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...
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