Re: [PATCH V2] efi/libstub/x86: Avoid EFI map buffer alloc in allocate_e820()

[Ard Biesheuvel]

On Thu, 7 May 2020 at 15:37, 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>
> --
> 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  |  2 ++
>  drivers/firmware/efi/libstub/mem.c      |  5 +++++
>  drivers/firmware/efi/libstub/x86-stub.c | 22 ++++++++--------------
>  3 files changed, 15 insertions(+), 14 deletions(-)
>
> diff --git a/drivers/firmware/efi/libstub/efistub.h b/drivers/firmware/efi/libstub/efistub.h
> index 67d26949fd26..191468b771b6 100644
> --- a/drivers/firmware/efi/libstub/efistub.h
> +++ b/drivers/firmware/efi/libstub/efistub.h
> @@ -616,6 +616,8 @@ void efi_printk(char *str);
>
>  void efi_free(unsigned long size, unsigned long addr);
>
> +unsigned int efi_get_mmap_nr_slack_slots(void);
> +
>  char *efi_convert_cmdline(efi_loaded_image_t *image, int *cmd_line_len,
>                           unsigned long max_addr);
>
> diff --git a/drivers/firmware/efi/libstub/mem.c b/drivers/firmware/efi/libstub/mem.c
> index 869a79c8946f..fc8b4fb08505 100644
> --- a/drivers/firmware/efi/libstub/mem.c
> +++ b/drivers/firmware/efi/libstub/mem.c
> @@ -7,6 +7,11 @@
>
>  #define EFI_MMAP_NR_SLACK_SLOTS        8
>
> +unsigned int efi_get_mmap_nr_slack_slots(void)
> +{
> +       return EFI_MMAP_NR_SLACK_SLOTS;
> +}
> +

Can we just move this #define to efistub.h and use it directly?

>  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..9a5ee0678434 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_get_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
>