process: implement thread-backed fork()/vfork()/execve()/waitpid() (opt-in)

OSv is a single-address-space unikernel, so a literal copy-on-write fork() is
not the model.  This adds a thread-backed fork() emulation that covers the
useful, compatible subset of fork semantics, gated entirely behind a new
CONFIG_fork kconfig option that defaults to OFF - so a default OSv build is
unchanged (none of this code is compiled in and fork()/vfork() return ENOSYS as
before).

With CONFIG_fork enabled:
- fork()/vfork() create a child OSv thread that resumes in fork()'s caller and
  returns 0 in the child / the child pid in the parent (the classic twin
  return).  The child runs on a private copy of the parent's user stack, so
  parent and child have independent locals after the return; it gets its own
  fresh OSv per-thread TLS block (own errno etc).
  Implemented in libc/process/fork.cc + arch/{x64,aarch64}/fork.cc.
- execve() launches the target as a fresh OSv application (its own ELF
  namespace) and does not return, making fork()+exec() work.
- waitpid()/wait4()/wait() reap a child's exit status via a pid->child
  registry; SIGCHLD is raised to the parent on child exit; SIGCHLD/SIGURG/
  SIGWINCH now correctly default to ignore (not poweroff).
- exit()/_exit() in a fork child ends only that child, not the whole unikernel.
- pthread_atfork prepare/parent/child handlers are now actually run around
  fork() (they were a no-op stub) - glibc/musl register these internally.
- sys_clone() routes the non-CLONE_THREAD (fork) case here when enabled.

Validated: tst-fork passes 10/10 on both x86-64 and aarch64 (twin return,
private-stack isolation, fork+exec, vfork, waitpid reaping).

Documented limitations (documentation/fork.md): the child shares the parent's
heap/globals (no per-process memory isolation in one address space - a follow-up
adds per-child copy-on-write address spaces behind the same flag); deep-call-
chain child unwind and fork-as-memory-snapshot (Redis BGSAVE) are not carried by
this base; execve()'s new-ELF-namespace path has a separate pre-existing fault.

Copyright (C) 2026 Greg Burd

Author: gburd

Makefile

@@ -1057,6 +1057,9 @@ objects += arch/$(arch)/firmware.o
 objects += arch/$(arch)/hypervisor.o
 objects += arch/$(arch)/interrupt.o
 objects += arch/$(arch)/clone.o
+ifeq ($(conf_fork),1)
+objects += arch/$(arch)/fork.o
+endif
 ifeq ($(conf_drivers_pci),1)
 objects += arch/$(arch)/pci.o
 objects += arch/$(arch)/msi.o
@@ -1660,11 +1663,20 @@ musl += prng/srand48.o
 libc += random.o
 
 libc += process/execve.o
+ifeq ($(conf_fork),1)
+libc += process/fork.o
+endif
 musl += process/execle.o
 musl += process/execv.o
 musl += process/execl.o
 libc += process/waitpid.o
+# When fork() is enabled, our waitpid.cc provides wait()/waitpid()/wait4()
+# backed by the fork() child registry, superseding musl's process/wait.o.
+# When fork() is disabled, waitpid.cc provides only the ECHILD waitpid() stub,
+# so keep musl's wait.o for wait()/wait4().
+ifneq ($(conf_fork),1)
 musl += process/wait.o
+endif
 
