[PATCH 0/3] kbuild: Avoid weak external linkage where possible

[PATCH 0/3] kbuild: Avoid weak external linkage where possible

[Ard Biesheuvel]

From: Ard Biesheuvel <ardb@kernel.org>

Weak external linkage is intended for cases where a symbol reference
can remain unsatisfied in the final link. Taking the address of such a
symbol should yield NULL if the reference was not satisfied.

Given that ordinary RIP or PC relative references cannot produce NULL,
some kind of indirection is always needed in such cases, and in position
independent code, this results in a GOT entry. In ordinary code, it is
arch specific but amounts to the same thing.

While unavoidable in some cases, weak references are currently also used
to declare symbols that are always defined in the final link, but not in
the first linker pass. This means we end up with worse codegen for no
good reason. So let's clean this up, by providing preliminary
definitions that are only used as a fallback.

Cc: Masahiro Yamada <masahiroy@kernel.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Martin KaFai Lau <martin.lau@linux.dev>
Cc: linux-arch@vger.kernel.org
Cc: linux-kbuild@vger.kernel.org
Cc: bpf@vger.kernel.org

Ard Biesheuvel (3):
  kallsyms: Avoid weak references for kallsyms symbols
  vmlinux: Avoid weak reference to notes section
  btf: Avoid weak external references

 include/asm-generic/vmlinux.lds.h | 21 ++++++++++++++
 kernel/bpf/btf.c                  |  4 +--
 kernel/kallsyms.c                 |  6 ----
 kernel/kallsyms_internal.h        | 30 ++++++++------------
 kernel/ksysfs.c                   |  4 +--
 lib/buildid.c                     |  4 +--
 6 files changed, 39 insertions(+), 30 deletions(-)

-- 
2.44.0.478.gd926399ef9-goog

[PATCH 1/3] kallsyms: Avoid weak references for kallsyms symbols

[Ard Biesheuvel]

From: Ard Biesheuvel <ardb@kernel.org>

kallsyms is a directory of all the symbols in the vmlinux binary, and so
creating it is somewhat of a chicken-and-egg problem, as its non-zero
size affects the layout of the binary, and therefore the values of the
symbols.

For this reason, the kernel is linked more than once, and the first pass
does not include any kallsyms data at all. For the linker to accept
this, the symbol declarations describing the kallsyms metadata are
emitted as having weak linkage, so they can remain unsatisfied. During
the subsequent passes, the weak references are satisfied by the kallsyms
metadata that was constructed based on information gathered from the
preceding passes.

Weak references lead to somewhat worse codegen, because taking their
address may need to produce NULL (if the reference was unsatisfied), and
this is not usually supported by RIP or PC relative symbol references.

Given that these references are ultimately always satisfied in the final
link, let's drop the weak annotation, and instead, provide fallback
definitions in the linker script that are only emitted if an unsatisfied
reference exists.

While at it, drop the FRV specific annotation that these symbols reside
in .rodata - FRV is long gone.

Tested-by: Nick Desaulniers <ndesaulniers@google.com> # Boot
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Link: https://lkml.kernel.org/r/20230504174320.3930345-1-ardb%40kernel.org
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
---
 include/asm-generic/vmlinux.lds.h | 19 +++++++++++++
 kernel/kallsyms.c                 |  6 ----
 kernel/kallsyms_internal.h        | 30 ++++++++------------
 3 files changed, 31 insertions(+), 24 deletions(-)

diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
index f7749d0f2562..e8449be62058 100644
--- a/include/asm-generic/vmlinux.lds.h
+++ b/include/asm-generic/vmlinux.lds.h
@@ -448,11 +448,30 @@
 #endif
 #endif
 
+/*
+ * Some symbol definitions will not exist yet during the first pass of the
+ * link, but are guaranteed to exist in the final link. Provide preliminary
+ * definitions that will be superseded in the final link to avoid having to
+ * rely on weak external linkage, which requires a GOT when used in position
+ * independent code.
+ */
+#define PRELIMINARY_SYMBOL_DEFINITIONS					\
+	PROVIDE(kallsyms_addresses = .);				\
+	PROVIDE(kallsyms_offsets = .);					\
+	PROVIDE(kallsyms_names = .);					\
+	PROVIDE(kallsyms_num_syms = .);					\
+	PROVIDE(kallsyms_relative_base = .);				\
+	PROVIDE(kallsyms_token_table = .);				\
+	PROVIDE(kallsyms_token_index = .);				\
+	PROVIDE(kallsyms_markers = .);					\
+	PROVIDE(kallsyms_seqs_of_names = .);
+
 /*
  * Read only Data
  */
 #define RO_DATA(align)							\
 	. = ALIGN((align));						\
+	PRELIMINARY_SYMBOL_DEFINITIONS					\
 	.rodata           : AT(ADDR(.rodata) - LOAD_OFFSET) {		\
 		__start_rodata = .;					\
 		*(.rodata) *(.rodata.*)					\
diff --git a/kernel/kallsyms.c b/kernel/kallsyms.c
index 18edd57b5fe8..22ea19a36e6e 100644
--- a/kernel/kallsyms.c
+++ b/kernel/kallsyms.c
@@ -325,12 +325,6 @@ static unsigned long get_symbol_pos(unsigned long addr,
 	unsigned long symbol_start = 0, symbol_end = 0;
 	unsigned long i, low, high, mid;
 
