CentOS - May 2019 - What happened if install a el7 package on a el6 system

Hi,

Recently, I encountered a interesting phenomenon that CentOS 6.3
running as normal even if I (my colleague, actually) installed a
kernel that build for CentOS 7.x (e.g. kernel-3.10.0-327.el7.x86_64.rpm).

I found kernel is mismatch accidentally when I using "uname -r" to
check kernel version. So my question is what the harmness we will
get if I install a el7 rpm into a el6 system?

Thanks.

On Tue, 7 May 2019 at 04:06, wuzhouhui <wuzhouhui14 at mails.ucas.ac.cn>
wrote:
> Hi,
>
> Recently, I encountered a interesting phenomenon that CentOS 6.3
> running as normal even if I (my colleague, actually) installed a
> kernel that build for CentOS 7.x (e.g. kernel-3.10.0-327.el7.x86_64.rpm).
>
> I found kernel is mismatch accidentally when I using "uname -r"
to
> check kernel version. So my question is what the harmness we will
> get if I install a el7 rpm into a el6 system?
>
>
Many CentOS-7 packages will not install because they will need dependencies
that the EL-6 does not have. The kernel is different because it is mostly
self-contained and meant to be parallel installed. In most cases, it should
result in an unbootable system because the boot is going to be
dracut+systemd bits and the EL-6 has none of that.



> Thanks.
> _______________________________________________
> CentOS mailing list
> CentOS at centos.org
> https://lists.centos.org/mailman/listinfo/centos
>

-- 
Stephen J Smoogen.

At Tue, 7 May 2019 05:50:35 -0400 CentOS mailing list <centos at
centos.org> wrote:
> 
> On Tue, 7 May 2019 at 04:06, wuzhouhui <wuzhouhui14 at
mails.ucas.ac.cn> wrote:
> 
> > Hi,
> >
> > Recently, I encountered a interesting phenomenon that CentOS 6.3
> > running as normal even if I (my colleague, actually) installed a
> > kernel that build for CentOS 7.x (e.g.
kernel-3.10.0-327.el7.x86_64.rpm).
> >
> > I found kernel is mismatch accidentally when I using "uname
-r" to
> > check kernel version. So my question is what the harmness we will
> > get if I install a el7 rpm into a el6 system?
> >
> >
> Many CentOS-7 packages will not install because they will need dependencies
> that the EL-6 does not have. The kernel is different because it is mostly
> self-contained and meant to be parallel installed. In most cases, it should
> result in an unbootable system because the boot is going to be
> dracut+systemd bits and the EL-6 has none of that.
And I wonder if the EL7 kernel will even show up as an available kernel.  EL7 
uses Grub 2 and EL6 uses Grub [1].  *Different* config files. Because (unlike 
Lilo), grub is not updated in the MBR each time its config is updated (eg 
installing a new kernel), it is very likely the EL7 kernel rpm install might 
not touch /boot/grub/grub.conf, since it is expecting to update 
/boot/grub/grub.cfg instead.

I know that when I installed Ubuntu 18.04 as a *second* OS, that even though 
the /boot file system is shared between CentOS 6 and Ubuntu 18.04 the Ubuntu 
18.04 installer did not touch /boot/grub/grub.conf and installed 
/boot/grub/grub.cfg along side (I manually reinstalled grub 1 and manually 
hacked /boot/grub/grub.conf to put the Ubuntu 18.04 boot option in).
> 
> 
> 
> 
> > Thanks.
> > _______________________________________________
> > CentOS mailing list
> > CentOS at centos.org
> > https://lists.centos.org/mailman/listinfo/centos
> >
> 
> 
-- 
Robert Heller             -- 978-544-6933
Deepwoods Software        -- Custom Software Services
http://www.deepsoft.com/  -- Linux Administration Services
heller at deepsoft.com       -- Webhosting Services

