Re: sizeof(struct ...)

["René Scharfe" <rene.scharfe@lsrfire.ath.cx>]

Gerrit Pape schrieb:
> Hi, I don't think we can rely on sizeof(struct ...) to be the exact size
> of the struct as defined.  As the selftests show, archive-zip doesn't
> work correctly on Debian/arm
> 
>  http://buildd.debian.org/fetch.cgi?&pkg=git-core&ver=1%3A1.4.4-1&arch=arm&stamp=1164122355&file=log
> 
> It's because sizeof(struct zip_local_header) is 32, zip_dir_header 48,
> and zip_dir_trailer 24, breaking the zip files.  Compiling with
> -fpack-struct seemed to break other things, so I for now I ended up with
> this (not so nice) workaround.

Hm, yes, this use sizeof() is not strictly correct.  But I'd very much
like to keep being lazy and let the compiler to do the summing.  How
about this patch instead?  Does it work for you, Gerrit?

Thanks,
René


 archive-zip.c |   24 ++++++++++++++++++------
 1 files changed, 18 insertions(+), 6 deletions(-)

diff --git a/archive-zip.c b/archive-zip.c
index ae5572a..4caaec4 100644
--- a/archive-zip.c
+++ b/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[0];
 };
 
 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[0];
 };
 
 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[0];
 };
 
+/*
+ * 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 unsigne
 	}
 
 	/* 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 unsigne
 	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 unsigne
 	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 unsi
 	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);