string assignment invalid BPF_LD_IMM insn

[brendangregg]

When dealing with strings, I often want to switch to a default if a given char * is null.

Test program:

from __future__ import print_function
from bcc import BPF

# load BPF program
b = BPF(text = """
void kprobe__sys_sync(void *ctx) {
        char *out = NULL, *hello = "world";
        if (out == NULL) {
                out = hello;
        }
        bpf_trace_printk("sync() %s\\n", out);
};
""")

# header
print("%-18s %s" % ("TIME(s)", "CALL"))

# format output
while 1:
        (task, pid, cpu, flags, ts, msg) = b.trace_fields()
        print("%-18.9f %s" % (ts, msg))

# ./teststr 
bpf: Invalid argument
0: (b7) r1 = 0
1: (73) *(u8 *)(r10 -6) = r1
2: (b7) r1 = 2675
3: (6b) *(u16 *)(r10 -8) = r1
4: (18) r1 = 0x636e7973
6: (7b) *(u64 *)(r10 -16) = r1
7: (bf) r1 = r10
8: (07) r1 += -16
9: (b7) r2 = 11
10: (00) r0 = 0x0
invalid BPF_LD_IMM insn

Traceback (most recent call last):
  File "./teststr", line 26, in <module>
    """)
  File "/usr/lib/python2.7/dist-packages/bcc/__init__.py", line 353, in __init__
    self._trace_autoload()
  File "/usr/lib/python2.7/dist-packages/bcc/__init__.py", line 573, in _trace_autoload
    fn = self.load_func(func_name, BPF.KPROBE)
  File "/usr/lib/python2.7/dist-packages/bcc/__init__.py", line 388, in load_func
    raise Exception("Failed to load BPF program %s" % func_name)
Exception: Failed to load BPF program kprobe__sys_sync

[drzaeus77]

Global strings won't work. Try changing it to hello[] = "world"

[brendangregg]

Yes, that worked, thanks. We'll need to include an example of that somewhere at least.

[brendangregg]

FWIW, the current error this produces is:

# python ./test.py 
bpf: Failed to load program: Invalid argument
unknown opcode 00

Traceback (most recent call last):
  File "./test.py", line 13, in <module>
    """)
  File "/usr/lib/python2.7/dist-packages/bcc/__init__.py", line 339, in __init__
    self._trace_autoload()
  File "/usr/lib/python2.7/dist-packages/bcc/__init__.py", line 1067, in _trace_autoload
    fn = self.load_func(func_name, BPF.KPROBE)
  File "/usr/lib/python2.7/dist-packages/bcc/__init__.py", line 379, in load_func
    (func_name, errstr))
Exception: Failed to load BPF program kprobe__do_nanosleep: Invalid argument

For this issue I'd either have a better error message (somehow), or document "unknown opcode 00" in the BPF Errors section of the reference guide explaining what it's about.

[yonghong-song]

Agreed. We could add a little more explanation in libbpf.c bpf_print_hints() if we found unknown opcode 00. The reason is typically due to ignored relocation which has initial 0's in the original place.