Date:   Sat, 14 Nov 2020 20:13:54 -0800
From:   Yonghong Song <yhs@...com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Andrii Nakryiko <andrii.nakryiko@...il.com>
CC:     Alan Maguire <alan.maguire@...cle.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>, Martin Lau <kafai@...com>,
        Song Liu <songliubraving@...com>,
        john fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...omium.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>, Hao Luo <haoluo@...gle.com>,
        Jiri Olsa <jolsa@...nel.org>,
        Quentin Monnet <quentin@...valent.com>,
        open list <linux-kernel@...r.kernel.org>,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>
Subject: Re: [RFC bpf-next 1/3] bpf: add module support to btf display helpers



On 11/14/20 8:04 AM, Alexei Starovoitov wrote:
> On Fri, Nov 13, 2020 at 10:59 PM Andrii Nakryiko
> <andrii.nakryiko@...il.com> wrote:
>>
>> On Fri, Nov 13, 2020 at 10:11 AM Alan Maguire <alan.maguire@...cle.com> wrote:
>>>
>>> bpf_snprintf_btf and bpf_seq_printf_btf use a "struct btf_ptr *"
>>> argument that specifies type information about the type to
>>> be displayed.  Augment this information to include a module
>>> name, allowing such display to support module types.
>>>
>>> Signed-off-by: Alan Maguire <alan.maguire@...cle.com>
>>> ---
>>>   include/linux/btf.h            |  8 ++++++++
>>>   include/uapi/linux/bpf.h       |  5 ++++-
>>>   kernel/bpf/btf.c               | 18 ++++++++++++++++++
>>>   kernel/trace/bpf_trace.c       | 42 ++++++++++++++++++++++++++++++++----------
>>>   tools/include/uapi/linux/bpf.h |  5 ++++-
>>>   5 files changed, 66 insertions(+), 12 deletions(-)
>>>
>>> diff --git a/include/linux/btf.h b/include/linux/btf.h
>>> index 2bf6418..d55ca00 100644
>>> --- a/include/linux/btf.h
>>> +++ b/include/linux/btf.h
>>> @@ -209,6 +209,14 @@ static inline const struct btf_var_secinfo *btf_type_var_secinfo(
>>>   const struct btf_type *btf_type_by_id(const struct btf *btf, u32 type_id);
>>>   const char *btf_name_by_offset(const struct btf *btf, u32 offset);
>>>   struct btf *btf_parse_vmlinux(void);
>>> +#ifdef CONFIG_DEBUG_INFO_BTF_MODULES
>>> +struct btf *bpf_get_btf_module(const char *name);
>>> +#else
>>> +static inline struct btf *bpf_get_btf_module(const char *name)
>>> +{
>>> +       return ERR_PTR(-ENOTSUPP);
>>> +}
>>> +#endif
>>>   struct btf *bpf_prog_get_target_btf(const struct bpf_prog *prog);
>>>   #else
>>>   static inline const struct btf_type *btf_type_by_id(const struct btf *btf,
>>> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
>>> index 162999b..26978be 100644
>>> --- a/include/uapi/linux/bpf.h
>>> +++ b/include/uapi/linux/bpf.h
>>> @@ -3636,7 +3636,8 @@ struct bpf_stack_build_id {
>>>    *             the pointer data is carried out to avoid kernel crashes during
>>>    *             operation.  Smaller types can use string space on the stack;
>>>    *             larger programs can use map data to store the string
>>> - *             representation.
>>> + *             representation.  Module-specific data structures can be
>>> + *             displayed if the module name is supplied.
>>>    *
>>>    *             The string can be subsequently shared with userspace via
>>>    *             bpf_perf_event_output() or ring buffer interfaces.
>>> @@ -5076,11 +5077,13 @@ struct bpf_sk_lookup {
>>>    * potentially to specify additional details about the BTF pointer
>>>    * (rather than its mode of display) - is included for future use.
>>>    * Display flags - BTF_F_* - are passed to bpf_snprintf_btf separately.
>>> + * A module name can be specified for module-specific data.
>>>    */
>>>   struct btf_ptr {
>>>          void *ptr;
>>>          __u32 type_id;
>>>          __u32 flags;            /* BTF ptr flags; unused at present. */
>>> +       const char *module;     /* optional module name. */
>>
>> I think module name is a wrong API here, similarly how type name was
>> wrong API for specifying the type (and thus we use type_id here).
>> Using the module's BTF ID seems like a more suitable interface. That's
>> what I'm going to use for all kinds of existing BPF APIs that expect
>> BTF type to attach BPF programs.
>>
>> Right now, we use only type_id and implicitly know that it's in
>> vmlinux BTF. With module BTFs, we now need a pair of BTF object ID +
>> BTF type ID to uniquely identify the type. vmlinux BTF now can be
>> specified in two different ways: either leaving BTF object ID as zero
>> (for simplicity and backwards compatibility) or specifying it's actual
>> BTF obj ID (which pretty much always should be 1, btw). This feels
>> like a natural extension, WDYT?
>>
>> And similar to type_id, no one should expect users to specify these
>> IDs by hand, Clang built-in and libbpf should work together to figure
>> this out for the kernel to use.
>>
>> BTW, with module names there is an extra problem for end users. Some
>> types could be either built-in or built as a module (e.g., XFS data
>> structures). Why would we require BPF users to care which is the case
>> on any given host?
> 
> +1.
> As much as possible libbpf should try to hide the difference between
> type in a module vs type in the vmlinux, since that difference most of the
> time is irrelevant from bpf prog pov.

I just crafted a llvm patch where for __builtin_btf_type_id(), a 64bit 
value is returned instead of a 32bit value. libbpf can use the lower
32bit as the btf_type_id and upper 32bit as the kernel module btf id.

    https://reviews.llvm.org/D91489

feel free to experiment with it to see whether it helps.

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