diff --git a/expected/wasm32-wasi/defined-symbols.txt b/expected/wasm32-wasi/defined-symbols.txt
index 2b6a0af..d50f745 100644
--- a/expected/wasm32-wasi/defined-symbols.txt
+++ b/expected/wasm32-wasi/defined-symbols.txt
@@ -251,6 +251,8 @@ __uselocale
 __utc
 __wasilibc_fd_renumber
 __wasilibc_find_relpath
+__wasilibc_open_nomode
+__wasilibc_openat_nomode
 __wasilibc_populate_environ
 __wasilibc_register_preopened_fd
 __wasilibc_rmdirat
diff --git a/libc-bottom-half/cloudlibc/src/libc/dirent/opendirat.c b/libc-bottom-half/cloudlibc/src/libc/dirent/opendirat.c
index ecda800..c940d5f 100644
--- a/libc-bottom-half/cloudlibc/src/libc/dirent/opendirat.c
+++ b/libc-bottom-half/cloudlibc/src/libc/dirent/opendirat.c
@@ -2,6 +2,7 @@
 //
 // SPDX-License-Identifier: BSD-2-Clause
 
+#include <wasi/libc.h>
 #include <dirent.h>
 #include <fcntl.h>
 #include <stddef.h>
@@ -9,7 +10,11 @@
 
 DIR *opendirat(int dir, const char *dirname) {
   // Open directory.
+#ifdef __wasilibc_unmodified_upstream // avoid making a varargs call
   int fd = openat(dir, dirname, O_RDONLY | O_NONBLOCK | O_DIRECTORY);
+#else
+  int fd = __wasilibc_openat_nomode(dir, dirname, O_RDONLY | O_NONBLOCK | O_DIRECTORY);
+#endif
   if (fd == -1)
     return NULL;
 
diff --git a/libc-bottom-half/cloudlibc/src/libc/dirent/scandirat.c b/libc-bottom-half/cloudlibc/src/libc/dirent/scandirat.c
index 324c590..345234f 100644
--- a/libc-bottom-half/cloudlibc/src/libc/dirent/scandirat.c
+++ b/libc-bottom-half/cloudlibc/src/libc/dirent/scandirat.c
@@ -3,6 +3,7 @@
 // SPDX-License-Identifier: BSD-2-Clause
 
 #include <wasi/core.h>
+#include <wasi/libc.h>
 #include <dirent.h>
 #include <errno.h>
 #include <fcntl.h>
@@ -24,7 +25,11 @@ int scandirat(int dirfd, const char *dir, struct dirent ***namelist,
     sel = sel_true;
 
   // Open the directory.
+#ifdef __wasilibc_unmodified_upstream // avoid making a varargs call
   int fd = openat(dirfd, dir, O_RDONLY | O_NONBLOCK | O_DIRECTORY);
+#else
+  int fd = __wasilibc_openat_nomode(dirfd, dir, O_RDONLY | O_NONBLOCK | O_DIRECTORY);
+#endif
   if (fd == -1)
     return -1;
 
diff --git a/libc-bottom-half/cloudlibc/src/libc/fcntl/openat.c b/libc-bottom-half/cloudlibc/src/libc/fcntl/openat.c
index 790434d..0b62500 100644
--- a/libc-bottom-half/cloudlibc/src/libc/fcntl/openat.c
+++ b/libc-bottom-half/cloudlibc/src/libc/fcntl/openat.c
@@ -6,6 +6,7 @@
 
 #include <assert.h>
 #include <wasi/core.h>
+#include <wasi/libc.h>
 #include <errno.h>
 #include <fcntl.h>
 #include <string.h>
@@ -22,6 +23,13 @@ static_assert(O_EXCL >> 12 == __WASI_O_EXCL, "Value mismatch");
 static_assert(O_TRUNC >> 12 == __WASI_O_TRUNC, "Value mismatch");
 
