Message-ID: <CAMj1kXEUL-Uv4tCx5NLVHDRo-BdEK1xJdee-UYs-ymE-mLxv0Q@mail.gmail.com>
Date: Mon, 3 Nov 2025 09:19:25 +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] efistub/smbios: Add fallback for SMBIOS record lookup

Hello Francesco,

On Sun, 2 Nov 2025 at 01:14, Francesco Pompo <francescopompo2@...il.com> wrote:
>
> Some UEFI firmware implementations 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. Log when the fallback is used.
>
> Signed-off-by: Francesco Pompo <francescopompo2@...il.com>
> ---
>  drivers/firmware/efi/libstub/efistub.h | 17 +++++
>  drivers/firmware/efi/libstub/smbios.c  | 99 +++++++++++++++++++++++++-
>  2 files changed, 113 insertions(+), 3 deletions(-)
>

On which platform does this fix an actual existing issue?

> diff --git a/drivers/firmware/efi/libstub/efistub.h b/drivers/firmware/efi/libstub/efistub.h
> index 685098f9626f..68582ce81370 100644
> --- a/drivers/firmware/efi/libstub/efistub.h
> +++ b/drivers/firmware/efi/libstub/efistub.h
> @@ -1151,6 +1151,23 @@ void free_screen_info(struct screen_info *si);
>  void efi_cache_sync_image(unsigned long image_base,
>                           unsigned long alloc_size);
>
> +struct __packed 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;
> +};
> +
>  struct efi_smbios_record {
>         u8      type;
>         u8      length;
> diff --git a/drivers/firmware/efi/libstub/smbios.c b/drivers/firmware/efi/libstub/smbios.c
> index f31410d7e7e1..21f499035b37 100644
> --- a/drivers/firmware/efi/libstub/smbios.c
> +++ b/drivers/firmware/efi/libstub/smbios.c
> @@ -33,6 +33,93 @@ union efi_smbios_protocol {
>         } mixed_mode;
>  };
>
> +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);
> +}
> +
>  const struct efi_smbios_record *efi_get_smbios_record(u8 type)
>  {
>         struct efi_smbios_record *record;
> @@ -43,9 +130,15 @@ const struct efi_smbios_record *efi_get_smbios_record(u8 type)
>         status = efi_bs_call(locate_protocol, &EFI_SMBIOS_PROTOCOL_GUID, NULL,
>                              (void **)&smbios) ?:
>                  efi_call_proto(smbios, get_next, &handle, &type, &record, NULL);
> -       if (status != EFI_SUCCESS)
> -               return NULL;
> -       return record;
> +       if (status == EFI_SUCCESS)
> +               return record;
> +
> +       efi_info(
> +               "Cannot access SMBIOS protocol (status 0x%lx), parsing table directly\n",
> +               status
> +       );
> +
> +       return get_table_record(type);
>  }
>
>  const u8 *__efi_get_smbios_string(const struct efi_smbios_record *record,
> --
> 2.51.0
>

Powered by blists - more mailing lists