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

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Date:   Tue, 1 Oct 2019 14:25:22 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Song Liu <songliubraving@...com>
Cc:     Andrii Nakryiko <andriin@...com>, bpf <bpf@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...com>,
        "daniel@...earbox.net" <daniel@...earbox.net>,
        Kernel Team <Kernel-team@...com>
Subject: Re: [PATCH bpf-next 4/6] libbpf: add BPF_CORE_READ/BPF_CORE_READ_INTO helpers

On Tue, Oct 1, 2019 at 2:14 PM Song Liu <songliubraving@...com> wrote:
>
>
> > On Sep 30, 2019, at 11:58 AM, Andrii Nakryiko <andriin@...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@...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**.
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.

>
> Thanks,
> Song
>