Cannot build kgdboe against kernel headers

Question

I am trying to build kgdboe according to the following tutorial. I am running Ubuntu 20.04 with an updated kernel 5.9.0 as I needed a driver which was not available in stock kernel. I updated the kernel by downloading the headers, modules and kernel image .deb packages and installing them manually. My uname -r output is 5.9.0-050900-generic. When I try to build the module with make -C /lib/modules/$(uname -r)/build M=$(pwd), I get the error:

ERROR: modpost: "__module_address" [/home/ipc/Downloads/kgdboe/kgdboe.ko] undefined!

I have also manually downloaded complete Linux kernel source tree from GIT and checked out 5.9.0. If I make this module with path in make command pointing to the full source tree, the module buids. But since the module magic version and uname -r output doesn't match, I am unable to insert the module even with modprobe -f.

My question is, why am I not able to build with only the linux headers? Am I missing some step that prepares the headers folder for building modules? I am trying to find a way of building this module against the kernel headers or if I could update the version magic of built module (5.9.0) to uname -r output.

Answer 1

(Sorry for crappy editing)

KGBoE is not compatible with Linux kernel > v5.7.0 (see link). I had to patch the Linux kernel and KGDBoE source code to get KGDBoE to work as following:

1. Export the missing symbols again in Kernel, that were unexported in recent kernel builds:

diff --git a/kernel/kallsyms.c b/kernel/kallsyms.c

index 4fb15fa96734..4669f3cfb55b 100644
--- a/kernel/kallsyms.c

+++ b/kernel/kallsyms.c
@@ -176,6 +176,7 @@ unsigned long kallsyms_lookup_name(const char *name)
        }
        return module_kallsyms_lookup_name(name);
 }
+EXPORT_SYMBOL_GPL(kallsyms_lookup_name);
 int kallsyms_on_each_symbol(int (*fn)(void *, const char *, struct module *,
                                      unsigned long),
diff --git a/kernel/module.c b/kernel/module.c
index 1c5cff34d9f2..cd07a842bdb2 100644
--- a/kernel/module.c
+++ b/kernel/module.c
@@ -4517,7 +4517,7 @@ struct module *__module_address(unsigned long addr)
        }
        return mod;
 }
-
+EXPORT_SYMBOL_GPL(__module_address);

2. Disable KGDBoE Module parameter kallsyms_lookup_name_address by commenting the following:

   in file kgdboe_main.c

   //uint64_t kallsyms_lookup_name_address; //module_param(kallsyms_lookup_name_address, ullong, 0444);

   static int __init kgdboe_init(void) { // if (!kallsyms_lookup_name_address) // { // printk(KERN_ERR "kgdboe: this kernel version requires specifying the address of kallsyms_lookup_name explicitly. Please append 'kallsyms_lookup_name_address=0x...' to insmod command line.\n"); // return -EINVAL; // }

   in file tracewrapper.h: //extern uint64_t kallsyms_lookup_name_address; //#define kallsyms_lookup_name ((unsigned long(*)(const char *))kallsyms_lookup_name_address)