BaseTools/GenSec: Use SECTION_SIZE to read section size

Replace the open-coded read of the 3-byte section Size[] field with
the SECTION_SIZE() macro from Pi/PiFirmwareFile.h. The macro reads
exactly the three Size[] bytes (endianness-safe) instead of a 4-byte
load that over-reads the adjacent byte. No functional change.

Verified by building BaseTools and running GenSec:

  python3 BaseTools/Edk2ToolsBuild.py -t GCC
  GenSec -s EFI_SECTION_RAW -o out.sec in.bin   # in.bin: 100 bytes

out.sec is 104 bytes: the 100-byte payload plus the 4-byte
EFI_COMMON_SECTION_HEADER. The Size field of the header reads
0x000068, matching out.sec's size that is calculated by SECTION_SIZE.

Generated with the following Coccinelle semantic patch:

```smpl
@@
EFI_COMMON_SECTION_HEADER *E;
typedef UINT32;
@@
(
- *(UINT32 *)(E->Size) & 0x00FFFFFF
+ SECTION_SIZE (E)
|
- *(UINT32 *)E->Size & 0x00FFFFFF
+ SECTION_SIZE (E)
)
```

Signed-off-by: Mingjie Shen <shen497@purdue.edu>

1 files changed, 1 insertions, 1 deletions

diff --git a/BaseTools/Source/C/GenSec/GenSec.c b/BaseTools/Source/C/GenSec/GenSec.c
index f675338a46..25304d84a3 100644
--- a/BaseTools/Source/C/GenSec/GenSec.c
+++ b/BaseTools/Source/C/GenSec/GenSec.c
@@ -1936,7 +1936,7 @@ Returns:
   //

   if (SectType != EFI_SECTION_ALL) {

     SectionHeader = (EFI_COMMON_SECTION_HEADER *)OutFileBuffer;

-    InputLength = *(UINT32 *)SectionHeader->Size & 0x00ffffff;

+    InputLength = SECTION_SIZE (SectionHeader);

     if (InputLength == 0xffffff) {

       InputLength = ((EFI_COMMON_SECTION_HEADER2 *)SectionHeader)->ExtendedSize;

     }