wrestling dragons

so, next goal after doing a dummy subsystem is to start on the existing builtin subsystems. at least one should end up being modularized, and for others that I examine, analysis should be given on why they can't or would be difficult to be made into modules. so a quick glance over the existing subsystems turned into surprise progress when I discovered that net_cls (which additionally goes by cls_cgroup, net_cls_cgroup, and in all likelihood additional permutations thereof) not only is restricted to one file (devices and ns for example have miscellaneous function calls to them with ifdefs, in contrast), but also already has module_init() et.al. declarations at the bottom!

After a bit of hacking around with cgroup_load_subsys() (my new function) and the subsys_id, and also changing the Kconfig option to tristate, the build system happily modulificatarized net_cls for me. Booting up UML, I realized it was probably time to figure out how to make modprobe do the right thing - as it turns out, UML's documentation is very helpful in this regard. A quick hostfs mount and depmod later, and the module loads and runs just fine. Victoly!

In other news, binding GDB to UML yields some frightening results:

0x00007f614c38b420 in nanosleep () from /lib64/libc.so.6
(gdb) break cgroup_load_subsys
Breakpoint 1 at 0x60051819: file kernel/cgroup.c, line 3619.
(gdb) cont
Continuing.

Program received signal SIGSEGV, Segmentation fault.
memcpy () at arch/um/sys-x86_64/../../x86/lib/memcpy_64.S:68
68 movq %r11, 0*8(%rdi)
Current language: auto; currently asm
(gdb) cont
Continuing.

Program received signal SIGSEGV, Segmentation fault.
0x00007f614c36911b in memset () from /lib64/libc.so.6
(gdb)
Continuing.

Program received signal SIGSEGV, Segmentation fault.
0x00007f614c36873e in memset () from /lib64/libc.so.6
(gdb)
Continuing.

Breakpoint 1, cgroup_load_subsys (ss=0x62828fe0) at kernel/cgroup.c:3619
3619 if (ss->fork || ss->exit)
Current language: auto; currently c
(gdb)