-	/* This kernel should never had been booted. */
-	if (!IS_ENABLED(CONFIG_KALLSYMS_BASE_RELATIVE))
-		BUG_ON(!kallsyms_addresses);
-	else
-		BUG_ON(!kallsyms_offsets);
-
 	/* Do a binary search on the sorted kallsyms_addresses array. */
 	low = 0;
 	high = kallsyms_num_syms;
diff --git a/kernel/kallsyms_internal.h b/kernel/kallsyms_internal.h
index 27fabdcc40f5..85480274fc8f 100644
--- a/kernel/kallsyms_internal.h
+++ b/kernel/kallsyms_internal.h
@@ -5,27 +5,21 @@
 #include <linux/types.h>
 
 /*
- * These will be re-linked against their real values
- * during the second link stage.
+ * These will be re-linked against their real values during the second link
+ * stage. Preliminary values must be provided in the linker script using the
+ * PROVIDE() directive so that the first link stage can complete successfully.
  */
-extern const unsigned long kallsyms_addresses[] __weak;
-extern const int kallsyms_offsets[] __weak;
-extern const u8 kallsyms_names[] __weak;
+extern const unsigned long kallsyms_addresses[];
+extern const int kallsyms_offsets[];
+extern const u8 kallsyms_names[];
 
-/*
- * Tell the compiler that the count isn't in the small data section if the arch
- * has one (eg: FRV).
- */
-extern const unsigned int kallsyms_num_syms
-__section(".rodata") __attribute__((weak));
-
-extern const unsigned long kallsyms_relative_base
-__section(".rodata") __attribute__((weak));
+extern const unsigned int kallsyms_num_syms;
+extern const unsigned long kallsyms_relative_base;
 
-extern const char kallsyms_token_table[] __weak;
-extern const u16 kallsyms_token_index[] __weak;
+extern const char kallsyms_token_table[];
+extern const u16 kallsyms_token_index[];
 
-extern const unsigned int kallsyms_markers[] __weak;
-extern const u8 kallsyms_seqs_of_names[] __weak;
+extern const unsigned int kallsyms_markers[];
+extern const u8 kallsyms_seqs_of_names[];
 
 #endif // LINUX_KALLSYMS_INTERNAL_H_
-- 
2.44.0.478.gd926399ef9-goog

[PATCH 2/3] vmlinux: Avoid weak reference to notes section

[Ard Biesheuvel]

From: Ard Biesheuvel <ardb@kernel.org>

Weak references are references that are permitted to remain unsatisfied
in the final link. This means they cannot be implemented using place
relative relocations, resulting in GOT entries when using position
independent code generation.

The notes section should always exist, so the weak annotations can be
omitted.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
---
 kernel/ksysfs.c | 4 ++--
 lib/buildid.c   | 4 ++--
 2 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/kernel/ksysfs.c b/kernel/ksysfs.c
index 495b69a71a5d..07fb5987b42b 100644
--- a/kernel/ksysfs.c
+++ b/kernel/ksysfs.c
@@ -228,8 +228,8 @@ KERNEL_ATTR_RW(rcu_normal);
 /*
  * Make /sys/kernel/notes give the raw contents of our kernel .notes section.
  */
-extern const void __start_notes __weak;
-extern const void __stop_notes __weak;
+extern const void __start_notes;
+extern const void __stop_notes;
 #define	notes_size (&__stop_notes - &__start_notes)
 
 static ssize_t notes_read(struct file *filp, struct kobject *kobj,
diff --git a/lib/buildid.c b/lib/buildid.c
index 898301b49eb6..7954dd92e36c 100644
--- a/lib/buildid.c
+++ b/lib/buildid.c
@@ -182,8 +182,8 @@ unsigned char vmlinux_build_id[BUILD_ID_SIZE_MAX] __ro_after_init;
  */
 void __init init_vmlinux_build_id(void)
 {
-	extern const void __start_notes __weak;
-	extern const void __stop_notes __weak;
+	extern const void __start_notes;
+	extern const void __stop_notes;
 	unsigned int size = &__stop_notes - &__start_notes;
 
 	build_id_parse_buf(&__start_notes, vmlinux_build_id, size);
-- 
2.44.0.478.gd926399ef9-goog

