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

[Lenny Szubowicz <lszubowi@xxxxxxxxxx>]

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;
+}
+
 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