errno: -1 is EPERM, and four places were saying it by accident

Ctrl-C killed the shell. consoleread() breaks off the read in progress when
SIGINT arrives and returned -1 to do it -- and -1 is -EPERM by the time it
reaches a program, so every program that handles SIGINT by carrying on saw a
permanent error instead of an interrupted call:

    zsh: error on TTY read: operation not permitted
    schinitd: restarting os.schos.console-shell

and the next Ctrl-C did it to the replacement. EINTR now, which is what a
signal-interrupted read is required to give. Verified: Ctrl-C an interruptible
command, shell survives, zero restarts.

Same mistake, same file family: link, unlink, rename and renameat returned -1
on every error path. Callers branch on errno -- `if (unlink(p) < 0 && errno !=
ENOENT)` is the standard shape, and against EPERM it always complains -- which
is why zsh reported failures for things that had worked. They return ENOENT,
EEXIST, EXDEV, EISDIR, ENOTEMPTY and EFAULT now. About ninety other `return -1`s
remain in sys_fs.c; these four are done because these four were what a program
tripped over. /dev/mem's unknown minors return ENODEV rather than -1 too.

/dev/urandom was a linear congruential generator seeded with a **constant**, so
every boot produced the same bytes in the same order -- not weak, the same. A
key, a nonce, a temporary filename: shared with every SchOS machine that had
ever run. Seeded at boot from RDRAND where the CPU has it, from the cycle
counter and the wall clock where it does not, and re-stirred per byte. Still an
LCG and still not a CSPRNG; what changed is the part that was dangerous rather
than merely poor.

**Not fixed, and recorded as such:** rename() still will not replace an existing
target, so zsh's history save fails on every exit after the first. An attempt to
unlink the target first was reverted -- the directory is an HTree, dirunlink
followed by dirlink of the same name did not behave, and it panicked in
filewrite with the old file already gone. Losing both files is worse than
refusing, so it refuses until the HTree's slot reuse is understood.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VQ7Qx8E1q1M1bbCq8QLHS2

docs/architecture/filesystem.md

@@ -189,8 +189,50 @@ selftest, `t` text specimen, `n` the netdev ARP selftest.
 
 ---
 
+## Errno, and why `return -1` is a bug
+
+A syscall that fails returns a negative errno. `-1` is therefore **`-EPERM`**,
+"operation not permitted" — and about ninety error paths in `kernel/sys/sys_fs.c`
+still say `return -1` because that is what xv6 did, where the convention was
+just "negative means no".
+
+It is not a cosmetic difference. Callers branch on `errno`, and the standard
+shape for removing a file you are not sure about is:
+
+```c
+if (unlink(p) < 0 && errno != ENOENT)
+    complain();
+```
+
+Against `-1` every such test takes the complaining branch. That is how it was
+found — zsh, saving its history, reported failures for things that had
+succeeded:
+
+```
+zsh: locking failed for /root/.zsh_history: operation not permitted
+zsh: failed to write history file /root/.zsh_history.new: operation not permitted
+```
+
+`link`, `unlink`, `rename` and `renameat` return real values now — `ENOENT`,
+`EEXIST`, `EXDEV`, `EISDIR`, `ENOTEMPTY`, `EFAULT`. **The rest of the file does
+not**, and that is the honest state: these four were corrected because these
+four were the ones a program tripped over.
+
 ## What it does not do
 
+- **`rename()` will not replace an existing target.** POSIX says it must, and
+  write-to-a-temporary-then-rename is how every careful program saves a file —
+  so the second save of anything fails with `EEXIST` where it should succeed.
+  It is the reason `zsh: can't rename /root/.zsh_history.new to $HISTFILE`
+  appears on every shell exit after the first.
+
+  An attempt to fix it by unlinking the target first was **reverted**: the
+  directory is an HTree, `dirunlink` followed by `dirlink` of the same name did
+  not behave, and the failure mode was the worst possible one — the old file
+  removed and the new link then refused, losing both. Doing this properly means
+  understanding the HTree's slot reuse, not patching around it.
+
+
 - **No page cache.** `mmap` of a file reads the whole thing in at map time, and
   writable `MAP_SHARED` of a file is refused rather than faked. This is the
   single largest gap in the layer.