linux-libc-headers - how to handle for older kernels?

linux-libc-headers - how to handle for older kernels?

[Peter Bergin]

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?

Best regards,
/Peter

Re: [yocto] linux-libc-headers - how to handle for older kernels?

[Mikko Rapeli]

Hi,

On Sun, Dec 01, 2019 at 10:57:15PM +0100, Peter Bergin 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?

I would fork linux-libc-headers to the kernel you are actually using despite the
comment in the file.

That's the only way to export headers from the correct kernel to everyone
who needs them.

There is no other documented way of exporting the actual kernel headers to users
in bitbake builds and SDK.

One optimization I would do is to fork the uapi headers from the actual kernel
recipe so that full system does not need to be recompiled when kernel changes.

One needs to manually track for kernel uapi header changes then.

Cheers,

-Mikko

Re: [yocto] linux-libc-headers - how to handle for older kernels?

[Mike Looijmans]

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...

Re: [yocto] linux-libc-headers - how to handle for older kernels?

[Mikko Rapeli]

Hi,

On Mon, Dec 02, 2019 at 09:13:47AM +0000, Mike Looijmans 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...

How to find a Linux kernel uapi header file with custom ioctl() etc definitions which
are missing form the default linux-libc-headers?

Other recipes and SDK builds need these.

Copying the needed header files into the recipes who needs them is not ok.

Cheers,

-Mikko

Re: [yocto] linux-libc-headers - how to handle for older kernels?

[Mike Looijmans]

On 02-12-19 10:19, Mikko.Rapeli@bmw.de wrote:
> Hi,
> 
> On Mon, Dec 02, 2019 at 09:13:47AM +0000, Mike Looijmans 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...
> 
> How to find a Linux kernel uapi header file with custom ioctl() etc definitions which
> are missing form the default linux-libc-headers?
> 
> Other recipes and SDK builds need these.
> 
> Copying the needed header files into the recipes who needs them is not ok.

One solution I can think of is to put the header into it's own 
recipe/repository and then refer to it like any other library would. Refer to 
that recipe from the module (or kernel) recipe that needs it. This way you 
have your header in a single maintainable location and dependencies properly 
taken care of.

If that's not something you could live with, share your recipes, since vague 
problem descriptions will only get you vague solutions...

Re: [yocto] linux-libc-headers - how to handle for older kernels?

[Mikko Rapeli]

Hi,

On Mon, Dec 02, 2019 at 09:28:03AM +0000, Mike Looijmans wrote:
> One solution I can think of is to put the header into it's own 
> recipe/repository and then refer to it like any other library would. Refer to 
> that recipe from the module (or kernel) recipe that needs it. This way you 
> have your header in a single maintainable location and dependencies properly 
> taken care of.
> 
> If that's not something you could live with, share your recipes, since vague 
> problem descriptions will only get you vague solutions...

This is the problem I see with multiple BSPs. In the end for every one of them
the best solution is to fork linux-libc-headers.

Adding custom recipes to export headers for every BSP kernel is not scaling so well.

Sharing the BSPs and hacks around them would be nice but they are not public sadly :(

Cheers,

-Mikko

Re: [yocto] linux-libc-headers - how to handle for older kernels?

[Bruce Ashfield]

On Mon, Dec 2, 2019 at 4:14 AM Mike Looijmans <mike.looijmans@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.

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.

Like Mike is saying, if you probably have a feeling if your BSP is
doing anything exotic and can explicitly test those problem areas, or
put in place some specific workarounds if there are some changed or
different interfaces.

Cheers,

Bruce

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thee at its end
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Re: [yocto] linux-libc-headers - how to handle for older kernels?

[Adrian Bunk]

On Mon, Dec 02, 2019 at 09:33:33AM +0000, Mikko Rapeli wrote:
>...
> On Mon, Dec 02, 2019 at 09:28:03AM +0000, Mike Looijmans wrote:
> > One solution I can think of is to put the header into it's own 
> > recipe/repository and then refer to it like any other library would. Refer to 
> > that recipe from the module (or kernel) recipe that needs it. This way you 
> > have your header in a single maintainable location and dependencies properly 
> > taken care of.
> > 
> > If that's not something you could live with, share your recipes, since vague 
> > problem descriptions will only get you vague solutions...
> 
> This is the problem I see with multiple BSPs. In the end for every one of them
> the best solution is to fork linux-libc-headers.
>...

