kernel debug:"Symbol file not found for *.ko"

When I debug vmlinux with gdb, after it connect to the target, I got below output:

usbcore.ko: No such file or directory.
Error while mapping shared library sections:
ohci_hcd.ko: No such file or directory.
Symbol file not found for usbcore.ko
Symbol file not found for ohci_hcd.ko
Symbol file not found for binfmt_misc.ko
Symbol file not found for edac_core.ko
Symbol file not found for k8_edac.ko
Symbol file not found for usbhid.ko
Symbol file not found for sg.ko
Symbol file not found for usb_storage.ko
Symbol file not found for ipmi_msghandler.ko
Symbol file not found for ipmi_si.ko
Symbol file not found for ipmi_devintf

I konw we can set the path in the gdb to avoid this, but I wonder what cause above output message?
any idears?

This is the standard output from gdb in this circumstance. I believe you can turn off these messages with "set verbose off" but I am not in front on a system where I can check this at present.

any one know what cause "symbol not found"?