Date:   Wed, 28 Jul 2021 15:29:20 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Quentin Monnet <quentin@...valent.com>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>
Subject: Re: [PATCH bpf-next v2 0/5] libbpf: rename btf__get_from_id() and
 btf__load() APIs, support split BTF

On Wed, Jul 28, 2021 at 2:54 PM Quentin Monnet <quentin@...valent.com> wrote:
>
> On Tue, 27 Jul 2021 at 21:49, Andrii Nakryiko <andrii.nakryiko@...il.com> wrote:
>
> > > >>>
> > > >>> LIBBPF_DEPRECATED_SINCE(0, 6, "API that will be marked deprecated in v0.6")
> > >
> > > So I've been looking into this, and it's not _that_ simple to do. Unless
> > > I missed something about preprocessing macros, I cannot bake a "#if" in
> > > a "#define", to have the attribute printed if and only if the current
> > > version is >= 0.6 in this example.
> > >
> > > I've come up with something, but it is not optimal because I have to
> > > write a check and macros for each version number used with the
> > > LIBBPF_DEPRECATED_SINCE macro. If we really wanted to automate that part
> > > I guess we could generate a header with those macros from the Makefile
> > > and include it in libbpf_common.h, but that does not really look much
> > > cleaner to me.
> >
> > Yeah, let's not add unnecessary code generation. It sucks, of course,
> > that we can't do #ifdef inside a macro :(
> >
> > So it's either do something like what you did with defining
> > version-specific macros, which is actually not too bad, because it's
> > not like we have tons of those versions anyways.
> >
> > LIBBPF_DEPRECATED_SINCE(0, 6, "use btf__load_from_kernel_by_id instead")
> > LIBBPF_API int btf__get_from_id(__u32 id, struct btf **btf);
> >
> > Alternatively, we can go with:
> >
> > #if LIBBPF_AT_OR_NEWER(0, 6)
> > LIBBPF_DEPRECATED("use btf__load_from_kernel_by_id instead")
> > #endif
> > LIBBPF API int btf__get_from_id(__u32 id, struct btf **btf);
> >
> > I don't really dislike the second variant too much either, but
> > LIBBPF_DEPRECATED_SINCE() reads nicer. Let's go with that. See some
> > comments below about implementation.
>
> Ok.
>
> >
> > >
> > > Here's my current code, below - does it correspond to what you had in
> > > mind? Or did you think of something else?
> > >
> > > ------
> > >
> > > diff --git a/tools/lib/bpf/Makefile b/tools/lib/bpf/Makefile
> > > index ec14aa725bb0..095d5dc30d50 100644
> > > --- a/tools/lib/bpf/Makefile
> > > +++ b/tools/lib/bpf/Makefile
> > > @@ -8,6 +8,7 @@ LIBBPF_VERSION := $(shell \
> > >         grep -oE '^LIBBPF_([0-9.]+)' libbpf.map | \
> > >         sort -rV | head -n1 | cut -d'_' -f2)
> > >  LIBBPF_MAJOR_VERSION := $(firstword $(subst ., ,$(LIBBPF_VERSION)))
> > > +LIBBPF_MINOR_VERSION := $(firstword $(subst ., ,$(subst $(LIBBPF_MAJOR_VERSION)., ,$(LIBBPF_VERSION))))
> >
> > Given all this is for internal use, I'd instead define something like
> > __LIBBPF_CURVER as an integer that is easy to compare against:
> >
> > #define __LIBBPF_CURVER (LIBBPF_MAJOR_VERSION * 100 +
> > LIBBPF_MINOR_VERSION) * 100 + LIBBPF_PATCH_VERSION
> >
> > That will simplify some stuff below and is generally easier to use in
> > code, if we will need this somewhere to use explicitly.
>
> Did you mean computing __LIBBPF_CURVER in the Makefile, or in the
> header?

I was thinking Makefile, but if it's simpler to do in the header
that's fine as well.

>
> I can do that if you want, although I'm not convinced it will simplify
> much. Instead of having one long-ish condition, we'll have to compute
> the integer for the current version, as well as for each of the versions
> that we list for deprecating functions. I suppose I can add another
> dedicated macro.

feels like if we need to do some comparisons, then writing

#if __LIBBPF_VER > 102
/* do something */
#endif

is much simpler than comparing MAJOR_VERSION and MINOR_VERSION
separately. It's just that currently with 0 major version it might
look a bit awkward right now, but that's temporary.

