Re: [PATCH bpf-next v2] bpftool: Fix missing pids during link show

[Yonghong Song <yonghong.song@linux.dev>]

On 3/12/24 4:11 PM, Andrii Nakryiko wrote:
> On Tue, Mar 12, 2024 at 8:24 AM Yonghong Song <yonghong.song@linux.dev> wrote:
>>
>> On 3/12/24 7:01 AM, Quentin Monnet wrote:
>>> 2024-03-12 02:32 UTC+0000 ~ Yonghong Song <yonghong.song@linux.dev>
>>>> Current 'bpftool link' command does not show pids, e.g.,
>>>>     $ tools/build/bpftool/bpftool link
>>>>     ...
>>>>     4: tracing  prog 23
>>>>           prog_type lsm  attach_type lsm_mac
>>>>           target_obj_id 1  target_btf_id 31320
>>>>
>>>> Hack the following change to enable normal libbpf debug output,
>>>>     --- a/tools/bpf/bpftool/pids.c
>>>>     +++ b/tools/bpf/bpftool/pids.c
>>>>     @@ -121,9 +121,9 @@ int build_obj_refs_table(struct hashmap **map, enum bpf_obj_type type)
>>>>             /* we don't want output polluted with libbpf errors if bpf_iter is not
>>>>              * supported
>>>>              */
>>>>     -       default_print = libbpf_set_print(libbpf_print_none);
>>>>     +       /* default_print = libbpf_set_print(libbpf_print_none); */
>>>>             err = pid_iter_bpf__load(skel);
>>>>     -       libbpf_set_print(default_print);
>>>>     +       /* libbpf_set_print(default_print); */
>>>>
>>>> Rerun the above bpftool command:
>>>>     $ tools/build/bpftool/bpftool link
>>>>     libbpf: prog 'iter': BPF program load failed: Permission denied
>>>>     libbpf: prog 'iter': -- BEGIN PROG LOAD LOG --
>>>>     0: R1=ctx() R10=fp0
>>>>     ; struct task_struct *task = ctx->task; @ pid_iter.bpf.c:69
>>>>     0: (79) r6 = *(u64 *)(r1 +8)          ; R1=ctx() R6_w=ptr_or_null_task_struct(id=1)
>>>>     ; struct file *file = ctx->file; @ pid_iter.bpf.c:68
>>>>     ...
>>>>     ; struct bpf_link *link = (struct bpf_link *) file->private_data; @ pid_iter.bpf.c:103
>>>>     80: (79) r3 = *(u64 *)(r8 +432)       ; R3_w=scalar() R8=ptr_file()
>>>>     ; if (link->type == bpf_core_enum_value(enum bpf_link_type___local, @ pid_iter.bpf.c:105
>>>>     81: (61) r1 = *(u32 *)(r3 +12)
>>>>     R3 invalid mem access 'scalar'
>>>>     processed 39 insns (limit 1000000) max_states_per_insn 0 total_states 3 peak_states 3 mark_read 2
>>>>     -- END PROG LOAD LOG --
>>>>     libbpf: prog 'iter': failed to load: -13
>>>>     ...
>>>>
>>>> The 'file->private_data' returns a 'void' type and this caused subsequent 'link->type'
>>>> (insn #81) failed in verification.
>>>>
>>>> To fix the issue, restore the previous BPF_CORE_READ so old kernels can also work.
>>>> With this patch, the 'bpftool link' runs successfully with 'pids'.
>>>>     $ tools/build/bpftool/bpftool link
>>>>     ...
>>>>     4: tracing  prog 23
>>>>           prog_type lsm  attach_type lsm_mac
>>>>           target_obj_id 1  target_btf_id 31320
>>>>           pids systemd(1)
>>>>
>>>> Fixes: 44ba7b30e84f ("bpftool: Use a local copy of BPF_LINK_TYPE_PERF_EVENT in pid_iter.bpf.c")
>>>> Cc: Quentin Monnet <quentin@isovalent.com>
>>>> Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
>>>> ---
>>>>    tools/bpf/bpftool/skeleton/pid_iter.bpf.c | 2 +-
>>>>    1 file changed, 1 insertion(+), 1 deletion(-)
>>>>
>>>> diff --git a/tools/bpf/bpftool/skeleton/pid_iter.bpf.c b/tools/bpf/bpftool/skeleton/pid_iter.bpf.c
>>>> index 26004f0c5a6a..1eb756f8d02e 100644
>>>> --- a/tools/bpf/bpftool/skeleton/pid_iter.bpf.c
>>>> +++ b/tools/bpf/bpftool/skeleton/pid_iter.bpf.c
>>>> @@ -102,7 +102,7 @@ int iter(struct bpf_iter__task_file *ctx)
>>>>                                      BPF_LINK_TYPE_PERF_EVENT___local)) {
>>>>               struct bpf_link *link = (struct bpf_link *) file->private_data;
>>>>
>>>> -            if (link->type == bpf_core_enum_value(enum bpf_link_type___local,
>>>> +            if (BPF_CORE_READ(link, type) == bpf_core_enum_value(enum bpf_link_type___local,
>>>>                                                     BPF_LINK_TYPE_PERF_EVENT___local)) {
>>> Looks good, thank you! Could you please just realign the
>>> BPF_LINK_TYPE_PERF_EVENT___local argument on the next line properly?
>> Will do. thanks.
>>
> We can fix it up while applying, so don't send another revision. We
> are waiting for trees to converge, so applying patches will be delayed
> a bit.

Thanks!

>
>>> With that:
>>>
>>> Tested-by: Quentin Monnet <quentin@isovalent.com>
>>> Reviewed-by: Quentin Monnet <quentin@isovalent.com>