linux-kernel - Re: [PATCH] x86/efi: Don't require non-blocking EFI callbacks

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Date:   Thu, 26 Sep 2019 17:29:57 +0200
From:   Ard Biesheuvel <ard.biesheuvel@...aro.org>
To:     Ross Lagerwall <ross.lagerwall@...rix.com>
Cc:     linux-efi <linux-efi@...r.kernel.org>,
        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>,
        "H. Peter Anvin" <hpa@...or.com>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        platform-driver-x86@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Sai Praneeth <sai.praneeth.prakhya@...el.com>
Subject: Re: [PATCH] x86/efi: Don't require non-blocking EFI callbacks

On Thu, 26 Sep 2019 at 16:12, Ross Lagerwall <ross.lagerwall@...rix.com> wrote:
>
> If a backend does not implement non-blocking EFI operations, it implies
> that the normal operations are non-blocking.

Is that documented anywhere?

> Instead of crashing
> dereferencing a NULL pointer, fallback to the normal operations since it
> is safe to do so.
>

I agree that crashing is never the right thing to do, but I wonder
whether we shouldn't just bail instead. If the provided default
operation is non-blocking, the platform can populate the function
pointer with a reference to the default implementation.


> Fixes: 5a58bc1b1edc ("efi/x86: Use non-blocking SetVariable() for efi_delete_dummy_variable()")
> Fixes: ca0e30dcaa53 ("efi: Add nonblocking option to efi_query_variable_store()")
> Signed-off-by: Ross Lagerwall <ross.lagerwall@...rix.com>
> ---
>  arch/x86/platform/efi/quirks.c | 20 ++++++++++++--------
>  1 file changed, 12 insertions(+), 8 deletions(-)
>
> diff --git a/arch/x86/platform/efi/quirks.c b/arch/x86/platform/efi/quirks.c
> index 3b9fd679cea9..4167f5e8f3e8 100644
> --- a/arch/x86/platform/efi/quirks.c
> +++ b/arch/x86/platform/efi/quirks.c
> @@ -106,11 +106,13 @@ early_param("efi_no_storage_paranoia", setup_storage_paranoia);
>  */
>  void efi_delete_dummy_variable(void)
>  {
> -       efi.set_variable_nonblocking((efi_char16_t *)efi_dummy_name,
> -                                    &EFI_DUMMY_GUID,
> -                                    EFI_VARIABLE_NON_VOLATILE |
> -                                    EFI_VARIABLE_BOOTSERVICE_ACCESS |
> -                                    EFI_VARIABLE_RUNTIME_ACCESS, 0, NULL);
> +       efi_set_variable_t *set_variable = efi.set_variable_nonblocking ?:
> +                                          efi.set_variable;
> +
> +       set_variable((efi_char16_t *)efi_dummy_name, &EFI_DUMMY_GUID,
> +                    EFI_VARIABLE_NON_VOLATILE |
> +                    EFI_VARIABLE_BOOTSERVICE_ACCESS |
> +                    EFI_VARIABLE_RUNTIME_ACCESS, 0, NULL);
>  }
>
>  /*
> @@ -127,10 +129,12 @@ query_variable_store_nonblocking(u32 attributes, unsigned long size)
>  {
>         efi_status_t status;
>         u64 storage_size, remaining_size, max_size;
> +       efi_query_variable_info_t *query_variable_info =
> +               efi.query_variable_info_nonblocking ?:
> +               efi.query_variable_info;
>
> -       status = efi.query_variable_info_nonblocking(attributes, &storage_size,
> -                                                    &remaining_size,
> -                                                    &max_size);
> +       status = query_variable_info(attributes, &storage_size,
> +                                    &remaining_size, &max_size);
>         if (status != EFI_SUCCESS)
>                 return status;
>
> --
> 2.21.0
>