dev: read /dev/zero and /dev/urandom from them, not from the keyboard

consoleinit() put consoleread/consolewrite in devsw[1], over memread and
memwrite. Major 1 holds the whole mem major (null, zero, full, random,
urandom) and memread already sends the console's minors to the console,
so the override only broke the others: a read of /dev/zero waited for a
typed line. dd and head on either device hung, and `head -c 10 /dev/zero`
only finished because it swallowed the next command.

The console now installs just its open and close hooks. /dev/keyboard
writes are discarded, as consolewrite used to do.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01S1giNvrHTrPMPw7V6x1kQ4

docs/architecture/devices.md

@@ -355,6 +355,14 @@ from the cycle counter on each byte so two readers do not continue one
 sequence. What changed is the property that made it dangerous rather than
 merely poor.
 
+**For a long time it could not be read at all**, and neither could `/dev/zero`.
+Both are minors of major 1, and `devsw[1]` is `memread`/`memwrite`, which hand
+the console's minors to the console and the rest to their own functions. But
+`consoleinit()` installed `consoleread` over the whole major, so a read of
+`/dev/zero` waited for a line from the keyboard. `head -c 10 /dev/zero`
+"worked" by taking the next command typed. The console now installs only its
+`open` and `close` hooks.
+
 ## What it does not do
 
 - **It is a flat list, not a device tree.** There is no parent/child nesting,

kernel/core/console.c

@@ -820,8 +820,12 @@ consoleinit(void)
   initlock(&cons.lock, "console");
   initlock(&input_lock, "input");
 
-  devsw[CONSOLE].write = consolewrite;
+  // Not read and write. Major 1 is the whole mem major -- null, zero, full,
-  devsw[CONSOLE].read  = consoleread;
+  // random, urandom as well as the console and the ttys -- and devsw[1] is
+  // memread/memwrite, which send the console's minors here and the rest
+  // where they belong. Installing consoleread over it made /dev/zero and
+  // /dev/urandom wait for a line from the keyboard, which is why neither
+  // had ever seemed to work.
   devsw[CONSOLE].open  = consoleopen;
   devsw[CONSOLE].close = consoleclose;
   cons.locking = 1;

kernel/dev/cdev.c

@@ -170,6 +170,7 @@ memwrite(struct inode *ip, uint64 off, char *dst, int n)
     case 10: case 11: case 12: case 13: case 14: case 15: case 16:
             return consolewrite(ip, off, dst, n);
     case 7: return fullwrite(ip, off, dst, n);   // /dev/full: always ENOSPC
+    case 3:                                      // /dev/keyboard: discarded
     case 2:
     case 5:
     case 8: