bpf_printk can't create data on trace_pipe

[Arvintian]

Version

Microsoft Windows [版本 10.0.22000.708]

WSL Version

• WSL 2
• WSL 1

Kernel Version

5.10.102.1

Distro Version

Ubuntu 20.04

Other Software

No response

Repro Steps

• Setup libbpf-bootstrap and make minimal.
• Mount debugfs sudo mount -t debugfs debugfs /sys/kernel/debug
• exec example ebpf program sudo ./minimal
• Cat&Tail ebpf program log sudo cat /sys/kernel/debug/tracing/trace_pipe

Expected Behavior

sudo cat /sys/kernel/debug/tracing/trace_pipe should echo the ebpf program's log.

Actual Behavior

Nothing has be print out.

Diagnostic Logs

libbpf: loading object 'minimal_bpf' from buffer
libbpf: elf: section(2) tp/syscalls/sys_enter_write, size 104, link 0, flags 6, type=1
libbpf: sec 'tp/syscalls/sys_enter_write': found program 'handle_tp' at insn offset 0 (0 bytes), code size 13 insns (104 bytes)
libbpf: elf: section(3) license, size 13, link 0, flags 3, type=1
libbpf: license of minimal_bpf is Dual BSD/GPL
libbpf: elf: section(4) .bss, size 4, link 0, flags 3, type=8
libbpf: elf: section(5) .rodata, size 28, link 0, flags 2, type=1
libbpf: elf: section(6) .BTF, size 606, link 0, flags 0, type=1
libbpf: elf: section(7) .BTF.ext, size 160, link 0, flags 0, type=1
libbpf: elf: section(8) .symtab, size 192, link 13, flags 0, type=2
libbpf: elf: section(9) .reltp/syscalls/sys_enter_write, size 32, link 8, flags 0, type=9
libbpf: looking for externs among 8 symbols...
libbpf: collected 0 externs total
libbpf: map 'minimal_.bss' (global data): at sec_idx 4, offset 0, flags 400.
libbpf: map 0 is "minimal_.bss"
libbpf: map 'minimal_.rodata' (global data): at sec_idx 5, offset 0, flags 480.
libbpf: map 1 is "minimal_.rodata"
libbpf: sec '.reltp/syscalls/sys_enter_write': collecting relocation for section(2) 'tp/syscalls/sys_enter_write'
libbpf: sec '.reltp/syscalls/sys_enter_write': relo #0: insn #2 against 'my_pid'
libbpf: prog 'handle_tp': found data map 0 (minimal_.bss, sec 4, off 0) for insn 2
libbpf: sec '.reltp/syscalls/sys_enter_write': relo #1: insn #6 against '.rodata'
libbpf: prog 'handle_tp': found data map 1 (minimal_.rodata, sec 5, off 0) for insn 6
libbpf: map 'minimal_.bss': created successfully, fd=4
libbpf: map 'minimal_.rodata': created successfully, fd=5
Successfully started! Please run `sudo cat /sys/kernel/debug/tracing/trace_pipe` to see output of the BPF programs.

[X-QIANG]

I had the same problem, but if you remove

if (pid != my_pid)  
    return 0;

you can see the bpf_printk print event about other process, probably userspace code fprintf didn't trigger the event.

[X-QIANG]

I had the same problem, but if you remove

if (pid != my_pid)  
    return 0;

you can see the bpf_printk print event about other process, probably userspace code fprintf didn't trigger the event.

actually, process in wsl run in a pid namespace, so the pid get from userspace is the pid in namesapce, and the pid get by ebpf code 'bpf_get_current_pid_tgid' is the global pid,.They can only be consistent if you get the pid inside the namespace in kernel mode.