From: Jiri Olsa <olsajiri@gmail.com>
To: Ard Biesheuvel <ardb@kernel.org>
Cc: Jiri Olsa <olsajiri@gmail.com>,
Ard Biesheuvel <ardb+git@google.com>,
linux-kernel@vger.kernel.org,
Masahiro Yamada <masahiroy@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>
Subject: Re: [PATCH v2 3/3] btf: Avoid weak external references
Date: Wed, 10 Apr 2024 11:30:58 +0200 [thread overview]
Message-ID: <ZhZcUkU6PoGNwTdQ@krava> (raw)
In-Reply-To: <CAMj1kXHuHVJn9H62GeA8kHNXm8L39BdCowebFkeCcNfYN29DjA@mail.gmail.com>
On Wed, Apr 10, 2024 at 10:37:42AM +0200, Ard Biesheuvel wrote:
> On Wed, 10 Apr 2024 at 10:22, Jiri Olsa <olsajiri@gmail.com> wrote:
> >
> > On Tue, Apr 09, 2024 at 05:01:36PM +0200, Ard Biesheuvel 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 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 pass, and superseded by the actual definitions in the subsequent ones.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@kernel.org>
> > > Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
> > > ---
> > > 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);
prev parent reply other threads:[~2024-04-10 9:31 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-04-09 15:01 [PATCH v2 0/3] kbuild: Avoid weak external linkage where possible Ard Biesheuvel
2024-04-09 15:01 ` [PATCH v2 1/3] kallsyms: Avoid weak references for kallsyms symbols Ard Biesheuvel
2024-04-09 15:04 ` Arnd Bergmann
2024-04-09 15:01 ` [PATCH v2 2/3] vmlinux: Avoid weak reference to notes section Ard Biesheuvel
2024-04-09 15:04 ` Arnd Bergmann
2024-04-09 15:01 ` [PATCH v2 3/3] btf: Avoid weak external references Ard Biesheuvel
2024-04-09 15:05 ` Arnd Bergmann
2024-04-10 8:21 ` Jiri Olsa
2024-04-10 8:37 ` Ard Biesheuvel
2024-04-10 9:30 ` Jiri Olsa [this message]
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