diff --git a/libc/calls/readv-metal.c b/libc/calls/readv-metal.c
index d7476f9bb..3e283a1f3 100644
--- a/libc/calls/readv-metal.c
+++ b/libc/calls/readv-metal.c
@@ -37,8 +37,8 @@ ssize_t sys_readv_metal(struct Fd *fd, const struct iovec *iov, int iovlen) {
        * escape sequences, in response to requests sent to it via write().
        * Read & return these if they are available.
        */
-      if (_weaken(sys_readv_vga)) {
-        ssize_t res = _weaken(sys_readv_vga)(fd, iov, iovlen);
+      if (weaken(sys_readv_vga)) {
+        ssize_t res = weaken(sys_readv_vga)(fd, iov, iovlen);
         if (res > 0) return res;
       }
       /* fall through */
diff --git a/libc/sock/sendfile.c b/libc/sock/sendfile.c
index 2a1350a33..4908c2f23 100644
--- a/libc/sock/sendfile.c
+++ b/libc/sock/sendfile.c
@@ -23,6 +23,7 @@
 #include "libc/calls/syscall-sysv.internal.h"
 #include "libc/calls/syscall_support-nt.internal.h"
 #include "libc/dce.h"
+#include "libc/intrin/asan.internal.h"
 #include "libc/intrin/safemacros.internal.h"
 #include "libc/nt/enum/wait.h"
 #include "libc/nt/errors.h"
@@ -106,10 +107,13 @@ static ssize_t sendfile_linux2bsd(int outfd, int infd,
   if (IsFreebsd()) {
     rc = sys_sendfile_freebsd(infd, outfd, offset, uptobytes, 0, &sbytes, 0);
   } else {
+    sbytes = uptobytes;
     rc = sys_sendfile_xnu(infd, outfd, offset, &sbytes, 0, 0);
   }
   if (rc != -1) {
-    if (inout_opt_inoffset) *inout_opt_inoffset += sbytes;
+    if (inout_opt_inoffset) {
+      *inout_opt_inoffset += sbytes;
+    }
     return sbytes;
   } else {
     return -1;
@@ -131,15 +135,25 @@ static ssize_t sendfile_linux2bsd(int outfd, int infd,
  */
 ssize_t sendfile(int outfd, int infd, int64_t *inout_opt_inoffset,
                  size_t uptobytes) {
-  if (!uptobytes) return einval();
-  if (uptobytes > 0x7ffffffe /* Microsoft's off-by-one */) return eoverflow();
-  if (IsLinux()) {
-    return sys_sendfile(outfd, infd, inout_opt_inoffset, uptobytes);
+  int rc;
+  if (!uptobytes) {
+    rc = einval();
+  } else if (IsAsan() && inout_opt_inoffset &&
+             !__asan_is_valid(inout_opt_inoffset,
+                              sizeof(*inout_opt_inoffset))) {
+    rc = efault();
+  } else if (uptobytes > 0x7ffffffe /* Microsoft's off-by-one */) {
+    rc = eoverflow();
+  } else if (IsLinux()) {
+    rc = sys_sendfile(outfd, infd, inout_opt_inoffset, uptobytes);
   } else if (IsFreebsd() || IsXnu()) {
-    return sendfile_linux2bsd(outfd, infd, inout_opt_inoffset, uptobytes);
+    rc = sendfile_linux2bsd(outfd, infd, inout_opt_inoffset, uptobytes);
   } else if (IsWindows()) {
-    return sendfile_linux2nt(outfd, infd, inout_opt_inoffset, uptobytes);
+    rc = sendfile_linux2nt(outfd, infd, inout_opt_inoffset, uptobytes);
   } else {
-    return copyfd(infd, inout_opt_inoffset, outfd, NULL, uptobytes, 0);
+    rc = copyfd(infd, inout_opt_inoffset, outfd, NULL, uptobytes, 0);
   }
+  STRACE("sendfile(%d, %d, %p, %'zu) → %ld% m", outfd, infd, inout_opt_inoffset,
+         uptobytes, rc);
+  return rc;
 }
diff --git a/tool/emacs/cosmo-stuff.el b/tool/emacs/cosmo-stuff.el
index a10d4b269..d7241f477 100644
--- a/tool/emacs/cosmo-stuff.el
+++ b/tool/emacs/cosmo-stuff.el
@@ -637,7 +637,7 @@
       (let ((default-directory root))
         (save-buffer)
         (cond ((memq major-mode '(c-mode c++-mode asm-mode fortran-mode))
-               (let* ((mode (cosmo--make-mode arg ""))
+               (let* ((mode (cosmo--make-mode arg "fastbuild"))
                       (compile-command (cosmo--compile-command this root 'test mode "" "" ".ok")))
                  (compile compile-command)))
               ('t
@@ -653,7 +653,7 @@
       (let ((default-directory root))
         (save-buffer)
         (cond ((memq major-mode '(c-mode c++-mode asm-mode fortran-mode))
-               (let* ((mode (cosmo--make-mode arg ""))
+               (let* ((mode (cosmo--make-mode arg "fastbuild"))
                       (compile-command (cosmo--compile-command this root 'run-win10 mode "" "" "")))
                  (compile compile-command)))
               ('t