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Aiden Luo commented on GUACAMOLE-428: ------------------------------------- Ok, I just want to clarify the root cause. > segfault in guac_vnc_user_leave_handler caused by concurrent problem > -------------------------------------------------------------------- > > Key: GUACAMOLE-428 > URL: https://issues.apache.org/jira/browse/GUACAMOLE-428 > Project: Guacamole > Issue Type: Bug > Components: VNC > Affects Versions: 0.9.13-incubating > Environment: guacd with container > Reporter: Aiden Luo > Priority: Major > Attachments: guacamole-server-call-flow.png > > > 1. core dump > {code:c} > Program terminated with signal SIGSEGV, Segmentation fault. > #0 0x00007f932c1763dc in guac_vnc_user_leave_handler (user=0x7f9328004980) > at user.c:116 > 116 guac_common_cursor_remove_user(vnc_client->display->cursor, user); > (gdb) bt > #0 0x00007f932c1763dc in guac_vnc_user_leave_handler (user=0x7f9328004980) > at user.c:116 > #1 0x00007f93322d1f7f in guac_client_remove_user (client=0x7f92b800ad90, > user=0x7f9328004980) at client.c:339 > #2 0x0000000000405c60 in guacd_handle_user (user=0x7f9328004980) at > user.c:304 > #3 0x0000000000404ca2 in guacd_user_thread (data=0x7f92b801bac0) at proc.c:95 > #4 0x00007f9331922064 in start_thread (arg=0x7f92b37fe700) at > pthread_create.c:309 > #5 0x00007f9330b0562d in clone () at > ../sysdeps/unix/sysv/linux/x86_64/clone.S:111 > (gdb) l > 111 int guac_vnc_user_leave_handler(guac_user* user) { > 112 > 113 guac_vnc_client* vnc_client = (guac_vnc_client*) > user->client->data; > 114 > 115 /* Update shared cursor state */ > 116 guac_common_cursor_remove_user(vnc_client->display->cursor, user); > 117 > 118 /* Free settings if not owner (owner settings will be freed with > client) */ > 119 if (!user->owner) { > 120 guac_vnc_settings* settings = (guac_vnc_settings*) user->data; > (gdb) p *vnc_client > $1 = {client_thread = 140268061849344, rfb_client = 0x0, > rfb_MallocFrameBuffer = 0x7f931f5f5030, copy_rect_used = 0, > settings = 0x7f9328006890, display = 0x0, clipboard = 0x7f92b8035b60, audio > = 0x0, sftp_user = 0x0, sftp_session = 0x0, > sftp_filesystem = 0x0, clipboard_reader = 0x7f932c1782de > <GUAC_READ_ISO8859_1>, > clipboard_writer = 0x7f932c178414 <GUAC_WRITE_ISO8859_1>} > (gdb) p *user > $2 = {client = 0x7f92b800ad90, socket = 0x7f932800af30, user_id = > 0x7f9328004c30 "@b4e03b65-8317-4049-9f73-61c9a92272f9", > owner = 1, active = 0, __prev = 0x0, __next = 0x0, last_received_timestamp > = 1586735641, last_frame_duration = 0, > processing_lag = 0, info = {optimal_width = 977, optimal_height = 668, > audio_mimetypes = 0x7f932800b000, > video_mimetypes = 0x7f932800b020, image_mimetypes = 0x7f9328004ae0, > optimal_resolution = 96}, > __stream_pool = 0x7f9328004a90, __output_streams = 0x7f9328005670, > __input_streams = 0x7f9328004c60, > __object_pool = 0x7f932800afb0, __objects = 0x7f9328006080, data = > 0x7f9328006890, > mouse_handler = 0x7f932c17592e <guac_vnc_user_mouse_handler>, key_handler = > 0x7f932c1759a7 <guac_vnc_user_key_handler>, > clipboard_handler = 0x7f932c174ea1 <guac_vnc_clipboard_handler>, > size_handler = 0x0, file_handler = 0x0, > pipe_handler = 0x0, ack_handler = 0x0, blob_handler = 0x0, end_handler = > 0x0, sync_handler = 0x0, leave_handler = 0x0, > get_handler = 0x0, put_handler = 0x0, audio_handler = 0x0} > {code} > 2. analysis > When user connect to guacd, guacd will create a new process to handle the > connection. In new process then create +static void* guacd_user_thread(void* > data)+ thread to do real work. > In guacd_user_thread thread, will create a +void* > guac_vnc_client_thread(void* data)+ thread to read frame from VNC server and > write to guac_socket. +vnc_client->clipboard_reader+, > +vnc_client->rfb_client+ and +vnc_client->display+ will be setup in this > thread. > Also guacd_user_thread will create another +void* > guacd_user_input_thread(void* data)+ thread to receive data from client and > send to VNC sever. > guacd_user_thread will wait guacd_user_input_thread finished, then call +void > guac_client_remove_user(guac_client* client, guac_user* user)+ function to > remove user which will call +int guac_vnc_user_leave_handler(guac_user* > user)+. > The segfault error caused by line > +guac_common_cursor_remove_user(vnc_client->display->cursor, user)+ in > function +guac_vnc_user_leave_handler+,because +vnc_client->display+ is NLL. > Why this segfault will happen? > My guess is that there exist concurrent problem. If guacd_user_input_thread > exit before +vnc_client->display+ gets initialized in guac_vnc_client_thread, > the +guac_common_cursor_remove_user+ function will do null pointer > dereference. > We should check +vnc_client->display+ if is null in function +void > guac_client_remove_user(guac_client* client, guac_user* user)+. -- This message was sent by Atlassian JIRA (v6.4.14#64029)