 musl += setjmp/$(musl_arch)/setjmp.o
 musl += setjmp/$(musl_arch)/longjmp.o

arch/aarch64/fork.cc

@@ -0,0 +1,83 @@
+/*
+ * Copyright (C) 2026 Greg Burd
+ *
+ * This work is open source software, licensed under the terms of the
+ * BSD license as described in the LICENSE file in the top-level directory.
+ *
+ * fork_thread() for aarch64 -- mirrors arch/x64/fork.cc.  fork() (fork.cc)
+ * passes the caller's return address and stack pointer; we copy the parent's
+ * user stack, bias the SP into the copy, and start a child thread that installs
+ * the copied stack, sets x0=0 (fork()'s return value in the child), and returns
+ * to fork()'s caller.
+ */
+
+#include "arch.hh"
+#include <errno.h>
+#include <string.h>
+#include <cstdlib>
+#include <osv/sched.hh>
+
+// pthread_atfork child-handler chain (defined in libc/pthread.cc), run in the
+// child's context before it resumes user code.
+extern "C" void __osv_run_atfork_child();
+
+sched::thread *fork_thread(void *caller_ret, void *caller_sp,
+                           void **out_stack_to_free)
+{
+    auto parent = sched::thread::current();
+    auto parent_pinned_cpu = parent->pinned() ? sched::cpu::current() : nullptr;
+
+    auto si = parent->get_stack_info();
+    char *stack_base = static_cast<char*>(si.begin) + si.size;
+    char *sp = static_cast<char*>(caller_sp);
+    if (sp < static_cast<char*>(si.begin) || sp > stack_base) {
+        return nullptr;
+    }
+    size_t stack_size = si.size;
+
+    char *child_stack_mem = static_cast<char*>(malloc(stack_size));
+    if (!child_stack_mem) {
+        return nullptr;
+    }
+    // Copy ONLY the live top [caller_sp .. stack_base) into the top of the
+    // child buffer (app stacks are demand-paged; copying from si.begin faults).
+    char *child_base = child_stack_mem + stack_size;
+    ptrdiff_t bias = child_base - stack_base;
+    size_t live = static_cast<size_t>(stack_base - sp);
+    memcpy(child_base - live, sp, live);
+    char *child_sp = sp + bias;
+
+    volatile u64 resume_sp = reinterpret_cast<u64>(child_sp);
+    volatile u64 resume_pc = reinterpret_cast<u64>(caller_ret);
+    char *stack_to_free = child_stack_mem;
+
+    // TLS: the child is a real OSv thread with its own fresh setup_tcb() block.
+    // Only override tpidr_el0 if the parent had installed its own app TCB via
+    // arch_prctl (parent_app_tcb != 0); otherwise keep the child's private OSv
+    // TLS (the clean case for a musl app built against OSv's libc).
+    u64 parent_app_tcb = parent->get_app_tcb();
+    auto t = sched::thread::make([resume_sp, resume_pc, parent_app_tcb] {
+        if (parent_app_tcb) {
+            asm volatile ("msr tpidr_el0, %0; isb" :: "r"(parent_app_tcb) : "memory");
+        }
+        // Run pthread_atfork child handlers in the child's context before
+        // resuming user code.
+        __osv_run_atfork_child();
+        asm volatile
+          ("mov sp, %0 \n\t"    // install the private copied stack
+           "mov x0, #0 \n\t"    // fork() returns 0 in the child
+           "br %1 \n\t"         // resume in fork()'s caller
+           : : "r"(resume_sp), "r"(resume_pc) : "x0", "memory");
+    }, sched::thread::attr().
+        stack(4096 * 4),
+        false,
+        true);
+    t->set_app_tcb(parent->get_app_tcb());
+    if (parent_pinned_cpu) {
+        t->pin(parent_pinned_cpu);
+    }
+    if (out_stack_to_free) {
+        *out_stack_to_free = stack_to_free;
+    }
+    return t;
+}

