archive-zip: don't use sizeof(struct ...)

We can't rely on sizeof(struct zip_*) returning the sum of
all struct members.  At least on ARM padding is added at the
end, as Gerrit Pape reported.  This fixes the problem but
still lets the compiler do the summing by introducing
explicit padding at the end of the structs and then taking
its offset as the combined size of the preceding members.

As Junio correctly notes, the _end[] marker array's size
must be greater than zero for compatibility with compilers
other than gcc.  The space wasted by the markers can safely
be neglected because we only have one instance of each
struct, i.e. in sum 3 wasted bytes on i386, and 0 on ARM. :)

We still rely on the compiler to not add padding between the
struct members, but that's reasonable given that all of them
are unsigned char arrays.

Signed-off-by: Rene Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: Junio C Hamano <junkio@cox.net>

archive-zip.c

@@ -35,6 +35,7 @@ struct zip_local_header {
 	unsigned char size[4];
 	unsigned char filename_length[2];
 	unsigned char extra_length[2];
+	unsigned char _end[1];
 };
 
 struct zip_dir_header {
@@ -55,6 +56,7 @@ struct zip_dir_header {
 	unsigned char attr1[2];
 	unsigned char attr2[4];
 	unsigned char offset[4];
+	unsigned char _end[1];
 };
 
 struct zip_dir_trailer {
@@ -66,8 +68,18 @@ struct zip_dir_trailer {
 	unsigned char size[4];
 	unsigned char offset[4];
 	unsigned char comment_length[2];
+	unsigned char _end[1];
 };
 
+/*
+ * On ARM, padding is added at the end of the struct, so a simple
+ * sizeof(struct ...) reports two bytes more than the payload size
+ * we're interested in.
+ */
+#define ZIP_LOCAL_HEADER_SIZE	offsetof(struct zip_local_header, _end)
+#define ZIP_DIR_HEADER_SIZE	offsetof(struct zip_dir_header, _end)
+#define ZIP_DIR_TRAILER_SIZE	offsetof(struct zip_dir_trailer, _end)
+
 static void copy_le16(unsigned char *dest, unsigned int n)
 {
 	dest[0] = 0xff & n;
@@ -211,7 +223,7 @@ static int write_zip_entry(const unsigned char *sha1,
 	}
 
 	/* make sure we have enough free space in the dictionary */
-	direntsize = sizeof(struct zip_dir_header) + pathlen;
+	direntsize = ZIP_DIR_HEADER_SIZE + pathlen;
 	while (zip_dir_size < zip_dir_offset + direntsize) {
 		zip_dir_size += ZIP_DIRECTORY_MIN_SIZE;
 		zip_dir = xrealloc(zip_dir, zip_dir_size);
@@ -234,8 +246,8 @@ static int write_zip_entry(const unsigned char *sha1,
 	copy_le16(dirent.attr1, 0);
 	copy_le32(dirent.attr2, attr2);
 	copy_le32(dirent.offset, zip_offset);
-	memcpy(zip_dir + zip_dir_offset, &dirent, sizeof(struct zip_dir_header));
-	zip_dir_offset += sizeof(struct zip_dir_header);
+	memcpy(zip_dir + zip_dir_offset, &dirent, ZIP_DIR_HEADER_SIZE);
+	zip_dir_offset += ZIP_DIR_HEADER_SIZE;
 	memcpy(zip_dir + zip_dir_offset, path, pathlen);
 	zip_dir_offset += pathlen;
 	zip_dir_entries++;
@@ -251,8 +263,8 @@ static int write_zip_entry(const unsigned char *sha1,
 	copy_le32(header.size, uncompressed_size);
 	copy_le16(header.filename_length, pathlen);
 	copy_le16(header.extra_length, 0);
-	write_or_die(1, &header, sizeof(struct zip_local_header));
-	zip_offset += sizeof(struct zip_local_header);
+	write_or_die(1, &header, ZIP_LOCAL_HEADER_SIZE);
+	zip_offset += ZIP_LOCAL_HEADER_SIZE;
 	write_or_die(1, path, pathlen);
 	zip_offset += pathlen;
 	if (compressed_size > 0) {
@@ -282,7 +294,7 @@ static void write_zip_trailer(const unsigned char *sha1)
 	copy_le16(trailer.comment_length, sha1 ? 40 : 0);
 
 	write_or_die(1, zip_dir, zip_dir_offset);
-	write_or_die(1, &trailer, sizeof(struct zip_dir_trailer));
+	write_or_die(1, &trailer, ZIP_DIR_TRAILER_SIZE);
 	if (sha1)
 		write_or_die(1, sha1_to_hex(sha1), 40);
 }