Hi All,   Both Ubuntu and Fedora have different GCC version. So different Kernel images will be created on both the OS.   -Anand Moon ________________________________ From: Andy Johnson <johnsonzjo@gmail.com> To: Valdis.Kletnieks@vt.edu Cc: kernelnewbies@kernelnewbies.org Sent: Friday, May 24, 2013 9:07 PM Subject: Re: building a kernel from a mounted partition Thanks Sorry for my ignorance - by checking timezone do you mean some environment variable ? I don't know much about admin tasks as you can figure... Andy On Fri, May 24, 2013 at 5:03 PM,  <Valdis.Kletnieks@vt.edu> wrote:

On Fri, 24 May 2013 11:05:57 +0300, Andy Johnson said:

...

Hello, I have a dual boot machine, Ubuntu/Fedora. I have a kernel tree on the fedora partition. Now, I had build a kernel on the Fedora partition successfully.

  I did **not** change anything in the tree, and then

When I boot into Ubuntu and mount the fedora partition, and go there and run "make" , it builds ** all **  the tree again ! any I idea why ?

And after the build on Ubuntu is over successfully, if I boot into Fedora again and run make, it **again** builds all the tree.

Is there a way to avoid this ?

Check that your timezone is set correctly in both installs - having the time ping-ponged back and forth at each reboot can cause such issues.

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On Sat, 25 May 2013 22:54:56 +0300, Andy Johnson said:

Hi, It seems to me that you are right. It seems that there is no problem with the timezone. I checked and indeed the gcc is slightly different in version is there a work around here ? I mean, I have gcc of ubuntu, and gcc of fedora in /mnt/sda3.

Aha. Found it. THe problem is that the file arch/x86/kernel/asm-offsets.s contains in it the data for 'struct utsname' - which includes the GCC version used to compile it (as seen in 'cat /proc/version'. However, due to what *else* is in that file, changing it (as happens when you change the compiler) will basically force a rebuild of the entire tree. This is somewhat by design - there have in the past been gcc compiler versions that had a sufficiently different ABI to cause problems (for instance, around 2.95 or so, some versions would pass a structure less than 8 bytes long in registers, while other versions passed a pointer just like all other structs). Hilarity results if you have a mix of .o's compiled with the two versions. Here's the command line I used to do kernel compiles while working around a few gcc 4.7 and .8 optimizer bugs (4.6.3 worked): make CC=/opt/gcc-4.6.3/bin/gcc-46 HOSTCC=/opt/gcc-4.6.3/bin/gcc-46 but of course, this assumes that gcc-46 was built with an installdir of /opt/ gcc-4.6.3. You'll probably need to do something to add a '-B /opt/whatever' flag to the gcc call so the main gcc program finds cpp, cc1, and the rest from "the other system's directory", rather than your running /usr/bin. At some point, maybe it's easier to just decide "I'll do all my builds under Fedora, even for kernels I'll use for Ubuntu".