 int openat(int fd, const char *path, int oflag, ...) {
+#ifdef __wasilibc_unmodified_upstream // fstat
+#else
+    return __wasilibc_openat_nomode(fd, path, oflag);
+}
+
+int __wasilibc_openat_nomode(int fd, const char *path, int oflag) {
+#endif
   // Compute rights corresponding with the access modes provided.
   // Attempt to obtain all rights, except the ones that contradict the
   // access mode provided to openat().
diff --git a/libc-bottom-half/headers/public/wasi/libc.h b/libc-bottom-half/headers/public/wasi/libc.h
index 643cc68..4bbc1dc 100644
--- a/libc-bottom-half/headers/public/wasi/libc.h
+++ b/libc-bottom-half/headers/public/wasi/libc.h
@@ -11,6 +11,8 @@ int __wasilibc_register_preopened_fd(int fd, const char *path);
 int __wasilibc_fd_renumber(int fd, int newfd);
 int __wasilibc_unlinkat(int fd, const char *path);
 int __wasilibc_rmdirat(int fd, const char *path);
+int __wasilibc_open_nomode(const char *path, int oflag);
+int __wasilibc_openat_nomode(int fd, const char *path, int oflag);
 off_t __wasilibc_tell(int fd);
 
 #ifdef __cplusplus
diff --git a/libc-bottom-half/libpreopen/libpreopen.c b/libc-bottom-half/libpreopen/libpreopen.c
index 2579110..8a17a42 100644
--- a/libc-bottom-half/libpreopen/libpreopen.c
+++ b/libc-bottom-half/libpreopen/libpreopen.c
@@ -65,6 +65,14 @@
 
 int
 open(const char *path, int flags, ...)
+{
+    // WASI libc's openat ignores the mode argument, so call a special
+    // entrypoint which avoids the varargs calling convention.
+    return __wasilibc_open_nomode(path, flags);
+}
+
+int
+__wasilibc_open_nomode(const char *path, int flags)
 {
     const char *relative_path;
     int dirfd = __wasilibc_find_relpath(path, __WASI_RIGHT_PATH_OPEN, 0,
@@ -77,9 +85,7 @@ open(const char *path, int flags, ...)
         return -1;
     }
 
-    // WASI libc's openat ignores the mode argument, so don't bother passing
-    // the actual mode value through.
-    return openat(dirfd, relative_path, flags);
+    return __wasilibc_openat_nomode(dirfd, relative_path, flags);
 }
 
 int
diff --git a/libc-bottom-half/sources/truncate.c b/libc-bottom-half/sources/truncate.c
index b56123d..24ae28f 100644
--- a/libc-bottom-half/sources/truncate.c
+++ b/libc-bottom-half/sources/truncate.c
@@ -1,23 +1,21 @@
 #include <unistd.h>
 #include <fcntl.h>
 #include <errno.h>
+#include <wasi/libc.h>
 
 int truncate(const char *path, off_t length)
 {
-    int fd = open(path, O_WRONLY | O_CLOEXEC | O_NOCTTY);
+    int fd = __wasilibc_open_nomode(path, O_WRONLY | O_CLOEXEC | O_NOCTTY);
     if (fd < 0)
         return -1;
 
     int result = ftruncate(fd, length);
-    if (result < 0) {
+    if (result != 0) {
         int save_errno = errno;
         (void)close(fd);
         errno = save_errno;
         return -1;
     }
 
-    if (close(fd) < 0)
-        return -1;
-
-    return 0;
+    return close(fd);
 }
diff --git a/libc-top-half/musl/src/stdio/fopen.c b/libc-top-half/musl/src/stdio/fopen.c
index 07f035d..670f438 100644
--- a/libc-top-half/musl/src/stdio/fopen.c
+++ b/libc-top-half/musl/src/stdio/fopen.c
@@ -1,6 +1,7 @@
 #ifdef __wasilibc_unmodified_upstream // WASI has no syscall
 #else
 #include <unistd.h>
+#include <wasi/libc.h>
 #endif
 #include "stdio_impl.h"
 #include <fcntl.h>
@@ -25,7 +26,8 @@ FILE *fopen(const char *restrict filename, const char *restrict mode)
 #ifdef __wasilibc_unmodified_upstream // WASI has no sys_open
 	fd = sys_open(filename, flags, 0666);
 #else
-	fd = open(filename, flags, 0666);
+	// WASI libc ignores the mode parameter anyway, so skip the varargs.
+	fd = __wasilibc_open_nomode(filename, flags);
 #endif
 	if (fd < 0) return 0;
 #ifdef __wasilibc_unmodified_upstream // WASI has no syscall