Re: BPF arena atomic example not working

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On 24/08/04 09:59AM, Jose Fernandez wrote:
> Hi folks,
> 
> I have not found BPF arena documentation, beyond the patches and selftests.
> I'm using the arena_atomics selftest as reference to create a simple BPF program
> that increments a value atomically.
> 
> bpf: https://github.com/jfernandez/bpf-playground/blob/main/arena.bpf.c
> userspace: https://github.com/jfernandez/bpf-playground/blob/main/arena.c
> common header: https://github.com/jfernandez/bpf-playground/blob/main/bpf_arena_common.h
> 
> I'm using the 6.10.2 kernel and libbpf 1.4.3.

I forgot to mention that I was using the latest clang release (18.1.8), and this
turned out to be the issue. The arena bpf program loaded after I used clang
compiled from the llvm-project master branch.

I now realise that __BPF_FEATURE_ADDR_SPACE_CAST flag is only available starting
with the 19.1.0 RC:

$ git --no-pager tag --contains 65b123e287d1320170bb3317179bc917f21852fa
llvmorg-19.1.0-rc1
llvmorg-20-init

> 
> The program does not load when I use the `__arena_global` macro. If fails with:
> 
> libbpf: map 'arena': at sec_idx 7, offset 0.
> libbpf: map 'arena': found type = 33.
> libbpf: map 'arena': found max_entries = 10.
> libbpf: map 'arena': found map_flags = 0x400.
> libbpf: map 'arena': found map_extra = 0x100000000000.
> libbpf: sec '.relraw_tp/sys_enter': collecting relocation for section(3) 'raw_tp/sys_enter'
> libbpf: sec '.relraw_tp/sys_enter': relo #0: insn #1 against 'add64_value'
> libbpf: sec '.relraw_tp/sys_enter': relo #1: insn #4 against 'add64_result'
> libbpf: object 'arena': failed (-95) to create BPF token from '/sys/fs/bpf', skipping optional step...
> libbpf: map 'arena': created successfully, fd=3
> libbpf: prog 'add': BPF program load failed: Permission denied
> libbpf: prog 'add': -- BEGIN PROG LOAD LOG --
> arg#0 reference type('UNKNOWN ') size cannot be determined: -22
> 0: R1=ctx() R10=fp0
> ; int add(const void *ctx) @ arena.bpf.c:17
> 0: (b7) r1 = 2                        ; R1_w=2
> ; add64_result = __sync_fetch_and_add(&add64_value, 2); @ arena.bpf.c:19
> 1: (18) r2 = 0x100000000000           ; R2_w=scalar()
> 3: (db) r1 = atomic64_fetch_add((u64 *)(r2 +0), r1)
> misaligned access off (0x0; 0xffffffffffffffff)+0+0 size 8
> processed 3 insns (limit 1000000) max_states_per_insn 0 total_states 0 peak_states 0 mark_read 0
> -- END PROG LOAD LOG --
> libbpf: prog 'add': failed to load: -13
> libbpf: failed to load object 'arena'
> libbpf: failed to load BPF skeleton 'arena': -13
> Failed to open BPF skeleton
> 
> I found Yonghong Song's patch that appears to address a similar issue. But I'm
> using a return value and not getting the `lock` instruction:
> https://lore.kernel.org/bpf/20240803025928.4184433-1-yonghong.song@xxxxxxxxx/
> 
> I have compared with code with the selftest and I can't spot any signifcant
> differences that may cause this issue. I would appreciate any help or guidance.
> 
> Thanks,
> Jose




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