win32: code shrink APE detection; avoid UB

Detecting Actually Portable Executable binaries used a longer
signature than seems necessary.  Six characters should be enough
for anyone.

When right shifting a byte by 24 bits, cast it to unsigned to avoid
undefined behaviour.

Saves 24-32 bytes.

(GitHub issue #424)

1 files changed, 7 insertions, 3 deletions

diff --git a/win32/mingw.c b/win32/mingw.c
index 26b046f1a..98254fdbe 100644
--- a/win32/mingw.c
+++ b/win32/mingw.c
@@ -473,16 +473,20 @@ static int has_exec_format(const char *name)
          * the magic from the file command.
          */
         if (buf[0] == 'M' && buf[1] == 'Z') {
+/* Convert four unsigned bytes to an unsigned int (little-endian) */
+#define LE4(b, o) (((unsigned)b[o+3] << 24) + (b[o+2] << 16) + \
+                                                (b[o+1] << 8) + b[o])
+
                 /* Actually Portable Executable */
                 /* See ape/ape.S at https://github.com/jart/cosmopolitan */
-                if (n > 9 && memcmp(buf + 2, "qFpD='\n", 7) == 0)
+                const unsigned char *qFpD = (unsigned char *)"qFpD";
+                if (n > 6 && LE4(buf, 2) == LE4(qFpD, 0))
                         return 1;
 
                 if (n > 0x3f) {
                         offset = (buf[0x19] << 8) + buf[0x18];
                         if (offset > 0x3f) {
-                                offset = (buf[0x3f] << 24) + (buf[0x3e] << 16) +
-                                                        (buf[0x3d] << 8) + buf[0x3c];
+                                offset = LE4(buf, 0x3c);
                                 if (offset < sizeof(buf)-100) {
                                         if (memcmp(buf+offset, "PE\0\0", 4) == 0) {
                                                 sig = (buf[offset+25] << 8) + buf[offset+24];