2021-07-23 08:57 UTC-0700 ~ Andrii Nakryiko <andrii.nakryiko@xxxxxxxxx>
> On Fri, Jul 23, 2021 at 2:52 AM Quentin Monnet <quentin@xxxxxxxxxxxxx> wrote:
>>
>> 2021-07-22 17:48 UTC-0700 ~ Andrii Nakryiko <andrii.nakryiko@xxxxxxxxx>
>>> On Wed, Jul 21, 2021 at 8:38 AM Quentin Monnet <quentin@xxxxxxxxxxxxx> wrote:
>>>>
>>>> Replace the calls to deprecated function btf__get_from_id() with calls
>>>> to btf__load_from_kernel_by_id() in tools/ (bpftool, perf, selftests).
>>>> Update the surrounding code accordingly (instead of passing a pointer to
>>>> the btf struct, get it as a return value from the function). Also make
>>>> sure that btf__free() is called on the pointer after use.
>>>>
>>>> v2:
>>>> - Given that btf__load_from_kernel_by_id() has changed since v1, adapt
>>>> the code accordingly instead of just renaming the function. Also add a
>>>> few calls to btf__free() when necessary.
>>>>
>>>> Signed-off-by: Quentin Monnet <quentin@xxxxxxxxxxxxx>
>>>> Acked-by: John Fastabend <john.fastabend@xxxxxxxxx>
>>>> ---
>>>> tools/bpf/bpftool/btf.c | 8 ++----
>>>> tools/bpf/bpftool/btf_dumper.c | 6 ++--
>>>> tools/bpf/bpftool/map.c | 16 +++++------
>>>> tools/bpf/bpftool/prog.c | 29 ++++++++++++++------
>>>> tools/perf/util/bpf-event.c | 11 ++++----
>>>> tools/perf/util/bpf_counter.c | 12 ++++++--
>>>> tools/testing/selftests/bpf/prog_tests/btf.c | 4 ++-
>>>> 7 files changed, 51 insertions(+), 35 deletions(-)
>>>>
>>>
>>> [...]
>>>
>>>> diff --git a/tools/bpf/bpftool/map.c b/tools/bpf/bpftool/map.c
>>>> index 09ae0381205b..12787758ce03 100644
>>>> --- a/tools/bpf/bpftool/map.c
>>>> +++ b/tools/bpf/bpftool/map.c
>>>> @@ -805,12 +805,11 @@ static struct btf *get_map_kv_btf(const struct bpf_map_info *info)
>>>> }
>>>> return btf_vmlinux;
>>>> } else if (info->btf_value_type_id) {
>>>> - int err;
>>>> -
>>>> - err = btf__get_from_id(info->btf_id, &btf);
>>>> - if (err || !btf) {
>>>> + btf = btf__load_from_kernel_by_id(info->btf_id);
>>>> + if (libbpf_get_error(btf)) {
>>>> p_err("failed to get btf");
>>>> - btf = err ? ERR_PTR(err) : ERR_PTR(-ESRCH);
>>>> + if (!btf)
>>>> + btf = ERR_PTR(-ESRCH);
>>>
>>> why not do a simpler (less conditionals)
>>>
>>> err = libbpf_get_error(btf);
>>> if (err) {
>>> btf = ERR_PTR(err);
>>> }
>>>
>>> ?
>>
>> Because if btf is NULL at this stage, this would change the return value
>> from -ESRCH to NULL. This would be problematic in mapdump(), since we
>> check this value ("if (IS_ERR(btf))") to detect a failure in
>> get_map_kv_btf().
>
> see my reply on previous patch. libbpf_get_error() handles this
> transparently regardless of CLEAN_PTRS mode, as long as it is called
> right after API call. So the above sample will work as you'd expect,
> preserving errors.
Right, it looks like I got confused on this one. I'll update it.
>
>>
>> I could change that check in mapdump() to use libbpf_get_error()
>> instead, but in that case it would similarly change the return value for
>> mapdump() (and errno), which I think would be propagated up to main()
>> and would return 0 instead of -ESRCH. This does not seem suitable and
>> would play badly with batch mode, among other things.
>>
>> So I'm considering keeping the one additional if.
>>
>>>
>>>> }
>>>> }
>>>>
>>>> @@ -1039,11 +1038,10 @@ static void print_key_value(struct bpf_map_info *info, void *key,
>>>> void *value)
>>>> {
>>>> json_writer_t *btf_wtr;
>>>> - struct btf *btf = NULL;
>>>> - int err;
>>>> + struct btf *btf;
>>>>
>>>> - err = btf__get_from_id(info->btf_id, &btf);
>>>> - if (err) {
>>>> + btf = btf__load_from_kernel_by_id(info->btf_id);
>>>> + if (libbpf_get_error(btf)) {
>>>> p_err("failed to get btf");
>>>> return;
>>>> }
>>>
>>> [...]
>>>
>>>>
>>>> func_info = u64_to_ptr(info->func_info);
>>>> @@ -781,6 +784,8 @@ prog_dump(struct bpf_prog_info *info, enum dump_mode mode,
>>>> kernel_syms_destroy(&dd);
>>>> }
>>>>
>>>> + btf__free(btf);
>>>> +
>>>
>>> warrants a Fixes: tag?
>>
>> I don't mind adding the tags, but do they have any advantage here? My
>> understanding is that they tend to be neon signs for backports to stable
>> branches, but this patch depends on btf__load_from_kernel_by_id(),
>> meaning more patches to pull. I'll see if I can move the btf__free()
>> fixes to a separate commit, maybe.
>
> Having Fixes: allows to keep track of where the issue originated. It
> doesn't necessarily mean something has to be backported, as far as I
> understand. So it's good to do regardless. Splitting fixes into a
> separate patch works for me as well, but I don't care all that much
> given they are small.
>
OK, thank you for the clarification :).
I'll keep a single patch in that case.
