llvm dev - Jun 2013 - [LLVMdev] Compile Linux Kernel module into LLVM bitcode

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 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
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. How can I change the
"make" command so that it produces .bc?

Thanks in advance,
Kevin

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
>

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
> >
> 
> _______________________________________________
> 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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