Re: [PATCH V3] efi/libstub/x86: Avoid EFI map buffer alloc in allocate_e820()

[Ard Biesheuvel]

On Thu, 7 May 2020 at 20:33, Lenny Szubowicz <lszubowi@xxxxxxxxxx> wrote:
>
> In allocate_e820(), call the EFI get_memory_map() service directly
> instead of indirectly via efi_get_memory_map(). This avoids allocation
> of a buffer and return of the full EFI memory map, which is not needed
> here and would otherwise need to be freed.
>
> Routine allocate_e820() only needs to know how many EFI memory
> descriptors there are in the map to allocate an adequately sized
> e820ext buffer, if it's needed. Note that since efi_get_memory_map()
> returns a memory map buffer sized with extra headroom, allocate_e820()
> now needs to explicitly factor that into the e820ext size calculation.
>
> Signed-off-by: Lenny Szubowicz <lszubowi@xxxxxxxxxx>
> Suggested-by: Ard Biesheuvel <ardb@xxxxxxxxxx>

Thanks Lenny. I'll get this queued up.

> --
> v3:
>   - Move define of EFI_MMAP_NR_SLACK_SLOTS to efistub.h instead
>     of providing a helper function to get it.
>
> v2:
>   - Instead of freeing the EFI memory map buffer allocated by
>     efi_get_memory_map(), avoid the allocation in the first place.
>
>   - Changed the title of the patch because the v1 title no longer
>     applies. v1 ref:
>     https://lore.kernel.org/lkml/20200505190016.4350-1-lszubowi@xxxxxxxxxx/
> --
> ---
>  drivers/firmware/efi/libstub/efistub.h  | 13 +++++++++++++
>  drivers/firmware/efi/libstub/mem.c      |  2 --
>  drivers/firmware/efi/libstub/x86-stub.c | 22 ++++++++--------------
>  3 files changed, 21 insertions(+), 16 deletions(-)
>
> diff --git a/drivers/firmware/efi/libstub/efistub.h b/drivers/firmware/efi/libstub/efistub.h
> index 67d26949fd26..62943992f02f 100644
> --- a/drivers/firmware/efi/libstub/efistub.h
> +++ b/drivers/firmware/efi/libstub/efistub.h
> @@ -92,6 +92,19 @@ extern __pure efi_system_table_t  *efi_system_table(void);
>  #define EFI_LOCATE_BY_REGISTER_NOTIFY          1
>  #define EFI_LOCATE_BY_PROTOCOL                 2
>
> +/*
> + * An efi_boot_memmap is used by efi_get_memory_map() to return the
> + * EFI memory map in a dynamically allocated buffer.
> + *
> + * The buffer allocated for the EFI memory map includes extra room for
> + * a minimum of EFI_MMAP_NR_SLACK_SLOTS additional EFI memory descriptors.
> + * This facilitates the reuse of the EFI memory map buffer when a second
> + * call to ExitBootServices() is needed because of intervening changes to
> + * the EFI memory map. Other related structures, e.g. x86 e820ext, need
> + * to factor in this headroom requirement as well.
> + */
> +#define EFI_MMAP_NR_SLACK_SLOTS        8
> +
>  struct efi_boot_memmap {
>         efi_memory_desc_t       **map;
>         unsigned long           *map_size;
> diff --git a/drivers/firmware/efi/libstub/mem.c b/drivers/firmware/efi/libstub/mem.c
> index 869a79c8946f..09f4fa01914e 100644
> --- a/drivers/firmware/efi/libstub/mem.c
> +++ b/drivers/firmware/efi/libstub/mem.c
> @@ -5,8 +5,6 @@
>
>  #include "efistub.h"
>
> -#define EFI_MMAP_NR_SLACK_SLOTS        8
> -
>  static inline bool mmap_has_headroom(unsigned long buff_size,
>                                      unsigned long map_size,
>                                      unsigned long desc_size)
> diff --git a/drivers/firmware/efi/libstub/x86-stub.c b/drivers/firmware/efi/libstub/x86-stub.c
> index 05ccb229fb45..a0aeb1cda8e3 100644
> --- a/drivers/firmware/efi/libstub/x86-stub.c
> +++ b/drivers/firmware/efi/libstub/x86-stub.c
> @@ -606,24 +606,18 @@ static efi_status_t allocate_e820(struct boot_params *params,
>                                   struct setup_data **e820ext,
>                                   u32 *e820ext_size)
>  {
> -       unsigned long map_size, desc_size, buff_size;
> -       struct efi_boot_memmap boot_map;
> -       efi_memory_desc_t *map;
> +       unsigned long map_size, desc_size;
>         efi_status_t status;
>         __u32 nr_desc;
>
> -       boot_map.map            = &map;
> -       boot_map.map_size       = &map_size;
> -       boot_map.desc_size      = &desc_size;
> -       boot_map.desc_ver       = NULL;
> -       boot_map.key_ptr        = NULL;
> -       boot_map.buff_size      = &buff_size;
> +       /* Only need the size of the mem map and size of each mem descriptor */
> +       map_size = 0;
> +       status = efi_bs_call(get_memory_map, &map_size, NULL, NULL,
> +                            &desc_size, NULL);
> +       if (status != EFI_BUFFER_TOO_SMALL)
> +               return (status != EFI_SUCCESS) ? status : EFI_UNSUPPORTED;
>
> -       status = efi_get_memory_map(&boot_map);
> -       if (status != EFI_SUCCESS)
> -               return status;
> -
> -       nr_desc = buff_size / desc_size;
> +       nr_desc = map_size / desc_size + EFI_MMAP_NR_SLACK_SLOTS;
>
>         if (nr_desc > ARRAY_SIZE(params->e820_table)) {
>                 u32 nr_e820ext = nr_desc - ARRAY_SIZE(params->e820_table);
> --
> 2.18.4
>