Re: [PATCH bpf] tools/bpftool: fix PID fetching with a lot of results

[Yonghong Song <yhs@fb.com>]

On 12/7/20 6:55 PM, Andrii Nakryiko wrote:
> On Sat, Dec 5, 2020 at 11:11 AM Yonghong Song <yhs@fb.com> wrote:
>>
>>
>>
>> On 12/4/20 3:20 PM, Andrii Nakryiko wrote:
>>> In case of having so many PID results that they don't fit into a singe page
>>> (4096) bytes, bpftool will erroneously conclude that it got corrupted data due
>>> to 4096 not being a multiple of struct pid_iter_entry, so the last entry will
>>> be partially truncated. Fix this by sizing the buffer to fit exactly N entries
>>> with no truncation in the middle of record.
>>>
>>> Fixes: d53dee3fe013 ("tools/bpftool: Show info for processes holding BPF map/prog/link/btf FDs")
>>> Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
>>
>> Ack with one nit below.
>>
>> Acked-by: Yonghong Song <yhs@fb.com>
>>
>>> ---
>>>    tools/bpf/bpftool/pids.c | 4 ++--
>>>    1 file changed, 2 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/tools/bpf/bpftool/pids.c b/tools/bpf/bpftool/pids.c
>>> index df7d8ec76036..477e55d59c34 100644
>>> --- a/tools/bpf/bpftool/pids.c
>>> +++ b/tools/bpf/bpftool/pids.c
>>> @@ -89,9 +89,9 @@ libbpf_print_none(__maybe_unused enum libbpf_print_level level,
>>>
>>>    int build_obj_refs_table(struct obj_refs_table *table, enum bpf_obj_type type)
>>>    {
>>> -     char buf[4096];
>>> -     struct pid_iter_bpf *skel;
>>>        struct pid_iter_entry *e;
>>> +     char buf[4096 / sizeof(*e) * sizeof(*e)];
>>> +     struct pid_iter_bpf *skel;
>>
>> No need to move "struct pid_iter_bpf *skel", right?
> 
> It's actually a move of `struct pid_iter_entry *e;` in from of char
> buf[], to be able to use sizeof(*e) instead of sizeof(struct
> pid_iter_bpf). It's just that diff tool didn't catch this properly :)

Indeed. Looking at the final code, no unnecessary code churn.

> 
>>
>>>        int err, ret, fd = -1, i;
>>>        libbpf_print_fn_t default_print;
>>>
>>>