Invalid argument (EINVAL) when trying to load a BPF program - c

Question

I am trying to load a BPF program using the bpf syscall but I am receiving invalid argument (EINVAL) on return. From the man page, the possible reasons for this are:
EINVAL
For BPF_PROG_LOAD, indicates an attempt to load an invalid program.
eBPF programs can be deemed invalid due to unrecognized instructions,
the use of reserved fields, jumps out of range, infinite loops or calls
of unknown functions.
So it seems there is something wrong with my BPF program. My BPF program is as follows:
#include <uapi/linux/bpf.h>
int prog(struct pt_regs *ctx)
{
return 0;
}
Which surely cannot have anything wrong with it.
I am compiling with the Makefile here (I removed most of the code from test_overhead_kprobe_kern.c to give a very simple program for testing).
What could be wrong with my program that caused it to get rejected?
uname -a: Linux ubuntu1710 4.13.0-32-generic #35-Ubuntu SMP Thu Jan 25 09:13:46 UTC 2018 x86_64 x86_64 x86_64 GNU/Linux
My full userspace code (in Go) is as follows:
package main
/*
#include <stdio.h>
#include <stdlib.h>
void print(char* s) {
printf("%s\n", s);
}
*/
import "C"
import (
"unsafe"
"golang.org/x/sys/unix"
"github.com/cilium/cilium/pkg/bpf"
)
import (
"fmt"
"io/ioutil"
)
const (
bufferSize = 256
sessionIDHTTPHeader = "X-Session-ID"
defaultServerAddress = "localhost"
defaultPort = 5050
)
const (
BPF_PROG_TYPE_UNSPEC = 0
BPF_PROG_TYPE_SOCKET_FILTER = 1
BPF_PROG_TYPE_KPROBE = 2
BPF_PROG_TYPE_SCHED_CLS = 3
BPF_PROG_TYPE_SCHED_ACT = 4
)
type ttyWrite struct {
Count int32
Buf [bufferSize]byte
SessionID int32
}
func main() {
//for i := 0; i < 6; i++ {
//b, err := ioutil.ReadFile(fmt.Sprintf("bpf/test%d.o", i))
b, err := ioutil.ReadFile("bpf/bpf_tty.o")
if err != nil {
fmt.Print(err)
}
err = loadProgram(BPF_PROG_TYPE_KPROBE, unsafe.Pointer(&b), len(b))
if err != nil {
fmt.Printf("%s\n", err)
}
//}
}
func loadProgram(progType int, insns unsafe.Pointer, insnCnt int) error {
licenseBuf := "GPL"
licenseStr := C.CString(licenseBuf)
defer C.free(unsafe.Pointer(licenseStr))
logStr := C.CString("XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX")
defer C.free(unsafe.Pointer(logStr))
lba := struct {
progType uint32
//pad0 [4]byte
insnCnt uint32
//pad1 [4]byte
insns uint64
license uint64
logLevel uint32
//pad2 [4]byte
logSize uint32
//pad3 [4]byte
logBuf uint64
kernVersion uint32
//pad4 [4]byte
}{
progType: uint32(progType),
insnCnt: uint32(insnCnt),
insns: uint64(uintptr(insns)),
license: uint64(uintptr(unsafe.Pointer(licenseStr))),
logLevel: uint32(1),
logSize: uint32(50),
logBuf: uint64(uintptr(unsafe.Pointer(logStr))),
//logBuf: uint64(uintptr(unsafe.Pointer(bufStr))),
// /usr/src/linux-headers-4.13.0-32-generic/include/generated/uapi/linux/version.h
kernVersion: uint32(265485),
}
ret, _, err := unix.Syscall(
unix.SYS_BPF,
bpf.BPF_PROG_LOAD,
uintptr(unsafe.Pointer(&lba)),
unsafe.Sizeof(lba),
)
//fmt.Printf("%s\n", logBuf)
//cs := C.CString("XXXXXXXXXX")
C.print(logStr)
//fmt.Printf("%c\n", *logStr)
if ret != 0 || err != 0 {
//fmt.Printf("%#v %d\n", logBuf, unsafe.Sizeof(lba))
return fmt.Errorf("Unable to load program: ret: %d: %s", int(ret), err)
}
return nil
}

Answer 1

As Qeole pointed out in your previous question, your userspace Go program needs to extract the BPF instructions (.text section) from the object file. Otherwise, the kernel will try to interpret the binary content as BPF instructions and inevitably fail.