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 pass, and superseded by the actual definitions in the subsequent ones.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);
+
env = kzalloc(sizeof(*env), GFP_KERNEL | __GFP_NOWARN);
if (!env)
return ERR_PTR(-ENOMEM);
