CODEWITHSUNDEEP
makefilekernelbpf
Maicake
Maicake

Reputation: 1136

How to compile tool and samples from within the kernel source tree? (e.g. bpftool, bpf samples)

GOAL: compile samples/bpf, compile bpf/bpftool and use them.

PROBLEM: on a VM with Ubuntu 18.04 bionic with a kernel 4.18.0-25-generic I've installed kernel src code executing apt install linux-source-4.18.0. Now I cd into /usr/src/linux-source-4.18.0/linux-source-4.18.0/samples/bpf and I run make and the result is

make -C ../../ /usr/src/linux-source-4.18.0/linux-source-4.18.0/samples/bpf/ BPF_SAMPLES_PATH=/usr/src/linux-source-4.18.0/linux-source-4.18.0/samples/bpf
make[1]: Entering directory '/usr/src/linux-source-4.18.0/linux-source-4.18.0'
scripts/kconfig/conf  --syncconfig Kconfig
***
*** Configuration file ".config" not found!
***
*** Please run some configurator (e.g. "make oldconfig" or
*** "make menuconfig" or "make xconfig").
***
scripts/kconfig/Makefile:40: recipe for target 'syncconfig' failed
make[3]: *** [syncconfig] Error 1
Makefile:562: recipe for target 'syncconfig' failed
make[2]: *** [syncconfig] Error 2
make[1]: *** No rule to make target 'include/config/auto.conf', needed by 'include/config/kernel.release'.  Stop.
make[1]: Leaving directory '/usr/src/linux-source-4.18.0/linux-source-4.18.0'
Makefile:203: recipe for target 'all' failed
make: *** [all] Error 2

If I cd into ../samples/bpf and I run sudo make the result is

Auto-detecting system features:
...                        libbfd: [ OFF ]
...        disassembler-four-args: [ OFF ]

  CC       map_perf_ring.o
  CC       xlated_dumper.o
  CC       perf.o
  CC       cfg.o
  CC       common.o
  CC       cgroup.o
  CC       main.o
main.c:36:10: fatal error: bfd.h: No such file or directory
 #include <bfd.h>
          ^~~~~~~
compilation terminated.
Makefile:92: recipe for target 'main.o' failed
make: *** [main.o] Error 1

QUESTIONS: what am I missing? After I compile them if I want to write a program which, for example, needs to use bpftool I have to write the program inside the source kernel directory or I can write it everywhere?

Upvotes: 0

Views: 5362

Answers (1)

Qeole
Qeole

Reputation: 9174

Build errors

The first case (Makefile:562: recipe for target 'syncconfig' failed) fails because you run make from the top of the linux kernel repository, and before trying to compile the samples, the build system tries to load a config file to use for your system (but does not find one).

Before trying to build the samples (make -C samples/bpf), you can create a .config file from your current kernel configuration like this:

$ cp /usr/src/linux-headers-$(uname -r)/.config <path to repo>/.config
$ make olddefconfig

Or even simply generate a default config file from scratch:

$ make defconfig

See make help from top directory to see the available make options.

Your second error, regarding bfd.h not found, is that you miss a library. Libbfd on Ubuntu comes with binutils-dev, so apt install binutils-dev should do the trick.

Compiling the programs

Finally, regarding your question on compiling the programs:

  • You can write and build program from the kernel repository, just by creating a new sample and reusing the existing Makefiles.
  • You can also write and compile programs outside of the kernel tree. The basic clang (v4.0 or above, if possible v6.0 or above) command to compile them usually looks something like this:
$ clang -O2 -emit-llvm -c my_bpf_prog.c -o - | \
          llc -march=bpf -filetype=obj -o my_bpf_prog.o

You can find examples of programs compiled out of the kernel tree in that repository (disclaimer: by my company) or in the XDP tutorial repo.

Upvotes: 3

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