[CHERRY-PICK] SecurityPkg/RngDxe: Simplify Rng algorithm selection fo…

…r Arm

The first element of mAvailableAlgoArray is defined as the default
Rng algorithm to use. Don't go through the array at each RngGetRNG()
call and just return the first element of the array.

Signed-off-by: Pierre Gondois <pierre.gondois@arm.com>
Reviewed-by: Sami Mujawar <sami.mujawar@arm.com>
Acked-by: Ard Biesheuvel <ardb@kernel.org>
Acked-by: Jiewen Yao <Jiewen.yao@intel.com>
Tested-by: Kun Qin <kun.qin@microsoft.com>
(cherry picked from commit ff7ddc0)

SecurityPkg/RandomNumberGenerator/RngDxe/ArmRngDxe.c

@@ -77,7 +77,6 @@ RngGetRNG (
   )
 {
   EFI_STATUS  Status;
-  UINTN       Index;
   GUID        RngGuid;
 
   if ((This == NULL) || (RNGValueLength == 0) || (RNGValue == NULL)) {
@@ -88,21 +87,13 @@ RngGetRNG (
     //
     // Use the default RNG algorithm if RNGAlgorithm is NULL.
     //
-    for (Index = 0; Index < mAvailableAlgoArrayCount; Index++) {
-      if (!IsZeroGuid (&mAvailableAlgoArray[Index])) {
-        RNGAlgorithm = &mAvailableAlgoArray[Index];
-        goto FoundAlgo;
-      }
-    }
-
-    if (Index == mAvailableAlgoArrayCount) {
-      // No algorithm available.
-      ASSERT (Index != mAvailableAlgoArrayCount);
-      return EFI_DEVICE_ERROR;
+    if (mAvailableAlgoArrayCount != 0) {
+      RNGAlgorithm = &mAvailableAlgoArray[0];
+    } else {
+      return EFI_UNSUPPORTED;
     }
   }
 
-FoundAlgo:
   Status = GetRngGuid (&RngGuid);
   if (!EFI_ERROR (Status) &&
       CompareGuid (RNGAlgorithm, &RngGuid))