efi/libstub/x86: Avoid EFI map buffer alloc in allocate_e820()

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@redhat.com>
Suggested-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>

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;

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)

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, map_key;
 	efi_status_t status;
-	__u32 nr_desc;
+	__u32 nr_desc, desc_version;
 
-	boot_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, &map_key,
+			     &desc_size, &desc_version);
+	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);