On 2019-05-07 04:50, Stephen John Smoogen wrote:
> On Tue, 7 May 2019 at 04:06, wuzhouhui <wuzhouhui14 at
mails.ucas.ac.cn> wrote:
> 
>> Hi,
>>
>> Recently, I encountered a interesting phenomenon that CentOS 6.3
>> running as normal even if I (my colleague, actually) installed a
>> kernel that build for CentOS 7.x (e.g.
kernel-3.10.0-327.el7.x86_64.rpm).
>>
>> I found kernel is mismatch accidentally when I using "uname
-r" to
>> check kernel version. So my question is what the harmness we will
>> get if I install a el7 rpm into a el6 system?
>>
>>
> Many CentOS-7 packages will not install because they will need dependencies
> that the EL-6 does not have. The kernel is different because it is mostly
> self-contained and meant to be parallel installed. In most cases, it should
> result in an unbootable system because the boot is going to be
> dracut+systemd bits and the EL-6 has none of that.
> 
+1

One needs to rebuild (recompile) everything that was not built on 
"mismatched" version of the system. And this will constitute correct 
"installation using old UNIX way". Otherwise you quite likely will
have
problems (even if they are hidden, i.e. if it appears to you that "all 
works"). One of examples would be using some library that is of 
different version than your extra software was compiled against. The 
library might be found, and successfully loaded at an execution time, 
but it may have different subroutine entry names or some missing which 
may be discovered only under some circumstances. At a compilation time 
the headers will be what provides necessary information... Anyway, 
understanding this will tell you apart as a sysadmin from
"non-sysadmin"
person ;-)

Valeri
> 
> 
> 
>> Thanks.
>> _______________________________________________
>> CentOS mailing list
>> CentOS at centos.org
>> https://lists.centos.org/mailman/listinfo/centos
>>
> 
> 
-- 
++++++++++++++++++++++++++++++++++++++++
Valeri Galtsev
Sr System Administrator
Department of Astronomy and Astrophysics
Kavli Institute for Cosmological Physics
University of Chicago
Phone: 773-702-4247
++++++++++++++++++++++++++++++++++++++++

I found kernel is mismatch accidentally when I using "uname -r"
to
> check kernel version. So my question is what the harmness we will
> get if I install a el7 rpm into a el6 system?
>
I would say it depends on the dependencies. If its just some userspace
tooling then it will probably work ok. Its just x86_64 binaries after all.
When you start getting into drivers and system level libraries then your
probably gonna have a bad time but even then, the kernel doesn't change
that much. Most of the new kernel stuff is for obscure virtualisation /
containerisation techniques.

Of course daemons will be a difficult area as the SystemD stuff might not
there but it seems that there is still a lot of packages that ship with
init.d scripts.

I generally found el6 packages to be generally compatable with el7 and vice
versa but there is no guarantee that the next update wont horribly break
your system. In fact, it probably will :)

Security was often a second concern to ' this system will horribly break if
we update it so lets just turn off the internet!'

Reproduce steps:

  1. Prepare a CentOS 6.3 system on a VM (make sure grubby-7.0.15-3.el6.x86_64
has been installed)
  2. Download kernel-3.10.0-327.el7.x86_64.rpm from http://vault.centos.org/
  3.
    3.1 Create a symlink with "ln -s /sbin/new-kernel-pkg
/usr/sbin/new-kernel-pkg"
    3.2 Install kernel-3.10.0-327.el7.x86_64.rpm with "rpm -i --force
--nodeps"
  4. Reboot

Then we'll see the system boot and running as normal. And /etc/grub.conf
updated automatically.
> -----Original Messages-----
> From: wuzhouhui <wuzhouhui14 at mails.ucas.ac.cn>
> Sent Time: 2019-05-07 16:05:54 (Tuesday)
> To: centos at centos.org
> Cc: 
> Subject: [CentOS] What happened if install a el7 package on a el6 system
> 
> Hi,
> 
> Recently, I encountered a interesting phenomenon that CentOS 6.3
> running as normal even if I (my colleague, actually) installed a
> kernel that build for CentOS 7.x (e.g. kernel-3.10.0-327.el7.x86_64.rpm).
> 
> I found kernel is mismatch accidentally when I using "uname -r"
to
> check kernel version. So my question is what the harmness we will
> get if I install a el7 rpm into a el6 system?
> 
> Thanks.
> _______________________________________________
> CentOS mailing list
> CentOS at centos.org
> https://lists.centos.org/mailman/listinfo/centos