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Message-ID: <CAK7LNAQi33YR35QZi3gX8Gfe-J3mfuEB5GWjmfT7W07mjmgKYw@mail.gmail.com>
Date: Tue, 23 Apr 2024 22:44:26 +0900
From: Masahiro Yamada <masahiroy@...nel.org>
To: Ard Biesheuvel <ardb+git@...gle.com>
Cc: linux-kernel@...r.kernel.org, Ard Biesheuvel <ardb@...nel.org>,
Arnd Bergmann <arnd@...db.de>, Martin KaFai Lau <martin.lau@...ux.dev>, linux-arch@...r.kernel.org,
linux-kbuild@...r.kernel.org, bpf@...r.kernel.org,
Andrii Nakryiko <andrii@...nel.org>, Jiri Olsa <olsajiri@...il.com>,
Nick Desaulniers <ndesaulniers@...gle.com>, Kees Cook <keescook@...omium.org>
Subject: Re: [PATCH v4 1/3] kallsyms: Avoid weak references for kallsyms symbols
On Tue, Apr 16, 2024 at 1:20 AM Ard Biesheuvel <ardb+git@...gle.com> wrote:
>
> From: Ard Biesheuvel <ardb@...nel.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@...gle.com> # Boot
> Reviewed-by: Nick Desaulniers <ndesaulniers@...gle.com>
> Reviewed-by: Kees Cook <keescook@...omium.org>
> Acked-by: Arnd Bergmann <arnd@...db.de>
> Link: https://lkml.kernel.org/r/20230504174320.3930345-1-ardb%40kernel.org
> Signed-off-by: Ard Biesheuvel <ardb@...nel.org>
> ---
I dropped v5, and picked up this one.
Thanks.
> 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_
> --
> 2.44.0.683.g7961c838ac-goog
>
>
--
Best Regards
Masahiro Yamada
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