Question

I’m using gdb to attach a running process, however, it always stops at __kernel_vsyscall. It looks like it stopped at my system call msgrcv(). I have to constantly “cont” it and don’t know when it could jump out of kernel and go back to application. How can I make it continue? The following is my procedure.

1. How did I get this situation?
2. How to make it continue?

Thanks!

gdb
(gdb) attach PID
...
Reading symbols from /lib/ld-linux.so.2...(no debugging symbols found)...done.
Loaded symbols for /lib/ld-linux.so.2
0xffffe410 in __kernel_vsyscall ()

(gdb)bt

#0  0xffffe410 in __kernel_vsyscall ()
#1  0x009ed573 in msgrcv () from /lib/libc.so.6
#2  0xf7f3a487 in _UX_wgetmsg (mode=0, msgp=0xffbb4178, pmaxtime=0xffbb4164, 
    pdata=0xf7f7a860, ux_type=0) at ../../../ux/com_ux/libux/com/UXipc.c:2550
#3  0xf7f3ad05 in UX_wgetmsg_v2 (mode=0, msgp=0xffbb4178, maxtime=10000, 
    ux_type=0) at ../../../ux/com_ux/libux/com/UXipc.c:2237
#4  0x0804bb9b in main (argc=1, argv=0xffbb5394)
    at /path/to/my_application:243

Answer 1

How did I get this situation?

That situation is completely normal for when you attach to a process which is blocked in a system call (waiting for message, or for read to complete).

How to make it continue?

You type continue (at which point the application would again block, waiting for a message). If you want to debug some part of the application, set breakpoints before continuing.