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

[Ard Biesheuvel]

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.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
---
 include/asm-generic/vmlinux.lds.h | 2 ++
 kernel/bpf/btf.c                  | 4 ++--
 2 files changed, 4 insertions(+), 2 deletions(-)

diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
index e8449be62058..141bddb511ee 100644
--- a/include/asm-generic/vmlinux.lds.h
+++ b/include/asm-generic/vmlinux.lds.h
@@ -456,6 +456,8 @@
  * independent code.
  */
 #define PRELIMINARY_SYMBOL_DEFINITIONS					\
+	PROVIDE(__start_BTF = .);					\
+	PROVIDE(__stop_BTF = .);					\
 	PROVIDE(kallsyms_addresses = .);				\
 	PROVIDE(kallsyms_offsets = .);					\
 	PROVIDE(kallsyms_names = .);					\
diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
index 90c4a32d89ff..46a56bf067a8 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)
-- 
2.44.0.478.gd926399ef9-goog

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

[Andrii Nakryiko]

On Fri, Mar 29, 2024 at 2:35 AM Ard Biesheuvel <ardb+git@google.com> 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.
>
> Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
> ---
>  include/asm-generic/vmlinux.lds.h | 2 ++
>  kernel/bpf/btf.c                  | 4 ++--
>  2 files changed, 4 insertions(+), 2 deletions(-)
>
> diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
> index e8449be62058..141bddb511ee 100644
> --- a/include/asm-generic/vmlinux.lds.h
> +++ b/include/asm-generic/vmlinux.lds.h
> @@ -456,6 +456,8 @@
>   * independent code.
>   */
>  #define PRELIMINARY_SYMBOL_DEFINITIONS                                 \
> +       PROVIDE(__start_BTF = .);                                       \
> +       PROVIDE(__stop_BTF = .);                                        \

should this be guarded by CONFIG_DEBUG_INFO_BTF condition?

>         PROVIDE(kallsyms_addresses = .);                                \
>         PROVIDE(kallsyms_offsets = .);                                  \
>         PROVIDE(kallsyms_names = .);                                    \
> diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
> index 90c4a32d89ff..46a56bf067a8 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[];

kernel/bpf/sysfs_btf.c also defines __weak externs for these symbols,
you probably need to adjust that as well?

Other than that looks good to me:

Acked-by: Andrii Nakryiko <andrii@kernel.org>


>  extern struct btf *btf_vmlinux;
>
>  #define BPF_MAP_TYPE(_id, _ops)
> --
> 2.44.0.478.gd926399ef9-goog
>
>

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

[Ard Biesheuvel]

On Fri, 29 Mar 2024 at 20:24, Andrii Nakryiko <andrii.nakryiko@gmail.com> wrote:
>
> On Fri, Mar 29, 2024 at 2:35 AM Ard Biesheuvel <ardb+git@google.com> 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.
> >
> > Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
> > ---
> >  include/asm-generic/vmlinux.lds.h | 2 ++
> >  kernel/bpf/btf.c                  | 4 ++--
> >  2 files changed, 4 insertions(+), 2 deletions(-)
> >
> > diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
> > index e8449be62058..141bddb511ee 100644
> > --- a/include/asm-generic/vmlinux.lds.h
> > +++ b/include/asm-generic/vmlinux.lds.h
> > @@ -456,6 +456,8 @@
> >   * independent code.
> >   */
> >  #define PRELIMINARY_SYMBOL_DEFINITIONS                                 \
> > +       PROVIDE(__start_BTF = .);                                       \
> > +       PROVIDE(__stop_BTF = .);                                        \
>
> should this be guarded by CONFIG_DEBUG_INFO_BTF condition?
>

That shouldn't matter - the linker will not create the symbol unless
an unsatisfied reference to it exists anywhere in the input.

> >         PROVIDE(kallsyms_addresses = .);                                \
> >         PROVIDE(kallsyms_offsets = .);                                  \
> >         PROVIDE(kallsyms_names = .);                                    \
> > diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
> > index 90c4a32d89ff..46a56bf067a8 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[];
>
> kernel/bpf/sysfs_btf.c also defines __weak externs for these symbols,
> you probably need to adjust that as well?
>

Yes, thanks for pointing that out.

> Other than that looks good to me:
>
> Acked-by: Andrii Nakryiko <andrii@kernel.org>
>

Thanks!

>
> >  extern struct btf *btf_vmlinux;
> >
> >  #define BPF_MAP_TYPE(_id, _ops)
> > --
> > 2.44.0.478.gd926399ef9-goog
> >
> >