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 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. 

> Prefixing with _libbpf is an option, but it will make it super ugly
> and hard to follow (I've spent a bunch of time to even get it to the
> current state), so I'd like to avoid that.

BPF programs will not use these marcos directly, so I feel it is OK to 
pay the pain of _libbpf prefix, as it is contained within this file. 

Thanks,
Song