kernel: processes have an environment now, and zsh is the shell

Porting zsh found four defects in the kernel and one in the filesystem. None
of them were about zsh; all of them had been there the whole time.

**exec threw the environment away.** kernel/fs/exec.c wrote a single zero for
envp with the comment "(no environment)" -- not yet, but ever, for every
process. musl has been passing a perfectly good envp to execve (Linux 59, which
SchOS answers) and it was never read. So $TERM was unset, $PATH was whatever
each program invented, and a variable exported in a shell did not survive the
exec meant to carry it. exec() takes an envp, sys_exec reads argument three,
and schinit manifests grew an Environment key. The userland two-argument
exec(path, argv) stays as a wrapper that passes null explicitly -- the raw stub
is execve now, so a short caller cannot leave %rdx holding a stray pointer.

**Which overflowed the kernel stack.** Three pointer arrays, ~1.8 KB, on a
4096-byte stack, under exec -> loaduvm -> readi -> SchFS -> btree. Pid 1 went
down on the first exec. The canary caught it and named the process, which is
what it is for. They live in one kalloc'd page now.

**waitpid returned a bare -1** for both "no children" and "interrupted". musl
reads [-4095,-1] as -errno, so userspace saw EPERM; zsh printed "wait failed:
operation not permitted" and abandoned the rest of /etc/zsh/zshrc. -ECHILD and
-EINTR now. Third time a bare -1 has cost real time -- on a system with no
permission model, EPERM should be read as "a syscall returned -1".

**NVMA was 64.** musl's allocator takes a mapping per size class and does not
release them eagerly, so compinit ran out of slots and got -1, which is EPERM,
which malloc turns into NULL, which zsh reports as "fatal error: out of memory"
-- on a machine with 1000 MB free. 256 slots, and -ENOMEM.

**And a small write to SchFS costs 58 ms.** 200 writes to one open file take
11.75 s; the same to /dev/null take 0.14 s. That is the design stated at the
top of kmount.c -- one transaction group per operation, and a commit is two
flushes and an uberblock -- not a bug. But compdump writes its digest with
thousands of separate prints, so it takes minutes and looks like a hang.
compinit -D, which skips the dump, finishes in 3.6 s. This one is a decision
rather than a fix, so it is written up and left: /etc/zsh/zshrc keeps compinit
switched off and explains exactly why, with the measurements.

zsh runs as /bin/zsh and is the console shell. ZLE works -- arrow keys recall
history through terminfo, $COLUMNS reads the real 160 because TIOCGWINSZ was
fixed, colour and prompt expansion work, startup is 0.14 s. mksh keeps /bin/sh,
so schinit's no-jobs fallback is a different program from the one that could
have failed.

Also fixed while here: AT_CLKTCK was the literal 100 and is USER_HZ; the
extract rule in four ports could move a source tree inside itself when make
re-ran it, which is how zsh's configure came to write modfile=zsh-5.9/... into
config.modules and fail three steps later.

schfs-check passes. Full writeup in docs/history/kernel.md section 14.

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

lib/glue.c

@@ -22,3 +22,15 @@ __stack_chk_fail(void)
     _xv6_exit();
     for (;;);
 }
+
+// exec() without an environment.
+//
+// The syscall is execve, three arguments, because that is what Linux numbers
+// 59 and what musl calls. The two-argument spelling stays because most of
+// cmd/ has no environment to pass and should not have to say so -- but it
+// says so *here*, explicitly, rather than leaving %rdx to chance.
+int
+exec(char *path, char **argv)
+{
+    return execve(path, argv, 0);
+}

lib/usys.S

@@ -22,7 +22,15 @@ SYSCALL(read)
 SYSCALL(write)
 SYSCALL(close)
 SYSCALL(kill)
-SYSCALL(exec)
+/* The raw stub is execve: three arguments, the Linux shape. exec() is a C
+   wrapper in glue.c that passes a null environment, so the 2-argument callers
+   in cmd/ cannot leave %rdx holding whatever happened to be there. */
+.global execve
+execve:
+    mov $SYS_exec, %rax
+    mov %rcx, %r10
+    syscall
+    ret
 SYSCALL(open)
 SYSCALL(mknod)
 SYSCALL(unlink)