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From: Masahiro Yamada <masahiroy@kernel.org>
Date: Tue, 16 Apr 2024 00:31:44 +0900
Message-ID: <CAK7LNARH45pmfdAYcJucsVPa_HJnQEh5=hcdO5u38yrhMX1wJw@mail.gmail.com>
Subject: Re: [PATCH v3 3/3] btf: Avoid weak external references
To: Ard Biesheuvel <ardb@kernel.org>
Cc: Ard Biesheuvel <ardb+git@google.com>, linux-kernel@vger.kernel.org, 
	Arnd Bergmann <arnd@arndb.de>, Martin KaFai Lau <martin.lau@linux.dev>, linux-arch@vger.kernel.org, 
	linux-kbuild@vger.kernel.org, bpf@vger.kernel.org, 
	Andrii Nakryiko <andrii@kernel.org>, Jiri Olsa <olsajiri@gmail.com>
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On Mon, Apr 15, 2024 at 11:59=E2=80=AFPM Ard Biesheuvel <ardb@kernel.org> w=
rote:
>
> On Mon, 15 Apr 2024 at 16:55, Masahiro Yamada <masahiroy@kernel.org> wrot=
e:
> >
> > On Mon, Apr 15, 2024 at 4:58=E2=80=AFPM Ard Biesheuvel <ardb+git@google=
com> wrote:
> > >
> > > From: Ard Biesheuvel <ardb@kernel.org>
> > >
> > > If the BTF code is enabled in the build configuration, the start/stop
> > > BTF markers are guaranteed to exist in the final link but not during =
the
> > > first linker pass.
> > >
> > > Avoid GOT based relocations to these markers in the final executable =
by
> > > providing preliminary definitions that will be used by the first link=
er
> > > pass, and superseded by the actual definitions in the subsequent ones=
.
> > >
> > > Make the preliminary definitions dependent on CONFIG_DEBUG_INFO_BTF s=
o
> > > that inadvertent references to this section will trigger a link failu=
re
> > > if they occur in code that does not honour CONFIG_DEBUG_INFO_BTF.
> > >
> > > Note that Clang will notice that taking the address of__start_BTF can=
not
> > > yield NULL any longer, so testing for that condition is no longer
> > > needed.
> > >
> > > Acked-by: Andrii Nakryiko <andrii@kernel.org>
> > > Acked-by: Arnd Bergmann <arnd@arndb.de>
> > > Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
> > > ---
> > >  include/asm-generic/vmlinux.lds.h | 9 +++++++++
> > >  kernel/bpf/btf.c                  | 7 +++++--
> > >  kernel/bpf/sysfs_btf.c            | 6 +++---
> > >  3 files changed, 17 insertions(+), 5 deletions(-)
> > >
> > > diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/=
vmlinux.lds.h
> > > index e8449be62058..4cb3d88449e5 100644
> > > --- a/include/asm-generic/vmlinux.lds.h
> > > +++ b/include/asm-generic/vmlinux.lds.h
> > > @@ -456,6 +456,7 @@
> > >   * independent code.
> > >   */
> > >  #define PRELIMINARY_SYMBOL_DEFINITIONS                              =
   \
> > > +       PRELIMINARY_BTF_DEFINITIONS                                  =
   \
> > >         PROVIDE(kallsyms_addresses =3D .);                           =
     \
> > >         PROVIDE(kallsyms_offsets =3D .);                             =
     \
> > >         PROVIDE(kallsyms_names =3D .);                               =
     \
> > > @@ -466,6 +467,14 @@
> > >         PROVIDE(kallsyms_markers =3D .);                             =
     \
> > >         PROVIDE(kallsyms_seqs_of_names =3D .);
> > >
> > > +#ifdef CONFIG_DEBUG_INFO_BTF
> > > +#define PRELIMINARY_BTF_DEFINITIONS                                 =
   \
> > > +       PROVIDE(__start_BTF =3D .);                                  =
     \
> > > +       PROVIDE(__stop_BTF =3D .);
> > > +#else
> > > +#define PRELIMINARY_BTF_DEFINITIONS
> > > +#endif
> > > +
> >
> >
> >
> > Is this necessary?
> >
>
> I think so.
>
> This actually resulted in Jiri's build failure with v2, and the
> realization that there was dead code in btf_parse_vmlinux() that
> happily tried to load the contents of the BTF section if
> CONFIG_DEBUG_INFO_BTF was not enabled to begin with.
>
> So this is another pitfall with weak references: the symbol may
> unexpectedly be missing altogether rather than only during the first
> linker pass.
>
> >
> >
> > The following code (BOUNDED_SECTION_BY)
> > produces __start_BTF and __stop_BTF symbols
> > under the same condition.
> >
>
> Indeed. So if CONFIG_DEBUG_INFO_BTF=3Dn, code can still link to
> __start_BTF and __stop_BTF even though BTF is not enabled.


I am talking about the case for CONFIG_DEBUG_INFO_BTF=3Dy.

PROVIDE() is meaningless because __start_BTF and __stop_BTF
are produced by the existing code.

(The code was a bit clearer before commit
9b351be25360c5cedfb98b88d6dfd89327849e52)


So, v4 of this patch will look like 2/3, right?


Just drop __weak attribute.

You still need

if (!IS_ENABLED(CONFIG_DEBUG_INFO_BTF))
             return ERR_PTR(-ENOENT);

But, you do not need to modify vmlinux.lds.h


--
Best Regards
Masahiro Yamada