linux-kernel - Re: [PATCH v2] efi: fix -Wmissing-variable-declarations warning

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Message-ID: <CAMj1kXGbBx=gWsDJ7nXPwygOvX=Ek3tPQmZ_fQpBNmVg5_7u9A@mail.gmail.com>
Date:   Wed, 30 Aug 2023 19:54:14 +0200
From:   Ard Biesheuvel <ardb@...nel.org>
To:     Justin Stitt <justinstitt@...gle.com>
Cc:     Darren Hart <dvhart@...radead.org>,
        Andy Shevchenko <andy@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Tom Rix <trix@...hat.com>, linux-efi@...r.kernel.org,
        platform-driver-x86@...r.kernel.org, linux-kernel@...r.kernel.org,
        llvm@...ts.linux.dev
Subject: Re: [PATCH v2] efi: fix -Wmissing-variable-declarations warning

On Wed, 30 Aug 2023 at 19:51, Justin Stitt <justinstitt@...gle.com> wrote:
>
> When building x86/defconfig with Clang-18 I encounter the following warnings:
> | arch/x86/platform/efi/efi.c:934:23: warning: no previous extern declaration for non-static variable 'efi_attr_fw_vendor' [-Wmissing-variable-declarations]
> |   934 | struct kobj_attribute efi_attr_fw_vendor = __ATTR_RO(fw_vendor);
> | arch/x86/platform/efi/efi.c:935:23: warning: no previous extern declaration for non-static variable 'efi_attr_runtime' [-Wmissing-variable-declarations]
> |   935 | struct kobj_attribute efi_attr_runtime = __ATTR_RO(runtime);
> | arch/x86/platform/efi/efi.c:936:23: warning: no previous extern declaration for non-static variable 'efi_attr_config_table' [-Wmissing-variable-declarations]
> |   936 | struct kobj_attribute efi_attr_config_table = __ATTR_RO(config_table);
>
> These symbols are declared extern in drivers/firmware/efi/efi.c. Move
> the declarations to linux/efi.h to resolve these warnings.
>
> Also, trim a trailing space from efi_set_variable_t typedef.
>
> Link: https://github.com/ClangBuiltLinux/linux/issues/1920
> Suggested-by: Ard Biesheuvel <ardb@...nel.org>
> Signed-off-by: Justin Stitt <justinstitt@...gle.com>

Thanks, I'll take this as a fix.

> ---
> Changes in v2:
> - Don't use static. Instead move symbols to header file (thanks Ard)
> - Remove duplicated warnings output from b4 (thanks Ard)
> - Link to v1: https://lore.kernel.org/r/20230829-missingvardecl-efi-v1-1-13d055a55176@google.com
> ---
> Note: build-tested.
>
> Also, a question: Why does the compiler warn at all? The extern
> declarations _do_ exist as Ard points out [1].
>
> [1]: https://lore.kernel.org/all/CAMj1kXE_xs+wWno4G4U+6CFCOwO7JWcmNv4et5=W=ZRrQatWnQ@mail.gmail.com/

Those declarations are not observed by the compiler when building .c
files other than drivers/firmware/efi/efi.c, so they essentially don't
exist when compiling arch/x86/platform/efi/efi.c.


> ---
>  drivers/firmware/efi/efi.c | 3 ---
>  include/linux/efi.h        | 6 +++++-
>  2 files changed, 5 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/firmware/efi/efi.c b/drivers/firmware/efi/efi.c
> index 1599f1176842..7b4e59ccf09e 100644
> --- a/drivers/firmware/efi/efi.c
> +++ b/drivers/firmware/efi/efi.c
> @@ -161,9 +161,6 @@ static ssize_t fw_platform_size_show(struct kobject *kobj,
>         return sprintf(buf, "%d\n", efi_enabled(EFI_64BIT) ? 64 : 32);
>  }
>
> -extern __weak struct kobj_attribute efi_attr_fw_vendor;
> -extern __weak struct kobj_attribute efi_attr_runtime;
> -extern __weak struct kobj_attribute efi_attr_config_table;
>  static struct kobj_attribute efi_attr_fw_platform_size =
>         __ATTR_RO(fw_platform_size);
>
> diff --git a/include/linux/efi.h b/include/linux/efi.h
> index ab088c662e88..32f7ca60b0e9 100644
> --- a/include/linux/efi.h
> +++ b/include/linux/efi.h
> @@ -287,7 +287,7 @@ typedef efi_status_t efi_get_variable_t (efi_char16_t *name, efi_guid_t *vendor,
>                                          unsigned long *data_size, void *data);
>  typedef efi_status_t efi_get_next_variable_t (unsigned long *name_size, efi_char16_t *name,
>                                               efi_guid_t *vendor);
> -typedef efi_status_t efi_set_variable_t (efi_char16_t *name, efi_guid_t *vendor,
> +typedef efi_status_t efi_set_variable_t (efi_char16_t *name, efi_guid_t *vendor,
>                                          u32 attr, unsigned long data_size,
>                                          void *data);
>  typedef efi_status_t efi_get_next_high_mono_count_t (u32 *count);
> @@ -632,6 +632,10 @@ typedef struct {
>
>  extern unsigned long __ro_after_init efi_rng_seed;             /* RNG Seed table */
>
> +extern __weak struct kobj_attribute efi_attr_fw_vendor;
> +extern __weak struct kobj_attribute efi_attr_runtime;
> +extern __weak struct kobj_attribute efi_attr_config_table;
> +
>  /*
>   * All runtime access to EFI goes through this structure:
>   */
>
> ---
> base-commit: 706a741595047797872e669b3101429ab8d378ef
> change-id: 20230829-missingvardecl-efi-59306aacfed4
>
> Best regards,
> --
> Justin Stitt <justinstitt@...gle.com>
>