From: Ard Biesheuvel ardb@kernel.org

[ Upstream commit 951bcae6c5a0bfaa55b27c5f16178204988f0379 ]

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 Acked-by: Arnd Bergmann arnd@arndb.de Link: https://lkml.kernel.org/r/20230504174320.3930345-1-ardb%40kernel.org Signed-off-by: Ard Biesheuvel ardb@kernel.org Signed-off-by: Masahiro Yamada masahiroy@kernel.org Signed-off-by: Huacai Chen chenhuacai@loongson.cn --- 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 63029bc7c9dd..64a80059d7fa 100644 --- a/include/asm-generic/vmlinux.lds.h +++ b/include/asm-generic/vmlinux.lds.h @@ -447,11 +447,30 @@ #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 Provide preliminary + * definitions that will be superseded in the final link to avoid having to + * rely on weak external linkage, which requires a GOT when used in position + * independent code.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_

From: Ard Biesheuvel ardb@kernel.org

[ Upstream commit 377d9095117c084b835e38c020faf5a78e386f01 ]

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.

Acked-by: Arnd Bergmann arnd@arndb.de Signed-off-by: Ard Biesheuvel ardb@kernel.org Signed-off-by: Masahiro Yamada masahiroy@kernel.org Signed-off-by: Huacai Chen chenhuacai@loongson.cn --- 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 1d4bc493b2f4..347beb763c59 100644 --- a/kernel/ksysfs.c +++ b/kernel/ksysfs.c @@ -226,8 +226,8 @@ 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 9fc46366597e..ebfc8a28e46a 100644 --- a/lib/buildid.c +++ b/lib/buildid.c @@ -208,8 +208,8 @@ 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);

From: Ard Biesheuvel ardb@kernel.org

[ Upstream commit fc5eb4a84e4c063e75a6a6e92308e9533c0f19b5 ]

If the BTF code is enabled in the build configuration, the start/stop BTF markers are guaranteed to exist. Only when CONFIG_DEBUG_INFO_BTF=n, the references in btf_parse_vmlinux() will remain unsatisfied, relying on the weak linkage of the external references to avoid breaking the build.

Avoid GOT based relocations to these markers in the final executable by dropping the weak attribute and instead, make btf_parse_vmlinux() return ERR_PTR(-ENOENT) directly if CONFIG_DEBUG_INFO_BTF is not enabled to begin with. The compiler will drop any subsequent references to __start_BTF and __stop_BTF in that case, allowing the link to succeed.

Note that Clang will notice that taking the address of __start_BTF can no longer yield NULL, so testing for that condition becomes unnecessary.

Signed-off-by: Ard Biesheuvel ardb@kernel.org Signed-off-by: Daniel Borkmann daniel@iogearbox.net Acked-by: Andrii Nakryiko andrii@kernel.org Acked-by: Arnd Bergmann arnd@arndb.de Acked-by: Jiri Olsa jolsa@kernel.org Link: https://lore.kernel.org/bpf/20240415162041.2491523-8-ardb+git@google.com Signed-off-by: Huacai Chen chenhuacai@loongson.cn --- kernel/bpf/btf.c | 7 +++++-- kernel/bpf/sysfs_btf.c | 6 +++--- 2 files changed, 8 insertions(+), 5 deletions(-)

diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c index c8828016a66f..f231e2a273a1 100644 --- a/kernel/bpf/btf.c +++ b/kernel/bpf/btf.c @@ -5574,8 +5574,8 @@ 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) @@ -5724,6 +5724,9 @@ 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);