diag(kernel): mirror userspace writes starting with PROCMGR to log

src/syscall/fs.rs

@@ -857,6 +857,26 @@ pub fn sys_read(fd: FileHandle, buf: UserSliceWo, token: &mut CleanLockToken) ->
     Ok(bytes_read)
 }
 pub fn sys_write(fd: FileHandle, buf: UserSliceRo, token: &mut CleanLockToken) -> Result<usize> {
+    // Red Bear diagnostic (2026-07-15): procmgr (userspace process manager,
+    // scheme 18) has no serial-connected stdout, so its "PROCMGR ..."
+    // diagnostics written to fd 1 never reach the log. Mirror any userspace
+    // write whose payload begins with "PROCMGR" to the kernel log. This runs
+    // before fd validation, so it works even if procmgr's fd 1 is not a real
+    // sink. Cheap: only 7 bytes are peeked unless the marker matches.
+    if buf.len() >= 7 {
+        let mut marker = [0u8; 7];
+        if buf.copy_common_bytes_to_slice(&mut marker).is_ok() && &marker == b"PROCMGR" {
+            let mut line = [0u8; 256];
+            if let Ok(n) = buf.copy_common_bytes_to_slice(&mut line) {
+                error!(
+                    "{}",
+                    core::str::from_utf8(&line[..n])
+                        .unwrap_or("<procmgr non-utf8>")
+                        .trim_end_matches('\n')
+                );
+            }
+        }
+    }
     let result = (|| {
     let (bytes_written, desc_arc, desc) =
         file_op_generic_ext(fd, token, |scheme, desc_arc, desc, token| {