efi/libstub: Call get_memory_map() to obtain map and desc sizes

This fixes two minor issues in the implementation of get_memory_map():
- Currently, it assumes that sizeof(efi_memory_desc_t) == desc_size,
  which is usually true, but not mandated by the spec. (This was added
  intentionally to allow future additions to the definition of
  efi_memory_desc_t). The way the loop is implemented currently, the
  added slack space may be insufficient if desc_size is larger, which in
  some corner cases could result in the loop never terminating.
- It allocates 32 efi_memory_desc_t entries first (again, using the size
  of the struct instead of desc_size), and frees and reallocates if it
  turns out to be insufficient. Few implementations of UEFI have such small
  memory maps, which results in a unnecessary allocate/free pair on each
  invocation.

Fix this by calling the get_memory_map() boot service first with a '0'
input value for map size to retrieve the map size and desc size from the
firmware and only then perform the allocation, using desc_size rather
than sizeof(efi_memory_desc_t).

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Matt Fleming <matt.fleming@intel.com>

1 files changed, 10 insertions, 6 deletions

diff --git a/drivers/firmware/efi/libstub/efi-stub-helper.c b/drivers/firmware/efi/libstub/efi-stub-helper.c
index 9bd9fbb5bea8..d073e3946383 100644
--- a/drivers/firmware/efi/libstub/efi-stub-helper.c
+++ b/drivers/firmware/efi/libstub/efi-stub-helper.c
@@ -66,25 +66,29 @@ efi_status_t efi_get_memory_map(efi_system_table_t *sys_table_arg,
         unsigned long key;
         u32 desc_version;
 
-        *map_size = sizeof(*m) * 32;
-again:
+        *map_size = 0;
+        *desc_size = 0;
+        key = 0;
+        status = efi_call_early(get_memory_map, map_size, NULL,
+                                &key, desc_size, &desc_version);
+        if (status != EFI_BUFFER_TOO_SMALL)
+                return EFI_LOAD_ERROR;
+
         /*
          * Add an additional efi_memory_desc_t because we're doing an
          * allocation which may be in a new descriptor region.
          */
-        *map_size += sizeof(*m);
+        *map_size += *desc_size;
         status = efi_call_early(allocate_pool, EFI_LOADER_DATA,
                                 *map_size, (void **)&m);
         if (status != EFI_SUCCESS)
                 goto fail;
 
-        *desc_size = 0;
-        key = 0;
         status = efi_call_early(get_memory_map, map_size, m,
                                 &key, desc_size, &desc_version);
         if (status == EFI_BUFFER_TOO_SMALL) {
                 efi_call_early(free_pool, m);
-                goto again;
+                return EFI_LOAD_ERROR;
         }
 
         if (status != EFI_SUCCESS)