Configuring and building the kernel using clang

[Darryl L. Pierce]

I'm trying to get up to speed with Linux kernel development and have a
few questions:

1. According to the book I'm reading (_Linux Kernel Development_ by
Robert Love) the current kernel's configuration can be found at
/proc/config.gz. But on Fedora, that's not the case, and it's in /boot/
as config-{VERSION}. How do I use this in the same way that I would have
used /proc/config.gz?

2. On the kernel-janitors mailing list I asked how to get started, and
was recommended to use clang to build the kernel and submit patches to
fix errors reported. Has anybody successfully built the kernel usin
clang packaged for Fedora? If so, how? I tried:

make cc=clang

and the build immediately failed on inline assembler code. I'd
appreciate a recommendation on how to approach building that way.

Thanks in advance.

[Dave Jones]

On Wed, Jul 18, 2012 at 11:28:38AM -0400, Darryl L. Pierce wrote:
> I'm trying to get up to speed with Linux kernel development and have a
> few questions:
> According to the book I'm reading (_Linux Kernel Development_ by
>
> Robert Love) the current kernel's configuration can be found at
> /proc/config.gz. But on Fedora, that's not the case, and it's in /boot/
> as config-{VERSION}. How do I use this in the same way that I would have
> used /proc/config.gz?

It's the same format, but the one in boot isn't gzipped, and doesn't use
up kernel memory.
> On the kernel-janitors mailing list I asked how to get started, and
>
> was recommended to use clang to build the kernel and submit patches to
> fix errors reported. Has anybody successfully built the kernel usin
> clang packaged for Fedora? If so, how? I tried:
>
> make cc=clang
>
> and the build immediately failed on inline assembler code. I'd
> appreciate a recommendation on how to approach building that way.

If you're just starting out, I'd advise something simpler until you're
comfortable working on the kernel.  This isn't a small amount of work
and there's plenty of other low-hanging fruit.

If you're determined however, I'd start by reading up at http://llvm.linuxfoundation.org/index.php/Main_Page
The stock kernel.org tree lacks some of the work already done.
(I've no idea on the state/quality of the patches already done, and whether
 they're even mergable upstream)

Dave

[Darryl L. Pierce]

On Wed, Jul 18, 2012 at 11:46:39AM -0400, Dave Jones wrote:
> On Wed, Jul 18, 2012 at 11:28:38AM -0400, Darryl L. Pierce wrote:
> > I'm trying to get up to speed with Linux kernel development and have a
> > few questions:
> > According to the book I'm reading (_Linux Kernel Development_ by
> >
> > Robert Love) the current kernel's configuration can be found at
> > /proc/config.gz. But on Fedora, that's not the case, and it's in /boot/
> > as config-{VERSION}. How do I use this in the same way that I would have
> > used /proc/config.gz?
>
> It's the same format, but the one in boot isn't gzipped, and doesn't use
> up kernel memory.

When I copied it into my development tree as .config and did:

make oldconfig

the environment seemed to want to redo the whole thing. Maybe I'm doing
something wrong?
> > On the kernel-janitors mailing list I asked how to get started, and
> >
> > was recommended to use clang to build the kernel and submit patches to
> > fix errors reported. Has anybody successfully built the kernel usin
> > clang packaged for Fedora? If so, how? I tried:
> >
> > make cc=clang
> >
> > and the build immediately failed on inline assembler code. I'd
> > appreciate a recommendation on how to approach building that way.
>
> If you're just starting out, I'd advise something simpler until you're
> comfortable working on the kernel.  This isn't a small amount of work
> and there's plenty of other low-hanging fruit.

I'm more interested in doing this to get some experience with the code
and the overall workflow of submitting patches to the kernel developers.
> If you're determined however, I'd start by reading up at http://llvm.linuxfoundation.org/index.php/Main_Page
> The stock kernel.org tree lacks some of the work already done.
> (I've no idea on the state/quality of the patches already done, and whether
>  they're even mergable upstream)

That's my main concern about this approach, that I'm trying to do
something too cutting edge when I'm not quite ready for that.

[Dave Jones]

On Wed, Jul 18, 2012 at 12:01:55PM -0400, Darryl L. Pierce wrote:
> On Wed, Jul 18, 2012 at 11:46:39AM -0400, Dave Jones wrote:
> > On Wed, Jul 18, 2012 at 11:28:38AM -0400, Darryl L. Pierce wrote:
> > > I'm trying to get up to speed with Linux kernel development and have a
> > > few questions:
> > > According to the book I'm reading (_Linux Kernel Development_ by
> > >
> > > Robert Love) the current kernel's configuration can be found at
> > > /proc/config.gz. But on Fedora, that's not the case, and it's in /boot/
> > > as config-{VERSION}. How do I use this in the same way that I would have
> > > used /proc/config.gz?
> >
> > It's the same format, but the one in boot isn't gzipped, and doesn't use
> > up kernel memory.
>
> When I copied it into my development tree as .config and did:
>
> make oldconfig
>
> the environment seemed to want to redo the whole thing. Maybe I'm doing
> something wrong?

If the version of the config is older than the tree you're building,
it will ask you questions. This is expected.

Dave