arch/x64/fork.cc

@@ -0,0 +1,107 @@
+/*
+ * Copyright (C) 2026 Greg Burd
+ *
+ * This work is open source software, licensed under the terms of the
+ * BSD license as described in the LICENSE file in the top-level directory.
+ *
+ * fork_thread(): create a child thread that resumes in fork()'s CALLER, on a
+ * private copy of the parent's user stack, returning 0 from fork() in the child.
+ * The x86-64 arch half of the fork() emulation (see documentation/fork.md).
+ *
+ * fork() (libc/process/fork.cc) passes us the caller's resume point:
+ *   caller_ret  = the address fork() would return to  (__builtin_return_address)
+ *   caller_sp   = the parent's SP at fork()'s return   (fork()'s frame base)
+ * We copy the parent stack region [caller_sp .. stack_base) into a fresh stack,
+ * bias caller_sp into the copy, and start a child thread whose trampoline sets
+ * rsp=child_sp, rax=0, and jumps to caller_ret -- i.e. the child returns from
+ * fork() with value 0 on its own private stack, in the caller.
+ */
+
+#include "arch.hh"
+#include "tls-switch.hh"
+#include <errno.h>
+#include <string.h>
+#include <cstdlib>
+#include <osv/sched.hh>
+
+// pthread_atfork child-handler chain (defined in libc/pthread.cc), run in the
+// child's context before it resumes user code.
+extern "C" void __osv_run_atfork_child();
+
+sched::thread *fork_thread(void *caller_ret, void *caller_sp, void **out_stack_to_free)
+{
+    auto parent = sched::thread::current();
+    auto parent_pinned_cpu = parent->pinned() ? sched::cpu::current() : nullptr;
+
+    auto si = parent->get_stack_info();
+    char *stack_base = static_cast<char*>(si.begin) + si.size;
+    char *sp = static_cast<char*>(caller_sp);
+    if (sp < static_cast<char*>(si.begin) || sp > stack_base) {
+        return nullptr;   // caller SP not within the known user stack
+    }
+    size_t stack_size = si.size;
+
+    char *child_stack_mem = static_cast<char*>(malloc(stack_size));
+    if (!child_stack_mem) {
+        return nullptr;
+    }
+    // Copy ONLY the live top of the stack, [caller_sp .. stack_base), into the
+    // TOP of the child buffer.  App (pthread) stacks are demand-paged: only the
+    // used top is mapped, so copying from si.begin would fault on the first
+    // unmapped page.  Keeping the copy at the top of the child buffer preserves
+    // the base-relative bias so a biased SP resolves correctly.
+    char *child_base = child_stack_mem + stack_size;
+    ptrdiff_t bias = child_base - stack_base;
+    size_t live = static_cast<size_t>(stack_base - sp);
+    memcpy(child_base - live, sp, live);
+    char *child_sp = sp + bias;
+
+    volatile u64 resume_sp = reinterpret_cast<u64>(child_sp);
+    volatile u64 resume_pc = reinterpret_cast<u64>(caller_ret);
+    char *stack_to_free = child_stack_mem;
+
+    // TLS handling.  The child is a real OSv sched::thread, so its constructor
+    // already ran setup_tcb() and installed a FRESH, private OSv TLS block
+    // (with its own errno and all libc __thread state).  Two cases:
+    //
+    //  (1) The app uses OSv's libc TLS (the normal dynamically-linked path,
+    //      app_tcb == 0): the child's own fresh TCB is exactly right -- do NOT
+    //      touch fsbase, let the child run on its private per-thread TLS.  This
+    //      is the clean case and fork() "just works" for TLS.
+    //  (2) The app installed its own TCB via arch_prctl(SET_FS) (app_tcb != 0,
+    //      e.g. a glibc binary's __libc_setup_tls): the child would need a
+    //      private COPY of that app TCB.  We do not duplicate it here yet;
+    //      the child inherits the parent's app_tcb (shared), which is the
+    //      documented multi-process-glibc limitation.  A musl app built against
+    //      OSv's libc takes path (1) and avoids this entirely.
+    u64 parent_app_tcb = parent->get_app_tcb();
+
+    auto t = sched::thread::make([resume_sp, resume_pc, parent_app_tcb] {
+        // Only override the child's own (fresh) TLS if the parent had installed
+        // an app TCB via arch_prctl; otherwise keep the child's private OSv TCB.
+        if (parent_app_tcb) {
+            arch::set_fsbase(parent_app_tcb);
+        }
+        // Run pthread_atfork child handlers in the child's context (e.g. reset
+        // the malloc arena lock) before resuming user code.
+        __osv_run_atfork_child();
+        asm volatile
+          ("movq %0, %%rsp \n\t"    // install the private copied stack
+           "xorq %%rax, %%rax \n\t" // fork() returns 0 in the child
+           "jmpq *%1 \n\t"          // resume in fork()'s caller
+           : : "r"(resume_sp), "r"(resume_pc) : "memory");
+    }, sched::thread::attr().
+        stack(4096 * 4),
+        false,
+        true);
+    t->set_app_tcb(parent->get_app_tcb());
+    if (parent_pinned_cpu) {
+        t->pin(parent_pinned_cpu);
+    }
+    // The caller (fork.cc) owns the single cleanup; hand back the copied user
+    // stack so it can be freed when the child is reaped.
+    if (out_stack_to_free) {
+        *out_stack_to_free = stack_to_free;
+    }
+    return t;
+}