>
> Do you actually want the patch version? I chose to leave it aside
> because 1) I thought it would not be relevant for deprecating symbols,
> and 2) if anything like a -rc1 suffix is ever appended to the version,
> it makes it more complex to parse from the version string.

yeah, you are probably right. major  and minor should be enough

>
> >
> > >
> > >  MAKEFLAGS += --no-print-directory
> > >
> > > @@ -86,6 +87,8 @@ override CFLAGS += -Werror -Wall
> > >  override CFLAGS += $(INCLUDES)
> > >  override CFLAGS += -fvisibility=hidden
> > >  override CFLAGS += -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64
> > > +override CFLAGS += -DLIBBPF_MAJOR_VERSION=$(LIBBPF_MAJOR_VERSION)
> > > +override CFLAGS += -DLIBBPF_MINOR_VERSION=$(LIBBPF_MINOR_VERSION)
> > >
> > >  # flags specific for shared library
> > >  SHLIB_FLAGS := -DSHARED -fPIC
> > > diff --git a/tools/lib/bpf/btf.h b/tools/lib/bpf/btf.h
> > > index cf8490f95641..8b6b5442dbd8 100644
> > > --- a/tools/lib/bpf/btf.h
> > > +++ b/tools/lib/bpf/btf.h
> > > @@ -45,7 +45,8 @@ LIBBPF_API struct btf *btf__parse_raw(const char *path);
> > >  LIBBPF_API struct btf *btf__parse_raw_split(const char *path, struct btf *base_btf);
> > >  LIBBPF_API struct btf *btf__load_from_kernel_by_id(__u32 id);
> > >  LIBBPF_API struct btf *btf__load_from_kernel_by_id_split(__u32 id, struct btf *base_btf);
> > > -LIBBPF_API int btf__get_from_id(__u32 id, struct btf **btf);
> > > +LIBBPF_API LIBBPF_DEPRECATED_SINCE(0, 6, "use btf__load_from_kernel_by_id instead")
> >
> > nit: given how long those deprecations will be, let's keep them at a
> > separate (first) line and keep LIBBPF_API near the function
> > declaration itself
>
> I thought having the LIBBPF_API on a separate line would slightly reduce
> the risk, when moving lines around, to move the function prototype but
> not the deprecation attribute. But ok, fine.

highly improbable and then we'll most probably catch it during build anyways

>
> >
> > > +int btf__get_from_id(__u32 id, struct btf **btf);
> > >
> > >  LIBBPF_API int btf__finalize_data(struct bpf_object *obj, struct btf *btf);
> > >  LIBBPF_API int btf__load(struct btf *btf);
> > > diff --git a/tools/lib/bpf/libbpf_common.h b/tools/lib/bpf/libbpf_common.h
> > > index 947d8bd8a7bb..9ba9f8135dc8 100644
> > > --- a/tools/lib/bpf/libbpf_common.h
> > > +++ b/tools/lib/bpf/libbpf_common.h
> > > @@ -17,6 +17,28 @@
> > >
> > >  #define LIBBPF_DEPRECATED(msg) __attribute__((deprecated(msg)))
> > >
> > > +#ifndef LIBBPF_DEPRECATED_SINCE
> >
> > why #ifndef conditional?
>
> Right, we don't expect to have the macro defined elsewhere. I'll remove
> it.
>
> >
> > > +#define __LIBBPF_VERSION_CHECK(major, minor) \
> > > +       LIBBPF_MAJOR_VERSION > major || \
> > > +               (LIBBPF_MAJOR_VERSION == major && LIBBPF_MINOR_VERSION >= minor)
> >
> > so we don't need this if we do __LIBBPF_CURVER
>
> Right, but we do need to compute an integer for each of the versions
> listed below (0.6 for now). I'll see if I can come up with something
> short.

see above, I'd just do 102 etc. I wonder if 006 will be treated as an
octal number, in that case probably fine to do just 6. Or we can have
a small macro for this, of course. Don't know, doesn't seem to matter
all that much

>
> >
> > > +
> > > +/* Add checks for other versions below when planning deprecation of API symbols
> > > + * with the LIBBPF_DEPRECATED_SINCE macro.
> > > + */
> > > +#if __LIBBPF_VERSION_CHECK(0, 6)
> > > +#define __LIBBPF_MARK_DEPRECATED_0_6(X) X
> > > +#else
> > > +#define __LIBBPF_MARK_DEPRECATED_0_6(X)
> > > +#endif
> > > +
> > > +#define __LIBBPF_DEPRECATED_SINCE(major, minor, msg) \
> > > +       __LIBBPF_MARK_DEPRECATED_ ## major ## _ ## minor (LIBBPF_DEPRECATED("v" # major "." # minor "+, " msg))
> > > +
> > > +/* Mark a symbol as deprecated when libbpf version is >= {major}.{minor} */
> > > +#define LIBBPF_DEPRECATED_SINCE(major, minor, msg) \
> > > +       __LIBBPF_DEPRECATED_SINCE(major, minor, msg)
> >
> > Is it needed for some macro value concatenation magic to have this
> > nested __LIBBPF_DEPRECATED_SINCE?
>
> I double-checked (I needed to, anyway), and it seems not. It's a
> leftover from an earlier version of my code, I'll clean it up before
> the proper submission.

ok, thanks

>
> Thanks!
> Quentin

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