Re: [PATCH bpf-next 4/6] libbpf: add BPF_CORE_READ/BPF_CORE_READ_INTO helpers

[Song Liu <songliubraving@fb.com>]

> On Oct 1, 2019, at 8:36 PM, Andrii Nakryiko <andrii.nakryiko@gmail.com> wrote:
> 
> On Tue, Oct 1, 2019 at 4:44 PM Song Liu <songliubraving@fb.com> wrote:
>> 
>> 
>> 
>>> On Oct 1, 2019, at 3:42 PM, Andrii Nakryiko <andrii.nakryiko@gmail.com> wrote:
>>> 
>>> On Tue, Oct 1, 2019 at 2:46 PM Song Liu <songliubraving@fb.com> wrote:
>>>> 
>>>> 
>>>> 
>>>>> On Oct 1, 2019, at 2:25 PM, Andrii Nakryiko <andrii.nakryiko@gmail.com> wrote:
>>>>> 
>>>>> On Tue, Oct 1, 2019 at 2:14 PM Song Liu <songliubraving@fb.com> wrote:
>>>>>> 
>>>>>> 
>>>>>>> On Sep 30, 2019, at 11:58 AM, Andrii Nakryiko <andriin@fb.com> wrote:
>>>>>>> 
>>>>>>> Add few macros simplifying BCC-like multi-level probe reads, while also
>>>>>>> emitting CO-RE relocations for each read.
>>>>>>> 
>>>>>>> Signed-off-by: Andrii Nakryiko <andriin@fb.com>
>>>>>>> ---
>>>>>>> tools/lib/bpf/bpf_helpers.h | 151 +++++++++++++++++++++++++++++++++++-
>>>>>>> 1 file changed, 147 insertions(+), 4 deletions(-)
>>>>>>> 
>>>>>>> diff --git a/tools/lib/bpf/bpf_helpers.h b/tools/lib/bpf/bpf_helpers.h
>>>>>>> index a1d9b97b8e15..51e7b11d53e8 100644
>>>>>>> --- a/tools/lib/bpf/bpf_helpers.h
>>>>>>> +++ b/tools/lib/bpf/bpf_helpers.h
>>>>>>> @@ -19,6 +19,10 @@
>>>>>>> */
>>>>>>> #define SEC(NAME) __attribute__((section(NAME), used))
>>>>>>> 
>>>>>>> +#ifndef __always_inline
>>>>>>> +#define __always_inline __attribute__((always_inline))
>>>>>>> +#endif
>>>>>>> +
>>>>>>> /* helper functions called from eBPF programs written in C */
>>>>>>> static void *(*bpf_map_lookup_elem)(void *map, const void *key) =
>>>>>>>    (void *) BPF_FUNC_map_lookup_elem;
>>>>>>> @@ -505,7 +509,7 @@ struct pt_regs;
>>>>>>> #endif
>>>>>>> 
>>>>>>> /*
>>>>>>> - * BPF_CORE_READ abstracts away bpf_probe_read() call and captures offset
>>>>>>> + * bpf_core_read() abstracts away bpf_probe_read() call and captures field
>>>>>>> * relocation for source address using __builtin_preserve_access_index()
>>>>>>> * built-in, provided by Clang.
>>>>>>> *
>>>>>>> @@ -520,8 +524,147 @@ struct pt_regs;
>>>>>>> * actual field offset, based on target kernel BTF type that matches original
>>>>>>> * (local) BTF, used to record relocation.
>>>>>>> */
>>>>>>> -#define BPF_CORE_READ(dst, src)                                              \
>>>>>>> -     bpf_probe_read((dst), sizeof(*(src)),                           \
>>>>>>> -                    __builtin_preserve_access_index(src))
>>>>>>> +#define bpf_core_read(dst, sz, src)                                      \
>>>>>>> +     bpf_probe_read(dst, sz,                                             \
>>>>>>> +                    (const void *)__builtin_preserve_access_index(src))
>>>>>>> +
>>>>>>> +/*
>>>>>>> + * bpf_core_read_str() is a thin wrapper around bpf_probe_read_str()
>>>>>>> + * additionally emitting BPF CO-RE field relocation for specified source
>>>>>>> + * argument.
>>>>>>> + */
>>>>>>> +#define bpf_core_read_str(dst, sz, src)                                          \
>>>>>>> +     bpf_probe_read_str(dst, sz,                                         \
>>>>>>> +                        (const void *)__builtin_preserve_access_index(src))
>>>>>>> +
>>>>>>> +#define ___concat(a, b) a ## b
>>>>>>> +#define ___apply(fn, n) ___concat(fn, n)
>>>>>>> +#define ___nth(_1, _2, _3, _4, _5, _6, _7, _8, _9, _10, __11, N, ...) N
>>>>>> 
>>>>>> We are adding many marcos with simple names: ___apply(), ___nth. So I worry
>>>>>> they may conflict with macro definitions from other libraries. Shall we hide
>>>>>> them in .c files or prefix/postfix them with _libbpf or something?
>>>>> 
>>>>> Keep in mind, this is the header that's included from BPF code.
>>>>> 
>>>>> They are prefixed with three underscores, I was hoping it's good
>>>>> enough to avoid accidental conflicts. It's unlikely someone will have
>>>>> macros with the same names **in BPF-side code**.
>>>> 
>>>> BPF side code would include kernel headers. So there are many headers
>>>> to conflict with. And we won't know until somebody want to trace certain
>>>> kernel structure.
>>> 
>>> We have all the kernel sources at our disposal, there's no need to
>>> guess :) There is no instance of ___apply, ___concat, ___nth,
>>> ___arrow, ___last, ___nolast, or ___type, not even speaking about
>>> other more specific names. There are currently two instances of
>>> "____last_____" used in a string. And I'm certainly not afraid that
>>> user code can use triple-underscored identifier with exact those names
>>> and complain about bpf_helpers.h :)
>> 
>> I worry more about _future_ conflicts, that someone may add ___apply to
> 
> You can say the same about pretty much any name that user might use,
> that's just the fact of life with C language without namespaces. I
> don't think that justifies usage of obscure names.
> 
> Look at SEC macro, for instance. It's also an enum value in
> drivers/sbus/char/oradax.c, but it might some day end up in one of
> driver's headers. This is probably not a reason to rename it, though.
> 
>> some kernel header file and break some BPF programs. Since these BPF
>> programs are not in-tree, it is very difficult to test them properly.
>> We have had name conflicts from other libraries, so I hope we don't create
>> more ourselves.
> 
> Let's agree to come back to this problem when and if we ever encounter
> it. All those ___xxx macro are internal and users shouldn't rely on
> them, which means if we ever get a real conflict, we'll be able to
> rename them to avoid the conflict.

Well, if this really happens, we will have to fix them. 

I won't block this set just for this. If you insist, let's keep these 
as-is. 

Thanks,
Song