diff --git a/redox-rt/src/sys.rs b/redox-rt/src/sys.rs
index 25a0c41e90..b099181b99 100644
--- a/redox-rt/src/sys.rs
+++ b/redox-rt/src/sys.rs
@@ -4,6 +4,25 @@ use core::{
     sync::atomic::{AtomicU32, Ordering},
 };
 
+pub(crate) fn format_errno(errno: usize) -> [u8; 6] {
+    let mut buf = [0u8; 6];
+    let mut n = errno;
+    let mut idx = 5;
+    if n == 0 {
+        buf[5] = b'0';
+        return buf;
+    }
+    while n > 0 && idx > 0 {
+        buf[idx] = b'0' + (n % 10) as u8;
+        n /= 10;
+        idx -= 1;
+    }
+    let start = idx + 1;
+    let mut result = [0u8; 6];
+    result[..6 - start].copy_from_slice(&buf[start..]);
+    result
+}
+
 use ioslice::IoSlice;
 use syscall::{
     Call, CallFlags, EINVAL, ERESTART, StdFsCallKind, TimeSpec,
@@ -598,8 +617,27 @@ pub fn open<T: AsRef<str>>(path: T, flags: usize) -> Result<usize> {
         flags,
         0,
     );
-    if posix_fd.is_err() {
-        let _ = syscall::write(2, b"RB_OPEN_DIAG open failed\n");
+    if let Err(ref e) = posix_fd {
+        let p = path.as_ref();
+        let mut buf = [0u8; 256];
+        let prefix = b"RB_OPEN_DIAG err=";
+        let mut len = 0;
+        buf[..prefix.len()].copy_from_slice(prefix);
+        len += prefix.len();
+        let errno_bytes = crate::format_errno(e.errno);
+        buf[len..len + errno_bytes.len()].copy_from_slice(&errno_bytes);
+        len += errno_bytes.len();
+        buf[len] = b' ';
+        len += 1;
+        let pfx2 = b"path=";
+        buf[len..len + pfx2.len()].copy_from_slice(pfx2);
+        len += pfx2.len();
+        let copy = (p.len()).min(buf.len() - len - 1);
+        buf[len..len + copy].copy_from_slice(&p.as_bytes()[..copy]);
+        len += copy;
+        buf[len] = b'\n';
+        len += 1;
+        let _ = syscall::write(2, &buf[..len]);
     }
     let posix_fd = posix_fd?;
     register_external_fd(posix_fd)?;