On 2019-12-02 14:44, Bruce Ashfield wrote:
On Mon, Dec 2, 2019 at 4:14 AM Mike Looijmans <mike.looijm...@topic.nl> wrote:
On 01-12-19 22:57, Peter Bergin via Lists.Yoctoproject.Org wrote:
Hi,
I'm currently working in a project using Yocto 2.6 (thud) release. It has
default kernel v4.18 and also linux-libc-headers from kernel v4.18. In my
project we will use kernel v4.1. I would like advice how to handle the
linux-libc-headers package for my project, should I use the v4.18 headers or
should I use the v4.1 header files which matches the running kernel?
From https://www.kernel.org/doc/html/latest/kbuild/headers_install.html:
"Kernel headers are backwards compatible, but not forwards compatible. This
means that a program built against a C library using older kernel headers
should run on a newer kernel (although it may not have access to new
features), but a program built against newer kernel headers may not work on an
older kernel."
With the information from the quote above I would directly use v4.1 headers as
my linux-libc-headers. But then reading the information in the file
meta/recipes-kernel/linux-libc-headers/linux-libc-headers.inc makes me think
another round. It states:
"
# You're probably looking here thinking you need to create some new copy
# of linux-libc-headers since you have your own custom kernel. To put
# this simply, you DO NOT.
...
# There can also be a case where your kernel extremely old and you want
# an older libc ABI for that old kernel. The headers installed by this
# recipe should still be a standard mainline kernel, not your own custom
# one.
"
The first part states that I should not change linux-libc-headers. But when I
read the last part I'm not sure about the interpretation and it could be for
my case. Just a matter of definition if v4.1 is extremely old compared to v4.18.
Then another thing comes in to the equation; the LIBC ABI. When I look into
the configuration of the glibc package it uses the configure switch
"--enable-kernel=3.2" which means it shall be compatible with all kernel newer
than v3.2. Then probably glibc is fine if it is compiled with v4.18 and run on
v4.1?
If building all applications against v4.18 headers but run on v4.1 kernel. I
have a feeling that there potentially can be problems here.
Please help me with some information about this and share your opinions? Are
there any risks at all to use v4.1 as linux-libc-headers in my Yocto build?
The only drawback I see is that it will be a new configuration not well tested
by the community. Are there other risks or drawbacks using your own version of
linux-libc-headers?
It is not broken, so please don't fix it.
OpenPLi has been using kernels way older than 4.1 with the kernel-headers
generated by OE/yocto and did not experience any problems with that. There's
about 50+ machines in there that have pre-built binary drivers that only work
with a particular kernel config and hence the old stuff.
There are some corner-cases with exotic kernels and exotic exports and exotic
boot executables that use the kernel compiler, but I doubt that you're in
there...
If you have a kernel that exports something that's not in the regular headers,
it's way better to solve that using a syscall than trying to poke in low level
libc stuff.
So again, if you don't experience problems, please don't try to fix it...
This has been my experience as well.
That's appealing approach. The part with the custom kernel headers is in
my case solved that we have separate kernel headers that we add to the
SDK. It is how I have understood the recommendation to solve this.
I've run a really wide set of BSP kernel's against the various "much
newer" yocto/OE libc-headers over the years, and I've never hit an ABI
or otherwise incompatibility.
Actually there are issues with Yocto 2.6 together with v4.1 headers. I
did an experiment to use v4.1 as linux-libc-headers. For my image i hit
build failures in two packages, systemd and iptables. Those were easily
fixed by upstream commits referenced below. I can not by a quick review
say that those should cause run-time issues, probably not, but just
showing that there are cases where packages not is fully aligned with
all versions of kernel headers.
systemd:
https://github.com/systemd/systemd/commit/9c869d08d82c73f62ab3527567858ce4b0cf1257
iptables:
https://git.netfilter.org/iptables/commit/?id=5beb1582d13d3bfdd0d2b277f5f3154b2fbf4a8e
If you want the most deterministic build for one system and one setup I
can not see any other option than using the same version for the kernel
headers in both virtual/kernel and linux-libc-headers. Then if you want
flexibility and possibility to support many builds it is better to have
one common linux-libc-header version. Also from a test and validation
point of view it is better if many people using the same version of
linux-libc-headers instead of their own combination as we have a
situation where this works almost all the time. Can anyone comment on
this analyze, is it valid?
Thanks,
/Peter
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