Why are there no debug symbols in my vmlinux when using gdb with /proc/kcore?

Question

I've configure all CONFIG_DEBUG_ related options to y,but when I try to debug the kernel,it says no debug symbols found:

gdb /usr/src/linux-2.6.32.9/vmlinux /proc/kcore

Reading symbols from /usr/src/linux-2.6.32.9/vmlinux...(no debugging symbols found)...done.

Why?

Answer 1

Here is my best guess so far: I don't know, and it doesn't matter.

I don't know why GDB is printing the message "(no debugging symbols found)". I've actually seen this when building my own kernels. I configure a kernel to use debug symbols, but GDB still prints this message when it looks at the kernel image. I never bothered to look into it, because my image can still be debugged fine. Despite the message, GDB can still disassemble functions, add breakpoints, look up symbols, and single-step through functions. I never noticed a lack of debugging functionality. I'm guessing that the same thing is happening to you.

Edit: Based on the your comments to the question, it looks like you were searching for the wrong symbol with your debugger. System call handlers start with a prefix of sys_, but you can't tell from looking at the code. The macro SYSCALL_DEFINE4(ptrace, ...) just ends up declaring the function as asmlinkage long sys_ptrace(...), although it does some other crazy stuff if you have ftrace enabled.