[LLVMdev] Compile Linux Kernel module into LLVM bitcode

[Kevin Boos]

Ping,

Per Duncan's suggestion, I just ended up running a verbose make command
   "make V=1  drivers/gpu/drm/drm_drv.o"
on the kernel module that I needed (which was drm_drv). That make command
prints out  a series of long "gcc" commands, so you can see exactly how
each file is compiled. I then copied and pasted each gcc command for each
file (you could script this instead) and changed each "gcc" invocation to
"clang -emit-llvm" instead.

At this point, you have an output file (which by default is drm_drv.o), but
you can change the file extension to be .bc if you want. That drm_drv.o
file is the actual bitcode file that you can use with "opt" or "llvm-dis".

I never bothered figuring out how to do it with the default kbuild
Makefiles, if that's even possible.

Best of luck,
Kevin




On Sat, Jun 8, 2013 at 8:06 AM, chenping <chenping19851 at hotmail.com> wrote:

> Greatings,
>
>      I am encountering the same issue with Kevin. Does anyone know how to
> compile the linux kernel to .bc file?
>      Is there any oneline guide for this? Thanks!~
>
> Bests,
> Ping
>
>
> > Date: Sat, 1 Jun 2013 20:15:56 +0200
> > From: baldrick at free.fr
> > To: llvmdev at cs.uiuc.edu
> > Subject: Re: [LLVMdev] Compile Linux Kernel module into LLVM bitcode
>
> >
> > Hi Kevin,
> >
> > On 01/06/13 09:59, Kevin Boos wrote:
> > > Greetings,
> > >
> > > I am trying to compile a linux kernel module (currently a small part
> of the gpu driver) into the bitcode ".bc" format so that I can run a pass
> on it using the "opt" command. This pass will count the number of times
> copy_to/from_user() is invoked.
> > >
> > > Compiling the gnu driver kernel modules works with clang as a front
> end. However, I am unable to get the "-emit-llvm" flag to work.
> > >
> > > I am using the typical Makefile system (kbuild) included with the
> kernel, as such:
> > > $ make V=1 CC=clang CFLAGS="-emit-llvm -c" drivers/gpu/drm/drm_drv.o
> > >
> > > I have tried setting KBUILD_CFLAGS or KCFLAGS instead of CFLAGS, but
> nothing I do gets it to work. Trying these few different variations gets me
> to the point where it doesn't actually use the "-emit-llvm" flag in the
> clang (previously gcc) commands that are shown with the verbose V=1 option.
> If the "-emit-llvm" flag *does* make it into the clang command, it doesn't
> create the .bc file
> >
> > -emit-llvm doesn't change the name of the file output, it changes what
> is put
> > in the output file (bitcode rather than object code). So it is normal
> not to
> > see a file called xyz.bc, it will still be called xyz.o. If you look
> inside
> > it you should see that it starts with BC (followed by nasty stuff). You
> can
> > use llvm-dis to turn this into a file containing human readable IR. The
> > following error output shows that indeed bitcode is being output:
> >
> > and sometimes errors out like this:
> > >
> > > ====================
> > > objdump: scripts/mod/.tmp_empty.o: File format not recognized
> > > if [ "-pg" = "-pg" ]; then if [ scripts/mod/empty.o !=
> "scripts/mod/empty.o" ]; then
> /home/kevin/split_io_Linux/scripts/recordmcount "scripts/mod/empty.o"; fi;
> fi;
> > > gcc -Wp,-MD,scripts/mod/.mk_elfconfig.d -Wall -Wmissing-prototypes
> -Wstrict-prototypes -O2 -fomit-frame-pointer -o scripts/mod/mk_elfconfig
> scripts/mod/mk_elfconfig.c
> > > scripts/mod/mk_elfconfig < scripts/mod/empty.o >
> scripts/mod/elfconfig.h
> > > Error: not ELF
> >
> > Yup, it's not ELF, it's bitcode! This is going to be a problem for
> something
> > like the linux kernel that does a lot of tricky stuff with generated
> object
> > files. I suggest you only try compiling your particular module as
> bitcode,
> > and compiling everything else normally. For example, compile everything
> using
> > clang but without -emit-llvm, and tell make to produce verbose output.
> This
> > should show you the clang command line it uses to compile your module.
> Run
> > that command line by hand, but adding -emit-llvm. This should produce
> bitcode
> > for your module.
> >
> > > make[2]: *** [scripts/mod/elfconfig.h] Error 1
> > > make[1]: *** [scripts/mod] Error 2
> > > make: *** [scripts] Error 2
> > > ====================
> > >
> > >
> > > Essentially, I need to know how to use LLVM/clang with a linux kernel
> makefile to output the .bc bitcode file. I thought clang was a good drop-in
> replacement for gcc, but it hasn't worked out for me yet.
> >
> > You didn't just drop it in, you made a major change, telling it to output
> > bitcode rather than ELF.
> >
> > Ciao, Duncan.
> >
> > How can I change the "make" command so that it produces .bc?
> > >
> > > Thanks in advance,
> > > Kevin
> > > _______________________________________________
> > > LLVM Developers mailing list
> > > LLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.edu
> > > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
> > >
> >
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