PRoot allows running programs from a recent guest distro on an obsolete host distro by emulating kernel features and system calls not present in the older host kernel. It does this through syscall emulation, flag emulation, modifying ELF auxiliary vectors, and replacing heap allocation calls. This provides kernel compatibility that allows running a program like VLC 2.0.8 from Ubuntu 13.04 on Redhat 4.7, overcoming the "kernel too old" error, and works for any host/guest combination.

1. Running programs from
a recent guest distro
on an obsolete host distro

2. Why would you do such a thing?
•Validate programs for the guest distro
•Use programs available only for the guest distro
•Publish programs with their original environment
for the sake of reproducibility (CARE, CDE)
•…
2

3. How could you do such a thing?
• Full virtualisation : virtual hardware booting the
guest OS (VirtualBox, VMWare, …)
• Partial virtualisation : guest programs requests
are redirected to the host kernel
– Kernel based : chroot
– LD_PRELOAD based : ScratchBox2, fakechroot, …
– ptrace based : PRoot, CDE, fakeroot-ng, ...
3

4. Risk with partial virtualisation ... 4
“FATAL: kernel too old”

5. Solution ! 5
PRoot makes
your kernel
grow younger!

6. Kernel features emulated by PRoot 6
kernel
version
Emulated features
2.6.16 openat, linkat, ... syscalls
2.6.19 epoll_pwait syscall
2.6.23 CLOEXEC flag for open
2.6.24 DUPFD_CLOEXEC command for fcntl
2.6.27 CLOEXEC, NONBLOCK flags for many syscalls
2.6.28 accept4 syscall
2.6.29 AT_RANDOM ELF auxiliary vector
* AT_SYSINFO ELF auxiliary vector (discarded)
* heap allocation

7. How does it work?
•Syscall emulation : replace unsupported
syscalls with something equivalent.
Example :
openat(<fd>, <path>, …)
is converted into :
open(canon(
readlink(/proc/<pid>/fd/<fd>)
+ <path>), …)
7

8. How does it work?
•Flag emulation : append one or several calls to
fcntl right after the original syscall :
Example :
open(<path>, <flags> | O_CLOEXEC)
is converted into :
fcntl(open(<path>, <flags>), FD_CLOEXEC)
8

9. How does it work in PRoot?
•ELF auxiliary vectors : read/write the process
memory right after execve
•heap allocation: replace call to brk with call to
mmap or mremap
9

10. Demo time !
1.On Redhat 4.7, only VLC 0.8.6 is officially available
2.It’s way too old !
3.Let’s build the VLC 2.0.8 on Redhat 4.7 …
4.… never mind, 57 packages are missing or outdated
5.Let’s run VLC 2.0.8 from Ubuntu 13.04 (Linux 3.8.0, 2012-06-04)
on Redhat 4.7 (Linux 2.6.9, 2004-10-19):
1.without proot -k : « FATAL : kernel too old »
2.with proot -k : http://youtu.be/AJ5kaO1HKlU
6.It works whatever the host and guest systems are !
10

11. Demo dynamic statistics 11
●
115.000 syscalls
●
200 emulated syscalls
●
1700 emulated flags
●
50 ELF auxiliary vector changes
●
400 emulated heap syscalls

12. When? 12
This kernel compatibility support
will be available in
PRoot and CARE
by the end of Septembre (2013).