linux-kernel - Re: [PATCH v2] efistub/x86: Add fallback for SMBIOS record lookup

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Message-ID: <CAMj1kXHO8Mxk+zPEwx2+VP1FF_LoDu58tbDOm+-Y8m07d3nitw@mail.gmail.com>
Date: Mon, 3 Nov 2025 14:47:22 +0100
From: Ard Biesheuvel <ardb@...nel.org>
To: Francesco Pompo <francescopompo2@...il.com>
Cc: linux-efi@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] efistub/x86: Add fallback for SMBIOS record lookup

On Mon, 3 Nov 2025 at 13:33, Francesco Pompo <francescopompo2@...il.com> wrote:
>
> Some Apple EFI firmwares do not provide the SMBIOS Protocol,
> causing efi_get_smbios_record() to fail. This prevents retrieval of
> system information such as product name, which is needed by
> apple_set_os() to enable the integrated GPU on dual-graphics Intel
> MacBooks.
>
> Add a fallback that directly parses the SMBIOS entry point table when
> the protocol is unavailable.
>
> Signed-off-by: Francesco Pompo <francescopompo2@...il.com>
> ---
>  drivers/firmware/efi/libstub/x86-stub.c | 107 +++++++++++++++++++++++-
>  1 file changed, 106 insertions(+), 1 deletion(-)
>

OK, I've pushed this to the efi/next branch now. I did apply some
cosmetic tweaks, though, so please double check that I did not break
anything.


> diff --git a/drivers/firmware/efi/libstub/x86-stub.c b/drivers/firmware/efi/libstub/x86-stub.c
> index f8e465da344d..13059412fdb9 100644
> --- a/drivers/firmware/efi/libstub/x86-stub.c
> +++ b/drivers/firmware/efi/libstub/x86-stub.c
> @@ -225,6 +225,110 @@ static void retrieve_apple_device_properties(struct boot_params *boot_params)
>         }
>  }
>
> +struct smbios_entry_point {
> +       char anchor[4];
> +       u8 ep_checksum;
> +       u8 ep_length;
> +       u8 major_version;
> +       u8 minor_version;
> +       u16 max_size_entry;
> +       u8 ep_rev;
> +       u8 reserved[5];
> +       char int_anchor[5];
> +       u8 int_checksum;
> +       u16 st_length;
> +       u32 st_address;
> +       u16 number_of_entries;
> +       u8 bcd_rev;
> +};
> +
> +static bool verify_ep_checksum(const struct smbios_entry_point *ep)
> +{
> +       const u8 *ptr = (u8 *)ep;
> +       u8 sum = 0;
> +       int i;
> +
> +       for (i = 0; i < ep->ep_length; i++)
> +               sum += ptr[i];
> +
> +       return sum == 0;
> +}
> +
> +static bool verify_ep_int_checksum(const struct smbios_entry_point *ep)
> +{
> +       const u8 *ptr = (u8 *)&ep->int_anchor;
> +       u8 sum = 0;
> +       int i;
> +
> +       for (i = 0; i < 15; i++)
> +               sum += ptr[i];
> +
> +       return sum == 0;
> +}
> +
> +static bool verify_ep_integrity(const struct smbios_entry_point *ep)
> +{
> +       if (memcmp(ep->anchor, "_SM_", sizeof(ep->anchor)) != 0)
> +               return false;
> +
> +       if (memcmp(ep->int_anchor, "_DMI_", sizeof(ep->int_anchor)) != 0)
> +               return false;
> +
> +       if (!verify_ep_checksum(ep) || !verify_ep_int_checksum(ep))
> +               return false;
> +
> +       return true;
> +}
> +
> +static const struct efi_smbios_record *search_record(void *table, u32 length,
> +                                                    u8 type)
> +{
> +       const u8 *p, *end;
> +
> +       p = (u8 *)table;
> +       end = p + length;
> +
> +       while (p + sizeof(struct efi_smbios_record) < end) {
> +               const struct efi_smbios_record *hdr =
> +                       (struct efi_smbios_record *)p;
> +               const u8 *next;
> +
> +               if (hdr->type == type)
> +                       return hdr;
> +
> +               /* Type 127 = End-of-Table */
> +               if (hdr->type == 0x7F)
> +                       return NULL;
> +
> +               /* Jumping to the unformed section */
> +               next = p + hdr->length;
> +
> +               /* Unformed section ends with 0000h */
> +               while ((next[0] != 0 || next[1] != 0) && next + 1 < end)
> +                       next++;
> +
> +               next += 2;
> +               p = next;
> +       }
> +
> +       return NULL;
> +}
> +
> +static const struct efi_smbios_record *get_table_record(u8 type)
> +{
> +       const struct smbios_entry_point *ep;
> +
> +       ep = get_efi_config_table(SMBIOS_TABLE_GUID);
> +       if (!ep)
> +               return NULL;
> +
> +       if (!verify_ep_integrity(ep))
> +               return NULL;
> +
> +       return search_record((void *)(unsigned long)ep->st_address,
> +               ep->st_length, type);
> +}
> +
>  static bool apple_match_product_name(void)
>  {
>         static const char type1_product_matches[][15] = {
> @@ -240,7 +344,8 @@ static bool apple_match_product_name(void)
>         const struct efi_smbios_type1_record *record;
>         const u8 *product;
>
> -       record = (struct efi_smbios_type1_record *)efi_get_smbios_record(1);
> +       record = (struct efi_smbios_type1_record *)(efi_get_smbios_record(1) ?:
> +               get_table_record(1));
>         if (!record)
>                 return false;
>
> --
> 2.50.1
>