Re: [PATCH v3 3/3] btf: Avoid weak external references

[Ard Biesheuvel]

On Mon, 15 Apr 2024 at 16:55, Masahiro Yamada <masahiroy@xxxxxxxxxx> wrote:
>
> On Mon, Apr 15, 2024 at 4:58 PM Ard Biesheuvel <ardb+git@xxxxxxxxxx> wrote:
> >
> > From: Ard Biesheuvel <ardb@xxxxxxxxxx>
> >
> > 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 linker
> > pass, and superseded by the actual definitions in the subsequent ones.
> >
> > Make the preliminary definitions dependent on CONFIG_DEBUG_INFO_BTF so
> > that inadvertent references to this section will trigger a link failure
> > 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 cannot
> > yield NULL any longer, so testing for that condition is no longer
> > needed.
> >
> > Acked-by: Andrii Nakryiko <andrii@xxxxxxxxxx>
> > Acked-by: Arnd Bergmann <arnd@xxxxxxxx>
> > Signed-off-by: Ard Biesheuvel <ardb@xxxxxxxxxx>
> > ---
> >  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 = .);                                \
> >         PROVIDE(kallsyms_offsets = .);                                  \
> >         PROVIDE(kallsyms_names = .);                                    \
> > @@ -466,6 +467,14 @@
> >         PROVIDE(kallsyms_markers = .);                                  \
> >         PROVIDE(kallsyms_seqs_of_names = .);
> >
> > +#ifdef CONFIG_DEBUG_INFO_BTF
> > +#define PRELIMINARY_BTF_DEFINITIONS                                    \
> > +       PROVIDE(__start_BTF = .);                                       \
> > +       PROVIDE(__stop_BTF = .);
> > +#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=n, code can still link to
__start_BTF and __stop_BTF even though BTF is not enabled.

>
>
> /*
>  * .BTF
>  */
> #ifdef CONFIG_DEBUG_INFO_BTF
> #define BTF                                                                \
>         .BTF : AT(ADDR(.BTF) - LOAD_OFFSET) {                                \
>                 BOUNDED_SECTION_BY(.BTF, _BTF)                                \
>         }                                                                \
>         . = ALIGN(4);                                                        \
>         .BTF_ids : AT(ADDR(.BTF_ids) - LOAD_OFFSET) {                        \
>                 *(.BTF_ids)                                                \
>         }
> #else
> #define BTF
> #endif
>
>
>
>
>
>
>
>
>
>
>
> >  /*
> >   * Read only Data
> >   */
> > diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
> > index 90c4a32d89ff..6d46cee47ae3 100644
> > --- a/kernel/bpf/btf.c
> > +++ b/kernel/bpf/btf.c
> > @@ -5642,8 +5642,8 @@ static struct btf *btf_parse(const union bpf_attr *attr, bpfptr_t uattr, u32 uat
> >         return ERR_PTR(err);
> >  }
> >
> > -extern char __weak __start_BTF[];
> > -extern char __weak __stop_BTF[];
> > +extern char __start_BTF[];
> > +extern char __stop_BTF[];
> >  extern struct btf *btf_vmlinux;
> >
> >  #define BPF_MAP_TYPE(_id, _ops)
> > @@ -5971,6 +5971,9 @@ struct btf *btf_parse_vmlinux(void)
> >         struct btf *btf = NULL;
> >         int err;
> >
> > +       if (!IS_ENABLED(CONFIG_DEBUG_INFO_BTF))
> > +               return ERR_PTR(-ENOENT);
> > +
> >         env = kzalloc(sizeof(*env), GFP_KERNEL | __GFP_NOWARN);
> >         if (!env)
> >                 return ERR_PTR(-ENOMEM);
> > diff --git a/kernel/bpf/sysfs_btf.c b/kernel/bpf/sysfs_btf.c
> > index ef6911aee3bb..fedb54c94cdb 100644
> > --- a/kernel/bpf/sysfs_btf.c
> > +++ b/kernel/bpf/sysfs_btf.c
> > @@ -9,8 +9,8 @@
> >  #include <linux/sysfs.h>
> >
> >  /* See scripts/link-vmlinux.sh, gen_btf() func for details */
> > -extern char __weak __start_BTF[];
> > -extern char __weak __stop_BTF[];
> > +extern char __start_BTF[];
> > +extern char __stop_BTF[];
> >
> >  static ssize_t
> >  btf_vmlinux_read(struct file *file, struct kobject *kobj,
> > @@ -32,7 +32,7 @@ static int __init btf_vmlinux_init(void)
> >  {
> >         bin_attr_btf_vmlinux.size = __stop_BTF - __start_BTF;
> >
> > -       if (!__start_BTF || bin_attr_btf_vmlinux.size == 0)
> > +       if (bin_attr_btf_vmlinux.size == 0)
> >                 return 0;
> >
> >         btf_kobj = kobject_create_and_add("btf", kernel_kobj);
> > --
> > 2.44.0.683.g7961c838ac-goog
> >
>
>
> --
> Best Regards
> Masahiro Yamada