Re: [PATCH v2 3/3] btf: Avoid weak external references

[Jiri Olsa]

On Wed, Apr 10, 2024 at 10:37:42AM +0200, Ard Biesheuvel wrote:
> On Wed, 10 Apr 2024 at 10:22, Jiri Olsa <olsajiri@xxxxxxxxx> wrote:
> >
> > On Tue, Apr 09, 2024 at 05:01:36PM +0200, Ard Biesheuvel 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 pass, and superseded by the actual definitions in the subsequent ones.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>
> > > Signed-off-by: Ard Biesheuvel <ardb@xxxxxxxxxx>
> > > ---
> > >  include/asm-generic/vmlinux.lds.h | 9 +++++++++
> > >  kernel/bpf/btf.c                  | 4 ++--
> > >  kernel/bpf/sysfs_btf.c            | 6 +++---
> > >  3 files changed, 14 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
> >
> > hi,
> > I'm getting following compilation fail when CONFIG_DEBUG_INFO_BTF is disabled
> >
> >         [jolsa@krava linux-qemu]$ make
> >           CALL    scripts/checksyscalls.sh
> >           DESCEND objtool
> >           INSTALL libsubcmd_headers
> >           UPD     include/generated/utsversion.h
> >           CC      init/version-timestamp.o
> >           LD      .tmp_vmlinux.kallsyms1
> >         ld: kernel/bpf/btf.o: in function `btf_parse_vmlinux':
> >         /home/jolsa/kernel/linux-qemu/kernel/bpf/btf.c:5988: undefined reference to `__start_BTF'
> >         ld: /home/jolsa/kernel/linux-qemu/kernel/bpf/btf.c:5989: undefined reference to `__stop_BTF'
> >         ld: /home/jolsa/kernel/linux-qemu/kernel/bpf/btf.c:5989: undefined reference to `__start_BTF'
> >         make[2]: *** [scripts/Makefile.vmlinux:37: vmlinux] Error 1
> >         make[1]: *** [/home/jolsa/kernel/linux-qemu/Makefile:1160: vmlinux] Error 2
> >         make: *** [Makefile:240: __sub-make] Error 2
> >
> > maybe the assumption was that kernel/bpf/btf.o is compiled only
> > for CONFIG_DEBUG_INFO_BTF, but it's actually:
> >
> >   obj-$(CONFIG_BPF_SYSCALL) += btf.o memalloc.o
> >
> 
> Interesting. I did build test this with and without
> CONFIG_DEBUG_INFO_BTF, but not with CONFIG_BPF_SYSCALL=y and
> CONFIG_DEBUG_INFO_BTF=n.
> 
> > I guess we just need !CONFIG_DEBUG_INFO_BTF version of btf_parse_vmlinux
> > function
> >
> 
> The below gives me a working build.
> 
> --- a/kernel/bpf/btf.c
> +++ b/kernel/bpf/btf.c
> @@ -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);

nice, so this basically eliminates the rest of the function,
I did not know this would work

build's fine now, thanks

jirka

> +
>         env = kzalloc(sizeof(*env), GFP_KERNEL | __GFP_NOWARN);
>         if (!env)
>                 return ERR_PTR(-ENOMEM);