conf/kconfig/threads

@@ -6,6 +6,19 @@ config lazy_stack
   prompt "Use lazy stack"
   def_bool $(shell,grep -q ^conf_lazy_stack=1 conf/base.mk && echo y || echo n)
 
+config fork
+  prompt "Include fork()/vfork() support (per-child address space, off by default)"
+  bool
+  default n
+  help
+    Enable OSv's thread-backed fork()/vfork()/execve()/waitpid() emulation,
+    including the per-child address space with copy-on-write needed to give a
+    forked child private memory.  This changes OSv's usual single-address-space
+    model (address-space switches on context switch between fork children), so
+    it is OFF by default: with this disabled, none of the fork code is compiled
+    in and OSv behaves exactly as before (fork() returns ENOSYS).  Enable it
+    only for workloads that require fork() (e.g. multi-process programs).
+
 config lazy_stack_invariant
   prompt "Check lazy stack invariant"
   def_bool $(shell,grep -q ^conf_lazy_stack_invariant=1 conf/base.mk && echo y || echo n)

documentation/fork.md

@@ -0,0 +1,111 @@
+# fork() on OSv
+
+OSv is a single-address-space unikernel: all threads of all applications share
+one address space, with no MMU-enforced process isolation. This is deliberate,
+it is where OSv's performance comes from. It also means Linux `fork()`, which
+gives the child a **private copy-on-write duplicate of the parent's entire
+address space**, cannot be implemented literally.
+
+OSv instead provides a **thread-backed fork() emulation** that supports the
+common, compatible subset of fork semantics. This document describes what works,
+what does not, and why.
+
+## Off by default: the `fork` configure flag
+
+All of the fork() machinery - the fork()/vfork()/execve()/waitpid()
+implementations, the per-child address space, and the copy-on-write changes -
+is gated behind the `CONFIG_fork` kconfig option (make variable `conf_fork`),
+which defaults to **n**. When it is disabled (the default), NONE of this code
+is compiled into the kernel: the fork object files are excluded from the build,
+fork()/vfork() return ENOSYS as before, and OSv behaves exactly as it did with
+no change to its single-address-space model or performance. Build with
+`conf_fork=1` (or enable "Include fork() support" in `make menuconfig`) only for
+workloads that need fork(). This keeps the default OSv unchanged while offering
+fork() to those who require it.
+
+## What works
+
+- **`fork()` twin return.** `fork()` creates a new OSv thread (the "child") that
+  resumes at the `fork()` call site. `fork()` returns the child's pid to the
+  parent and `0` to the child, exactly like Linux. The child runs on a **private
+  copy of the parent's user stack**, so parent and child have independent local
+  variables and call chains after the return.
+
+- **`fork()` + `execve()`.** The child calls `execve()`, which launches the
+  requested program as a fresh OSv application (its own ELF namespace) and makes
+  the child thread that program's driver; a successful `execve()` never returns,
+  as on Linux. The parent is a separate thread and is unaffected.
+
+- **`waitpid()` / `wait4()` / `wait()`.** The parent reaps a child's exit status
+  (Linux-encoded, use `WIFEXITED`/`WEXITSTATUS`). `WNOHANG` is supported.
+  `SIGCHLD` is raised to the parent when a child exits.
+
+- **`vfork()`.** Maps to `fork()`. Because the child already shares the parent's
+  address space, this actually matches vfork's contract ("the child borrows the
+  parent's memory until it execs or exits") more faithfully than it matches
+  fork's copy contract.
+
+- **`_exit()` / `exit()` in a child** ends only that child "process" (thread /
