Re: [PATCH v5 1/3] Provide in-kernel headers to make extending kernel easier

[Alexei Starovoitov <alexei.starovoitov@gmail.com>]

On Tue, Apr 16, 2019 at 09:57:08AM -0700, Olof Johansson wrote:
> On Tue, Apr 16, 2019 at 9:46 AM Alexei Starovoitov
> <alexei.starovoitov@gmail.com> wrote:
> >
> > On Tue, Apr 16, 2019 at 04:22:40PM +0200, Greg Kroah-Hartman wrote:
> > > On Tue, Apr 16, 2019 at 09:45:09AM -0400, Steven Rostedt wrote:
> > > > On Tue, 16 Apr 2019 09:32:37 -0400
> > > > Karim Yaghmour <karim.yaghmour@opersys.com> wrote:
> > > >
> > > > > >>> Then we should perhaps make a new file system call tarballs ;-)
> > > > > >>>
> > > > > >>>   /sys/kernel/tarballs/
> > > > > >>>
> > > > > >>> and place everything there. That way it removes it from /proc (which is
> > > > > >>> the worse place for that) and also makes it something other than debug.
> > > > > >>> That's what I did for tracefs.
> > > > > >>
> > > > > >> As horrible as that suggestion is, it does kind of make sense :)
> > > > > >>
> > > > > >> We can't put this in debugfs as that's only for debugging and systems
> > > > > >> should never have that mounted for normal operations (users want to
> > > > > >> build ebpf programs), and /proc really should be for processes but that
> > > > > >> horse is long left the barn.
> > > > > >>
> > > > > >> But, I'm willing to consider putting this either in a system-fs-like
> > > > > >> filesystem, or just in sysfs itself, we do have /sys/kernel/ to play
> > > > > >> around in if the main objection is that we should not be cluttering up
> > > > > >> /proc with stuff like this.
> > > > > >>
> > > > > >
> > > > > > I am ok with the suggestion of /sys/kernel for the archive. That also seems
> > > > > > to fit well with the idea that the headers are kernel related and probably
> > > > > > belong here more strictly speaking, than /proc.
> > > > >
> > > > > This makes sense. And if it alleviates concerns regarding extending
> > > > > /proc ABIs then might as well switch to this.
> > > > >
> > > > > Olof, what do you think of this?
> > > >
> > > > BTW, the name "tarballs" was kind of a joke. Probably should come up
> > > > with a better name. Although, I'm fine with tarballsfs ;-)
> > >
> > > No need to have this be a separate filesystem, we can use a binary sysfs
> > > file in /sys/kernel/ for this as the kernel is not doing any "parsing"
> > > of the data, it is just dumping it out to userspace.
> >
> > What folks keep saying that an fs of header files is easier to use
> > than tarball from bcc and cleaner from architectural pov.
> > That's not the case.
> > From bcc side I'd rather have a single precompiled headers blob
> > that I can feed into clang and improve bpf program compilation time.
> > Having a set of headers is a step to generate such .pch file,
> > but once generated the headers can be removed from fs and kheaders
> > module unloaded.
> > The sequence is: bcc checks standard /lib/module location,
> > if not there loads kheader mod, extracts into known location, and unloads.
> 
> May I suggest keeping the bcc-populated headers somewhere else?

what do you mean by bcc-populated?
bcc keeps its own headers inside libbcc.so .data section and provides
them to clang as 'memory buffer' in clang's virtual file system.

> Ideally something cleaned out on every reboot in case kernel changes
> without version string doing it.
> 
> That way you can by default prefer the module-exported tarball, and
> fall back to /lib/module/$(uname -r)/ if not available, instead of the
> other way around and instead of having to check creation times on the
> dir vs boot time of the kernel, etc.

the order of checks is bcc implementation detail. we can change that later.
we've seen issues with /lib/modules/`uname -r` being corrupted by chef,
so we might actually extract from kheaders.tar.xz all the time and more
than once.
Like try-compiling a simple prog and if it doesn't work, do the extract.

> Anyway, that's just an implementation detail. But it's the kind of
> detail that all tools that use this would need to get right, instead
> of doing it right once by exporting it in a way that it can be
> directly used.

Today bcc is the only tool that interacts with clang this way.
There is enough complexity and plenty of complex issues with
on-the-fly recompile approach.
I strongly suggest anyone considering new on-the-fly recompile to work
with us on BTF instead.

The set of headers is not an ultimate goal. See the example with pch. 
bpf tracing needs three components:
- all types and layout of datastructures;
  including all function prototypes with arg names
- all macros
- all inline functions

The first one is solved by BTF based solution,
but macroses and infline functions have no substitute, but C header files.
That is today.
Eventually we might find a way to reduce dependency on headers and have
macroses and infline functions represented some other way.
Like mini-pch where only relevant bits of headers are represented as
clang's syntax tree or mini C code.
The key point is that having headers is not a goal.
Making kernel maintain an fs of headers is imo a waste of kernel code.
The most minimal approach of compressed tarball is preferred.

> 
> > The extraced headers are in plain fs cache and will be evicted from memory
> > when bcc is done compiling progs.
> > imo much cleaner than kernel maintaining headers-fs and wasting memory.
> 
> So, in my original proposal I recommended unmounting when not needing
> it, which would remove the memory usage as well.

and such header-fs would uncompress internal tarball, create inodes, dentries
and to make sure all that stuff is cleanly refcnted and freed.
imo that is plenty of kernel code for no good reason.

> > Where kheaders.tar.xz is placed doesn't really matter.
> > /proc or /sys/kernel makes no real difference.
> 
> If done in a location that isn't a perpetual ABI commitment, a tarball
> solution is something we can work with.

Fair enough. My guess that kheaders.tar.xz in this shape we would
need for at least 5 years. After that we'll come up with better approach.