diff --git a/src/libsec/386/md5block.s b/src/libsec/386/md5block.spp
similarity index 100%
rename from src/libsec/386/md5block.s
rename to src/libsec/386/md5block.spp
diff --git a/src/libsec/386/mkfile b/src/libsec/386/mkfile
index b3d560ad..d0c7cdb4 100644
--- a/src/libsec/386/mkfile
+++ b/src/libsec/386/mkfile
@@ -14,3 +14,8 @@ UPDATE=mkfile\
 	$SFILES\
 
 <$PLAN9/src/mksyslib
+
+%.s: %.spp
+	cpp $stem.spp >$stem.s
+
+CLEANFILES=md5block.s sha1block.s
diff --git a/src/libsec/386/sha1block.s b/src/libsec/386/sha1block.spp
similarity index 100%
rename from src/libsec/386/sha1block.s
rename to src/libsec/386/sha1block.spp