+  app) and records its status for the parent, rather than shutting down the
+  whole unikernel (which is what a top-level `exit()` still does).
+
+## What does NOT work (and why)
+
+- **Shared TLS (thread-local storage) - only for apps that install their own
+  TCB.** A forked child is a real OSv thread, so it gets its OWN fresh, private
+  OSv TLS block (its own `errno` and libc `__thread` state) automatically. For a
+  program that uses OSv's (musl-derived) libc the normal way, fork() TLS "just
+  works" - the child does not share the parent's TLS. The exception is a program
+  that installs its OWN thread pointer via `arch_prctl(ARCH_SET_FS)` (e.g. a
+  glibc-ABI binary's `__libc_setup_tls`): OSv records that as `app_tcb`, and the
+  fork child currently inherits the SAME `app_tcb` (shared), which collides. The
+  fix for such binaries is to build them against OSv's own musl libc instead of
+  glibc, so they take the clean per-thread-TLS path. (This was the wall stock
+  glibc-built PostgreSQL hit; a musl build avoids it.)
+
+- **Memory isolation.** The child shares the parent's heap and global variables
+  (only the stack is copied). A child that **writes** to shared globals or
+  heap-allocated data expecting a private copy will affect the parent. Code that
+  only reads shared state and writes to its own fds or freshly-`malloc`'d memory
+  before `exec`/`_exit` is fine; code that mutates shared state after fork is
+  not. This cannot be fixed without adding process isolation to OSv.
+
+- **`fork()` as a memory snapshot** (e.g. Redis `BGSAVE`, a GC that forks to walk
+  a frozen heap). These rely on the child seeing a *frozen* copy of the parent's
+  memory at fork time. On OSv the child sees live, shared memory. This is a
+  silent behavioral difference (it cannot be detected at the syscall boundary)
+  and is **unsupported**.
+
+- **Stack-internal pointers.** Because the child's stack is a byte copy of the
+  parent's biased to a new address, a pointer stored on the stack that points
+  *into the same stack* still points at the parent's stack in the child. Short
+  child code paths (the fork+exec and fork+work-then-_exit patterns) do not hit
+  this; long-lived divergent children can. A future refinement could scan and
+  fix up such pointers.
+
+- **`clone()` with namespace-unshare flags** (`CLONE_NEWNS`, `CLONE_NEWPID`,
+  etc.) returns `ENOSYS` — there are no namespaces to unshare.
+
+- **aarch64.** Implemented and validated: the stack-copy + `br` resume
+  trampoline works on aarch64 as well as x86-64. `tst-fork` passes
+  10/10 on both architectures.
+
+## Implementation
+
+- `libc/process/fork.cc` — `fork()`/`vfork()`, the child registry, and the
+  `waitpid()` backend + `SIGCHLD` notification.
+- `arch/x64/fork.cc` — `fork_thread()`: allocates a fresh stack, copies the
+  parent's current user stack into it, and returns a child thread that installs
+  the copied stack and returns `0` from `fork()`. Reuses the register/
+  continuation approach of `clone_thread()` (used by `pthread_create`).
+- `libc/process/execve.cc` — `execve()` via `osv::application::run()`.
+- `libc/process/waitpid.cc` — `wait`/`waitpid`/`wait4`.
+- `runtime.cc` — `exit()` ends a child rather than shutting down OSv.
+- `linux.cc` — `sys_clone()` routes the non-`CLONE_THREAD` (fork) case here.
+
+## Guidance
+
+For spawning helper programs, prefer `posix_spawn()` or `system()` (which route
+straight to `osv::application::run()` and skip the stack copy entirely) over
+`fork()`+`exec()` where you control the code. Use `fork()` for compatibility
+with existing Linux programs that expect it, within the limitations above.