Re: State of userland headers

[Randy.Dunlap]

On Fri, 24 Mar 2006 17:46:27 -0500 Kyle Moffett wrote:

> On Mar 24, 2006, at 16:48:47, Nix wrote:
> > On 24 Mar 2006, Rob Landley suggested tentatively:
> >> On Friday 24 March 2006 1:51 pm, Kyle Moffett wrote:
> >>> 1: Ewww, bad glibc!
> >>> 2: The symbols in kabi/*.h should probably all start with __kabi_
> >>
> >> Any grand new incompatible thing is something I will happily  
> >> ignore for as long as I am able to, and I'm not alone here.  Your  
> >> uptake will be zero.
> >
> > I concur. The purpose of this thing is by definition to provide  
> > libcs with the kernel/user interface stuff they need in order for  
> > userspace programs to be compiled. There's no point defining a new  
> > interface because there is a massive quantity of *existing* code  
> > out there that we must work with. (Plus, it can be, uh, difficult  
> > to get changes of this nature into glibc in particular, and glibc  
> > is the 300-pound gorilla in this particular room. If the headers  
> > don't have working with it as a goal, they are pointless.)
> 
> Hmm, I didn't really explain my idea very well.  Let me start with a  
> list of a facts.  If anybody disagrees with any part of this, please  
> let me know.
> 
> 1)  The <linux/*.h> headers include a lot of information essential to  
> compiling userspace applications and libraries (libcs in  
> particular).  That same information is also required while building  
> the kernel (IE: The ABI).
> 2)  Those headers have a lot of declarations and definitions which  
> must *not* be present while compiling userspace applications, and is  
> basically kernel-only stuff.
> 3)  Glibc is extremely large and complex 500-pound gorilla and  
> contains an ugly build process and a lot of static definitions in its  
> own header files that conflict with the definitions in the kernel  
> headers.
> 4)  UML runs into a lot of problems when glibc's headers and the  
> native kernel headers headers conflict.
> 
> Here's some of my opinions about this:
> 
> 1)  Trying to create and maintain 2 separate versions of an ABI as  
> large and complex as the kernel<=>userspace ABI across new versions  
> and features would be extremely difficult and result in subtle bugs  
> and missing features, even over a short period of time.
> 2)  Ideally there should be three distinct pieces, the kernel, the  
> ABI, and userspace.  Compiling either the kernel or userspace  
> requires the ABI, but the ABI depends only on the compiler.
> 3)  Breaking any compatibility is bad
> 4)  Trying to continue to maintain the glibc custom-header-file  
> status-quo as more APIs and architectures get added to the kernel is  
> going to become an increasingly difficult and tedious task.
> 
> My proposal (which I'm working on sample patches for) would be to  
> divide up the kernel headers into 2 parts.  The first part would be  
> <kabi/*.h>, and the second would be all the traditional kernel-only  
> headers.  The kabi headers would *only* define things that begin with  
> the prefix __kabi_.  This would mean that the kabi headers have no  
> risk of namespace contamination with anything else existing in the  
> kernel or userspace, and since they would depend only on the  
> compiler, they would be useable anywhere.
> 
> The second step would be to convert the traditional linux header to  
> include the corresponding kabi header, then redefine its own  
> structures and defines in terms of those in the kabi header.  This  
> would provide complete backwards compatibility to all kernel code, as  
> well as to anything that currently compiles using the existing kernel  
> headers.  The entire rest of the <linux/*.h> header file would be  
> wrapped in #ifdef __KERNEL__, as it should not be needed by anything  
> in userspace.
> 
> In the process of those two steps, we would relocate many of the  
> misplaced "#ifdef __KERNEL__" and "#endif /* __KERNEL__ */".  The  
> kabi headers should not mention __KERNEL__ at all, and the linux/*  
> headers should be almost completely wrapped in __KERNEL__ ifdefs.   
> That should be enough to make klibc build correctly, although from  
> the description glibc needs significantly more work.
> 
> Once a significant portion of the kernel headers have been split that  
> way (preserving complete backwards compatibility), external projects  
> _may_ be converted to #include <kabi/*.h> instead of #include <linux/ 
> *.h>, although this would require other changes to the source to  
> handle the __kabi_ prefix.  Most of those should be straightforward,  
> however.  Since the kabi/*.h headers would not be kernel-version- 
> specific, they could be copied to a system running an older kernel  
> and reused there without problems.  Even though some of the syscalls  
> and ioctls referenced in the kabi headers might not be present on the  
> running kernel, portable programs are expected to be able to sanely  
> handle older kernels.
> 
> Once the kabi headers are available, it would be possible to begin  
> cleaning up many of the glibc headers without worrying about  
> differences between architectures.  If all critical constants and  
> datatypes are already defined in <kabi/*.h> with __kabi_ or __KABI_  
> prefixes, it should be possible to import those definitions into  
> klibc and glibc without much effort.
> 
> UML has other issues with conflicts between the native kernel headers  
> and the GLIBC-provided stubs.  It's been mentioned on the prior  
> threads about this topic that this sort of system would ease most of  
> the issues that UML runs into.
> 
> I'm working on some sample patches now which I'll try to post in a  
> few days if I get the time.

Kyle,
Do you have (recorded) or recall any constraints or requirements
on this $subject from Linus or Andrew or others?
I mean just basic big items, like "thou shalt not mix foo and bar".

I'm just looking for the basic parameters of this task.

Thanks,
---
~Randy
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