sftp.c: Check Read Packet File Offset

This commit adds a simple check to see if the offset of the read
request matches the expected file offset.

We could try to recover, from this condition at some point in the future.
Right now it is better to return an error instead of corrupted data.

src/sftp.c

@@ -1377,6 +1377,7 @@ static ssize_t sftp_read(LIBSSH2_SFTP_HANDLE * handle, char *buffer,
                 return _libssh2_error(session, LIBSSH2_ERROR_ALLOC,
                                       "malloc fail for FXP_WRITE");
 
+            chunk->offset = filep->offset_sent;
             chunk->len = size;
             chunk->lefttosend = packet_len;
             chunk->sent = 0;
@@ -1397,7 +1398,9 @@ static ssize_t sftp_read(LIBSSH2_SFTP_HANDLE * handle, char *buffer,
             _libssh2_list_add(&handle->packet_list, &chunk->node);
             count -= size; /* deduct the size we used, as we might have
                               to create more packets */
-            _libssh2_debug(session, LIBSSH2_TRACE_SFTP, "read request id %d sent", request_id);
+           _libssh2_debug(session, LIBSSH2_TRACE_SFTP,
+                          "read request id %d sent (offset: %d, size: %d)",
+                          request_id, (int)chunk->offset, (int)chunk->len);
         }
 
     case libssh2_NB_state_sent:
@@ -1506,6 +1509,16 @@ static ssize_t sftp_read(LIBSSH2_SFTP_HANDLE * handle, char *buffer,
                 break;
 
             case SSH_FXP_DATA:
+                if (chunk->offset != filep->offset) {
+                    /* This could happen if the server returns less bytes than
+                       requested, which shouldn't happen for normal files. See:
+                       http://tools.ietf.org/html/draft-ietf-secsh-filexfer-02
+                       #section-6.4
+                    */
+                    return _libssh2_error(session, LIBSSH2_ERROR_SFTP_PROTOCOL,
+                                          "Read Packet At Unexpected Offset");
+                }
+
                 rc32 = _libssh2_ntohu32(data + 5);
                 if (rc32 > (data_len - 9))
                     return _libssh2_error(session, LIBSSH2_ERROR_SFTP_PROTOCOL,

src/sftp.h

@@ -52,6 +52,8 @@
 
 struct sftp_pipeline_chunk {
     struct list_node node;
+    uint64_t offset; /* READ: offset at which to start reading
+    					WRITE: not used */
     size_t len; /* WRITE: size of the data to write
                    READ: how many bytes that was asked for */
     size_t sent;