/urandom of=testdata bs=4096 count=$COUNT # random data
+dd if=testdata of=/dev/nbd0 bs=4096 seek=$OFFSET count=$COUNT
+dd if=/dev/nbd0 of=compdata bs=4096 skip=$OFFSET count=$COUNT
+sum testdata compdata
+
+The data will be different.
+
+If the kernel does a partition check on /dev/nbd0 it will fail with
+COUNT=1 as well.
+
+
+For the second case, with sparse_cow=true, we need to repeat the test
+with an offset > 0.  expwrite() calls write() instead of pwrite().
+
+export OFFSET=100
+export COUNT=3 # anything >= 1
+dd if=/dev/urandom of=testdata bs=4096 count=$COUNT # random data
+dd if=testdata of=/dev/nbd0 bs=4096 seek=$OFFSET count=$COUNT
+dd if=/dev/nbd0 of=compdata bs=4096 skip=$OFFSET count=$COUNT
+sum testdata compdata
+
+The first time it's run, it will result in an Input/Output error.
+
+The second time it's run, it will work.
+
+
+First patch:
+
+diff --git a/nbd-orig/nbd-server.c b/nbd-patched/nbd-server.c
+index 92fd141..18e5ddd 100644
+--- a/nbd-orig/nbd-server.c
++++ b/nbd-patched/nbd-server.c
+@@ -1582,6 +1582,7 @@ int expread(READ_CTX *ctx, CLIENT *client) {
+                        if (pread(client->difffile, buf, rdlen, client-
+>difmap[mapcnt]*DIFFPAGESIZE+offset) != rdlen) {
+                                goto fail;
+                        }
++                       ctx->current_offset += rdlen;
+                        confirm_read(client, ctx, rdlen);
+                } else { /* the block is not there */
+                        if ((client->server->flags & F_WAIT) &&
+(client->export == NULL)){
+
+
+Second patch:
+
+diff --git a/nbd-orig/nbd-server.c b/nbd-patched/nbd-server.c
+index 92fd141..9a57ad5 100644
+--- a/nbd-orig/nbd-server.c
++++ b/nbd-patched/nbd-server.c
+@@ -1669,7 +1669,7 @@ int expwrite(off_t a, char *buf, size_t len,
+CLIENT *client, int fua) {
+                                if(ret < 0 ) goto fail;
+                        }
+                        memcpy(pagebuf+offset,buf,wrlen) ;
+-                       if (write(client->difffile, pagebuf,
+DIFFPAGESIZE) != DIFFPAGESIZE)
++                       if (pwrite(client->difffile, pagebuf,
+DIFFPAGESIZE, client->difmap[mapcnt]*DIFFPAGESIZE) != DIFFPAGESIZE)
+                                goto fail;
+                }
+                if (!(client->server->flags & F_COPYONWRITE))
+
+
+Berend

nbd-server.c

@@ -1582,6 +1582,7 @@ int expread(READ_CTX *ctx, CLIENT *client) {
 			if (pread(client->difffile, buf, rdlen, client->difmap[mapcnt]*DIFFPAGESIZE+offset) != rdlen) {
 				goto fail;
 			}
+			ctx->current_offset += rdlen;
 			confirm_read(client, ctx, rdlen);
 		} else { /* the block is not there */
 			if ((client->server->flags & F_WAIT) && (client->export == NULL)){
@@ -1669,7 +1670,7 @@ int expwrite(off_t a, char *buf, size_t len, CLIENT *client, int fua) {
 				if(ret < 0 ) goto fail;
 			}
 			memcpy(pagebuf+offset,buf,wrlen) ;
-			if (write(client->difffile, pagebuf, DIFFPAGESIZE) != DIFFPAGESIZE)
+			if (pwrite(client->difffile, pagebuf, DIFFPAGESIZE, client->difmap[mapcnt]*DIFFPAGESIZE) != DIFFPAGESIZE)
 				goto fail;
 		}
 		if (!(client->server->flags & F_COPYONWRITE))