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

include/defs.h

@@ -79,7 +79,7 @@ void            consoleintr(int(*)(void));
 void            panic(char*) __attribute__((noreturn));
 
 // exec.c
-int             exec(char*, char**);
+int             exec(char*, char**, char**);
 
 // file.c
 struct file*    filealloc(void);

include/param.h

@@ -43,6 +43,7 @@
 #define ROOTDEV        1  // device number of file system root disk
 #endif
 #define MAXARG        32  // max exec arguments
+#define MAXENV        64  // max environment variables passed through exec
 #define MAXPATH       128 // max path 
 #define MAXOPBLOCKS   30  // max # of blocks any FS op writes
 #define LOGSIZE      (MAXOPBLOCKS*3)  // max data blocks in on-disk log

include/proc.h

@@ -3,7 +3,18 @@
 #include "types.h"
 #include "traps.h"
 
-#define NVMA 64
+// Simultaneous mmap() regions per process.
+//
+// 64 was enough for every first-party program and for toybox, and not enough
+// for zsh: musl's allocator takes a fresh mapping per size class and does not
+// release them eagerly, so compinit -- which reads twelve hundred completion
+// functions -- ran out of slots and reported "fatal error: out of memory" on
+// a machine with a gigabyte free. The slots were the limit, not the memory.
+//
+// The table is inline in struct proc, so this is NPROC * NVMA * sizeof(struct
+// vma) of kernel memory, always resident. At 256 that is about 1 MB total,
+// which is worth it to stop a shell dying on a machine that has 1000 MB spare.
+#define NVMA 256
 
 struct vma {
   uint64 addr;

include/user.h

@@ -18,7 +18,8 @@ int write(int, void*, int);
 int read(int, void*, int);
 int close(int);
 int kill(int);
-int exec(char*, char**);
+int exec(char*, char**);            /* execve(path, argv, 0) */
+int execve(char*, char**, char**);
 int open(char*, int);
 int mknod(char*, short, short);
 int unlink(char*);