netdev - Re: [PATCH v2 bpf-next 1/3] bpf: switch BPF UAPI #define constants to enums

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Date:   Mon, 2 Mar 2020 10:25:58 -0800
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Yonghong Song <yhs@...com>
Cc:     Andrii Nakryiko <andriin@...com>, bpf <bpf@...r.kernel.org>,
        Networking <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Kernel Team <kernel-team@...com>
Subject: Re: [PATCH v2 bpf-next 1/3] bpf: switch BPF UAPI #define constants to enums

On Mon, Mar 2, 2020 at 8:22 AM Yonghong Song <yhs@...com> wrote:
>
>
>
> On 2/29/20 10:24 PM, Andrii Nakryiko wrote:
> > Switch BPF UAPI constants, previously defined as #define macro, to anonymous
> > enum values. This preserves constants values and behavior in expressions, but
> > has added advantaged of being captured as part of DWARF and, subsequently, BTF
> > type info. Which, in turn, greatly improves usefulness of generated vmlinux.h
> > for BPF applications, as it will not require BPF users to copy/paste various
> > flags and constants, which are frequently used with BPF helpers.
> >
> > Signed-off-by: Andrii Nakryiko <andriin@...com>
> > ---
> >   include/uapi/linux/bpf.h              | 272 +++++++++++++++----------
> >   include/uapi/linux/bpf_common.h       |  86 ++++----
> >   include/uapi/linux/btf.h              |  60 +++---
> >   tools/include/uapi/linux/bpf.h        | 274 ++++++++++++++++----------
> >   tools/include/uapi/linux/bpf_common.h |  86 ++++----
> >   tools/include/uapi/linux/btf.h        |  60 +++---
> >   6 files changed, 497 insertions(+), 341 deletions(-)
> >
> > diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> > index 8e98ced0963b..03e08f256bd1 100644
> > --- a/include/uapi/linux/bpf.h
> > +++ b/include/uapi/linux/bpf.h
> > @@ -14,34 +14,36 @@
> >   /* Extended instruction set based on top of classic BPF */
> >
> >   /* instruction classes */
> > -#define BPF_JMP32    0x06    /* jmp mode in word width */
> > -#define BPF_ALU64    0x07    /* alu mode in double word width */
> > +enum {
> > +     BPF_JMP32       = 0x06, /* jmp mode in word width */
> > +     BPF_ALU64       = 0x07, /* alu mode in double word width */
>
> Not sure whether we have uapi backward compatibility or not.
> One possibility is to add
>    #define BPF_ALU64 BPF_ALU64
> this way, people uses macros will continue to work.

This is going to be a really ugly solution, though. I wonder if it was
ever an expected behavior of UAPI constants to be able to do #ifdef on
them.

Do you know any existing application that relies on those constants
being #defines?

>
> If this is an acceptable solution, we have a lot of constants
> in net related headers and will benefit from this conversion for
> kprobe/tracepoint of networking related functions.
>
> >
> >   /* ld/ldx fields */
> > -#define BPF_DW               0x18    /* double word (64-bit) */
> > -#define BPF_XADD     0xc0    /* exclusive add */
> > +     BPF_DW          = 0x18, /* double word (64-bit) */
> > +     BPF_XADD        = 0xc0, /* exclusive add */
> >
> >   /* alu/jmp fields */
> > -#define BPF_MOV              0xb0    /* mov reg to reg */
> > -#define BPF_ARSH     0xc0    /* sign extending arithmetic shift right */
> > +     BPF_MOV         = 0xb0, /* mov reg to reg */
> > +     BPF_ARSH        = 0xc0, /* sign extending arithmetic shift right */
> >
> >   /* change endianness of a register */
> > -#define BPF_END              0xd0    /* flags for endianness conversion: */
> > -#define BPF_TO_LE    0x00    /* convert to little-endian */
> > -#define BPF_TO_BE    0x08    /* convert to big-endian */
> > -#define BPF_FROM_LE  BPF_TO_LE
> > -#define BPF_FROM_BE  BPF_TO_BE
> > +     BPF_END         = 0xd0, /* flags for endianness conversion: */
> > +     BPF_TO_LE       = 0x00, /* convert to little-endian */
> > +     BPF_TO_BE       = 0x08, /* convert to big-endian */
> > +     BPF_FROM_LE     = BPF_TO_LE,
> > +     BPF_FROM_BE     = BPF_TO_BE,
> >
> >   /* jmp encodings */
> > -#define BPF_JNE              0x50    /* jump != */
> > -#define BPF_JLT              0xa0    /* LT is unsigned, '<' */
> > -#define BPF_JLE              0xb0    /* LE is unsigned, '<=' */
> > -#define BPF_JSGT     0x60    /* SGT is signed '>', GT in x86 */
> > -#define BPF_JSGE     0x70    /* SGE is signed '>=', GE in x86 */
> > -#define BPF_JSLT     0xc0    /* SLT is signed, '<' */
> > -#define BPF_JSLE     0xd0    /* SLE is signed, '<=' */
> > -#define BPF_CALL     0x80    /* function call */
> > -#define BPF_EXIT     0x90    /* function return */
> > +     BPF_JNE         = 0x50, /* jump != */
> > +     BPF_JLT         = 0xa0, /* LT is unsigned, '<' */
> > +     BPF_JLE         = 0xb0, /* LE is unsigned, '<=' */
> > +     BPF_JSGT        = 0x60, /* SGT is signed '>', GT in x86 */
> > +     BPF_JSGE        = 0x70, /* SGE is signed '>=', GE in x86 */
> > +     BPF_JSLT        = 0xc0, /* SLT is signed, '<' */
> > +     BPF_JSLE        = 0xd0, /* SLE is signed, '<=' */
> > +     BPF_CALL        = 0x80, /* function call */
> > +     BPF_EXIT        = 0x90, /* function return */
> > +};
> >
> [...]