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From: Ard Biesheuvel <ardb@kernel.org>
Date: Mon, 15 Apr 2024 17:54:16 +0200
Message-ID: <CAMj1kXGEbaqzD2nCqOxdBgVZ+sjwCSZyqthdb=FQx2xhySbDEQ@mail.gmail.com>
Subject: Re: [PATCH v3 3/3] btf: Avoid weak external references
To: Masahiro Yamada <masahiroy@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, 15 Apr 2024 at 17:32, Masahiro Yamada <masahiroy@kernel.org> wrote:
>
> On Mon, Apr 15, 2024 at 11:59=E2=80=AFPM Ard Biesheuvel <ardb@kernel.org>=
 wrote:
> >
> > On Mon, 15 Apr 2024 at 16:55, Masahiro Yamada <masahiroy@kernel.org> wr=
ote:
> > >
> > > On Mon, Apr 15, 2024 at 4:58=E2=80=AFPM Ard Biesheuvel <ardb+git@goog=
le.com> wrote:
> > > >
> > > > From: Ard Biesheuvel <ardb@kernel.org>
> > > >
> > > > If the BTF code is enabled in the build configuration, the start/st=
op
> > > > BTF markers are guaranteed to exist in the final link but not durin=
g the
> > > > first linker pass.
> > > >
> > > > Avoid GOT based relocations to these markers in the final executabl=
e by
> > > > providing preliminary definitions that will be used by the first li=
nker
> > > > pass, and superseded by the actual definitions in the subsequent on=
es.
> > > >
> > > > Make the preliminary definitions dependent on CONFIG_DEBUG_INFO_BTF=
 so
> > > > that inadvertent references to this section will trigger a link fai=
lure
> > > > 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 c=
annot
> > > > 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-generi=
c/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
>

OK, I see what you mean now.

The PROVIDE() is indeed unnecessary - I'll drop that bit in v4.