The fork can be a .bbappend with a patch adding the required
definitions to linux-libc-headers.

> Cheers,
> 
> -Mikko

cu
Adrian

Re: [yocto] linux-libc-headers - how to handle for older kernels?

[Mikko Rapeli]

On Mon, Dec 02, 2019 at 04:01:41PM +0200, Adrian Bunk wrote:
> On Mon, Dec 02, 2019 at 09:33:33AM +0000, Mikko Rapeli wrote:
> >...
> > On Mon, Dec 02, 2019 at 09:28:03AM +0000, Mike Looijmans wrote:
> > > One solution I can think of is to put the header into it's own 
> > > recipe/repository and then refer to it like any other library would. Refer to 
> > > that recipe from the module (or kernel) recipe that needs it. This way you 
> > > have your header in a single maintainable location and dependencies properly 
> > > taken care of.
> > > 
> > > If that's not something you could live with, share your recipes, since vague 
> > > problem descriptions will only get you vague solutions...
> > 
> > This is the problem I see with multiple BSPs. In the end for every one of them
> > the best solution is to fork linux-libc-headers.
> >...
> 
> The fork can be a .bbappend with a patch adding the required
> definitions to linux-libc-headers.

Yes, this is what I meant. Basically just adjust the SRC_URI and possibly PV
through a linux-libc-headers_%.bbappend in product or BSP specific meta layer.

Cheers,

-Mikko

Re: [yocto] linux-libc-headers - how to handle for older kernels?

[Peter Bergin]

On 2019-12-02 14:44, Bruce Ashfield wrote:
> On Mon, Dec 2, 2019 at 4:14 AM Mike Looijmans <mike.looijmans@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

Re: [yocto] linux-libc-headers - how to handle for older kernels?

[Bruce Ashfield]

On Tue, Dec 3, 2019 at 2:59 AM Peter Bergin <peter@berginkonsult.se> wrote:
>
> On 2019-12-02 14:44, Bruce Ashfield wrote:
> > On Mon, Dec 2, 2019 at 4:14 AM Mike Looijmans <mike.looijmans@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'm assuming that you mean those headers are in a separate package and
are installed into the dependent packages sysroot (or the SDK in your
descriptin) via other means that linux-libc-headers .. if so, yes,
that is the right thing to do.

> > 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.

These are one of the cases that do pop up, and yes, we normally fix by
updating (or patching) the packages. The same thing happens to me in
master each time I introduce the updated libc-headers for the release.

"They are normally just factoring/shuffling/churn on the header files
requiring some different includes (or dropped ones), and don't impact
the userspace -> kernel interfaces (i.e. the interface stability).

Looking at the ones you referenced below, I'd agree that they won't
impact the runtime.

>
> 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

I wouldn't say deterministic, but yes, if you are only ever concerned
about one kernel, one userspace, etc, then a more consistent build
would be against the 1:1 matching headers.

> flexibility and possibility to support many builds it is better to have
> one common linux-libc-header version. Also from a test and validation

The reality is that most builds run against any number of kernels (in
particular if you are getting into binary builds), so you do need a
stake in the ground for consistent headers -> many kernels, so yes,
more flexibility and reuse by not varying the libc-heades .. since
again, the are the libc-headers and the c-library does most of the
abstraction as well.

> 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?

That's the premise around bumping the libc-headers to a reference
version as I bump the various reference kernels. That we are getting
more coverage and collective benefits by using a common base across
many different userspace and kernel combinations.

Cheers,

Bruce

>
> Thanks,
> /Peter
>


-- 
- Thou shalt not follow the NULL pointer, for chaos and madness await
thee at its end
- "Use the force Harry" - Gandalf, Star Trek II

Re: [yocto] linux-libc-headers - how to handle for older kernels?

[Peter Bergin]

On 2019-12-03 16:28, Bruce Ashfield wrote:
> I'm assuming that you mean those headers are in a separate package and
> are installed into the dependent packages sysroot (or the SDK in your
> descriptin) via other means that linux-libc-headers .. if so, yes,
> that is the right thing to do.
Thanks Bruce!

Yes my plan is to create a new package in the 'virtual/kernel' recipe 
where I add the additional files to '/usr/include/linux/' and then add 
that new package to my SDK (or the sysroot if needed).

Best regards,
/Peter