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

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

On 3/11/24 4:13 PM, Andrii Nakryiko wrote:
> On Mon, Mar 11, 2024 at 2:41 PM Yonghong Song <yonghong.song@linux.dev> wrote:
>> 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, do a type cast from 'void *' to 'struct bpf_link *' for file->private_data
>> as in this patch, and 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..96ffcc4f0e67 100644
>> --- a/tools/bpf/bpftool/skeleton/pid_iter.bpf.c
>> +++ b/tools/bpf/bpftool/skeleton/pid_iter.bpf.c
>> @@ -100,7 +100,7 @@ int iter(struct bpf_iter__task_file *ctx)
>>          if (obj_type == BPF_OBJ_LINK &&
>>              bpf_core_enum_value_exists(enum bpf_link_type___local,
>>                                         BPF_LINK_TYPE_PERF_EVENT___local)) {
>> -               struct bpf_link *link = (struct bpf_link *) file->private_data;
>> +               struct bpf_link *link = bpf_core_cast(file->private_data, struct bpf_link);
> bpf_core_cast() (i.e., bpf_rdonly_cast()) is newer feature, so relying
> on it in bpftool will make PID printing not work on older kernels that
> generally do support iter/task_file iterators.
>
>>                  if (link->type == bpf_core_enum_value(enum bpf_link_type___local,
> the problem is that we dropped BPF_CORE_READ() here and went with
> `link->type`. Should we restore BPF_CORE_READ(link, type) here and
> solve the problem, while also supporting this feature on wider set of
> kernels? Quentin?

Good point. Using the latest fancy feature might not be the best way as old
kernels won't work and this is a strong argument. I will change
to use BPF_CORE_READ in v2.

>
>>                                                        BPF_LINK_TYPE_PERF_EVENT___local)) {
>> --
>> 2.43.0
>>