C/R 32-bit tasks on x86_64

[xemul]

For x86 we only dump and restore 64-bit tasks. Doing 32-bit should also be done, but keep in mind, that not only 64-bit tree OR 32-bit tree should be supported. There can be mixed 64-and-32-bit trees out there and CRIU should support those too.

[xemul]

@cyrillos in Cc :)

[cyrillos]

Yes, we've compat mode (ie 32bit tasks running on x86-64 kernel) in my todo list. It will require kernel patching. Thanks for reminder ;)

[xemul]

@0x7f454c46 is working on it

[xemul]

https://criu.org/32bit_tasks_C/R

[rockymtnman]

Hi guys, just curious if you have an estimate when this feature might be added. Guessing this is low priority fix?

[xemul]

Cc @0x7f454c46 and @cyrillos

[cyrillos]

We're working on it.

[rockymtnman]

Ok, thanks for the update. I'm a developer, if there are any small tasks that you think someone could easily pick up I'd be happy to help

[cyrillos]

Sure, thanks. Once I manage to split this task into pieces I'll ping you.

[0x7f454c46]

Hi @rockymtnman,
there are easy things to do, but to test them you'll need to patch the kernel.
Linux kernel from v4.9 supports 32-bit C/R on x86_64 platform. But, as it happens, there is a kernel bug which was found on ZDTM test suite. There are patches to fix that bug, but yet they haven't got into mainline. So, to correctly test 32-bit C/R, you'll need to get sources of kernel that's newer than v4.9, patch them. Also you'll need to boot that kernel with vsyscall=none boot parameter. That's the hardest part of preparations.
After you got a patched kernel running, you can run compatible ZDTM tests with:

make COMPAT_TEST=y -j9 zdtm
./test/zdtm.py run -a --keep-going

There are a couple of tests failing at this moment, you can find a list of them here. The easiest things to fix are:

1. futex-rl test - need to call 32-bit syscall for dumping and restoring robust list: the kernel keeps two lists per task - compatible and native futex lists. So, you'll need to change get_task_futex_robust_list() and restore_thread_common() to call 32-bit syscall for compatible tasks.
2. Even easier, if you know python: patch zdtm.py to add option, something like '--compat' to call make with COMPAT_TEST=y parameter and run compat tests. (but only if CONFIG_COMPAT defined)
3. Well, you can fix any ZDTM test that fails, but I don't promise that it'll be so easy as previous two.

[rockymtnman]

Ok, it'll take me a little bit to get up to speed, but I'll give it a try

[rockymtnman]

Hi @0x7f454c46 where can I find the patchfile or source for your patch? I see the affected files listed, but not sure how to get access to your source. I have v4.9.9 kernel building now (first time)

[cyrillos]

Dima's patches can be fetched here https://patchwork.kernel.org/project/LKML/list/?page=37