[bzimage] Align initrd images to page boundary

Some versions of Linux apparently complain if initrds are not aligned
to a page boundary.  Fix by changing INITRD_ALIGN from 4 bytes to 4096
bytes.

The amount of padding at the end of each initrd will now often be
sufficient to allow the cpio header to be prepended without crossing
an alignment boundary.  The final location of the initrd may therefore
end up being slightly higher than the post-shuffle location.
bzimage_load_initrd() must therefore now copy the initrd body prior to
copying the cpio header, otherwise the start of the initrd body may be
overwritten by the cpio header.  (Note that the guarantee that an
initrd will never need to overwrite an initrd at a higher location
still holds, since the overall length of each initrd cannot decrease
as a result of adding a cpio header.)

Reported-by: Dave Hansen <dave@sr71.net>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/arch/i386/image/bzimage.c

 	char *filename = initrd->cmdline;
 	char *cmdline;
 	struct cpio_header cpio;
-        size_t offset = 0;
+	size_t offset;
 	size_t name_len;
 	size_t pad_len;
 
 	if ( initrd == image )
 		return 0;
 
-	/* Create cpio header before non-prebuilt images */
+	/* Create cpio header for non-prebuilt images */
 	if ( filename && filename[0] ) {
 		cmdline = strchr ( filename, ' ' );
 		name_len = ( ( cmdline ? ( ( size_t ) ( cmdline - filename ) )
 			bzimage_parse_cpio_cmdline ( image, &cpio,
 						     ( cmdline + 1 /* ' ' */ ));
 		}
-		if ( address ) {
-			copy_to_user ( address, offset, &cpio,
-				       sizeof ( cpio ) );
-		}
-		offset += sizeof ( cpio );
-		if ( address ) {
-			memset_user ( address, offset, 0, name_len );
-			copy_to_user ( address, offset, filename,
-				       ( name_len - 1 /* NUL (or space) */ ) );
-		}
-		offset += name_len;
-		offset = ( ( offset + 0x03 ) & ~0x03 );
+		offset = ( ( sizeof ( cpio ) + name_len + 0x03 ) & ~0x03 );
+	} else {
+		offset = 0;
 	}
 
-	/* Copy in initrd image body */
-	if ( address )
-		memmove_user ( address, offset, initrd->data, 0, initrd->len );
+	/* Copy in initrd image body (and cpio header if applicable) */
 	if ( address ) {
+		memmove_user ( address, offset, initrd->data, 0, initrd->len );
+		if ( offset ) {
+			memset_user ( address, 0, 0, offset );
+			copy_to_user ( address, 0, &cpio, sizeof ( cpio ) );
+			copy_to_user ( address, sizeof ( cpio ), filename,
+				       ( name_len - 1 /* NUL (or space) */ ) );
+		}
 		DBGC ( image, "bzImage %p initrd %p [%#08lx,%#08lx,%#08lx)"
 		       "%s%s\n", image, initrd, user_to_phys ( address, 0 ),
 		       user_to_phys ( address, offset ),

src/arch/i386/include/initrd.h

 
 /** Minimum alignment for initrds
  *
- * Chosen to maximise memcpy() speeds
+ * Some versions of Linux complain about initrds that are not
+ * page-aligned.
  */
-#define INITRD_ALIGN 4
+#define INITRD_ALIGN 4096
 
 /** Minimum free space required to reshuffle initrds
  *