[ale] Kernel cross compilation

[Michael H. Warfield]

On Tue, 2012-09-18 at 17:26 +0000, Boris Borisov wrote:
> Never done it that is why I'm asking.

> If you build kernel for MIPS CPU on x86 host but you are using debian
> 6 installation on host for example and build the same kernel but on
> different distro ( Ubuntu ) are these two kernel will be indentical ?

I'm not totally sure I understand the question.

Are you asking if you build on Debian vs Ubuntu vs Fedora vs CentOS on
the same arch will the kernels be identical?

Are you asking if you cross compile a MIPS or ARM kernel on an x88_64
platform will it be identical to the one built natively on it's target
arch?  You specifically mention "cross compile" in the subject but that
is generally taken to mean compiling on one architecture with another
architecture as a target and that's not what you described above.

Either way...  The answer depends heavily by what you mean by
"identical".  Binaries do have timestamps in them and a variety of data
that may vary.  If you build two "identical" kernels at two different
times, you will get two different binaries that will not have a matching
hash.  Just the fact that the binaries will contain the build date and
time insure that they can not be identical if you were to hash them.  It
may (WILL) depend on variations and versions of the compilers you use.
So, two different versions of Ubuntu with two different versions of gcc
can result in different code which is, most of the time, equivalently
functional.  There have been times where "compiler bugs" have resulted
in non-functional kernel builds but the kernel stresses the limits of
what a compiler can do with volatile data and varieties of interrupts,
concurency, multiprocessing, and timing.

If everything, EVERYTHING, is held consistent, then you should get a
consistent kernel regardless of the distribution you use.  That means
the same gcc and development tools, libraries, and headers.  Will it be
"identical"?  Not by my definition of the word identical in that you
would not be able to run a sha1 / sha256 / sha512 hash on the two and
get the same numbers.  They may be functionally identical but they will
not be identical binaries.

Regards,
Mike
-- 
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