SchOS/include/proc.h

#pragma once

#include "types.h"
#include "traps.h"

// 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;
  uint64 length;
  int used;
  // The pages under this mapping are not ours to free on unmap: either they
  // are device memory (the framebuffer) or they belong to something with a
  // longer life than this process (a shared memory segment). This was
  // `is_fb`, which was the only such case until shared memory existed --
  // and freeing another process's shared pages on exit is exactly the kind
  // of bug that shows up somewhere else entirely, hours later.
  int no_free;
  int shmid;                     // shared segment this came from, or -1
};

// Per-CPU state
struct cpu {
  uchar id;
  uchar apicid;               // Local APIC ID
  struct context *scheduler;  // swtch() here to enter scheduler
  volatile uint64 started;      // Has the CPU started?
  int ncli;                   // Depth of pushcli nesting.
  int intena;                 // Were interrupts enabled before pushcli?
  void *local;                // CPU-local storage (GS base)
};

extern struct cpu cpus[NCPU];
extern int ncpu;        // processors present
extern int ncpu_online; // processors actually running

// --- per-CPU variables ------------------------------------------------------
//
// These were `__thread struct cpu *cpu` and `__thread struct proc *proc`, and
// that was wrong in a way that only appears once a second processor runs.
//
// `__thread` means *per thread*, and the compiler is entitled to assume a
// thread's pointer never changes: it reads %fs:0 once and keeps the value in a
// register or spilled on the stack across calls. clang did exactly that in
// sleep():
//
//     movq %fs:0x0, %rax          # read the thread pointer
//     movq %rax, -0x18(%rbp)      # spill it
//     callq sched                 # <-- the CPU can change here
//     movq -0x18(%rbp), %rax      # reload the OLD pointer
//     movq (%rax,%rcx), %rax      # proc, from the previous CPU's block
//     movq $0x0, 0x40(%rax)       # NULL, because that CPU set proc = 0
//
// But %fs here is *per CPU*, not per thread, and a kernel thread changes CPU at
// every sched(). So the assumption is false for exactly the variables that
// most need to be right, and the failure is a null dereference deep in the
// scheduler on a machine with more than one core.
//
// The fix is to read the value at each use, with no cached base: an explicit
// %gs-relative access that the compiler cannot hoist across a call. `volatile`
// is what forbids that, and the memory clobber on the setters keeps stores in
// order with the rest of the scheduler's bookkeeping.
//
// Layout of the per-CPU block, whose base is in both FS_BASE and GS_BASE, set
// by seginit() in kernel/arch/x86_64/vm.c:
//
//     +0   self-pointer     required by the x86-64 TLS ABI (%fs:0)
//     +8   syscall scratch  trapasm.S parks %rsp here across the stack switch
//     +16  struct cpu *
//     +24  struct proc *
//
// Offset 8 is NOT free. syscall_entry uses %fs:(8) as its scratch slot, and
// because seginit() sets GS base and FS base to the same address, %fs:8 and
// %gs:8 are the same memory. Putting `cpu` there made every syscall overwrite
// it with %rsp -- a #GP in syscall_entry itself, several layers below anything
// that could report it.
//
// trapasm.S reads %gs:24 for proc on the way out to user mode, so these three
// offsets are shared with assembly and must not be reordered.
#define PERCPU_SELF   0
#define PERCPU_SCRATCH 8
#define PERCPU_CPU    16
#define PERCPU_PROC   24

static inline struct cpu *
cpu_get(void)
{
  struct cpu *c;
  asm volatile("movq %%gs:16, %0" : "=r" (c));
  return c;
}

static inline void
cpu_set(struct cpu *c)
{
  asm volatile("movq %0, %%gs:16" :: "r" (c) : "memory");
}

static inline struct proc *
proc_get(void)
{
  struct proc *p;
  asm volatile("movq %%gs:24, %0" : "=r" (p));
  return p;
}

static inline void
proc_set(struct proc *p)
{
  asm volatile("movq %0, %%gs:24" :: "r" (p) : "memory");
}

// Named mycpu()/myproc(), as xv6-riscv does, and *not* spelled `cpu`/`proc`.
// A macro by those names would also rewrite every `struct cpu` and
// `struct proc`, because macros have no namespace and tags do -- which is
// exactly why the variables could carry those names in the first place.
#define mycpu()   cpu_get()
#define myproc()  proc_get()

// Saved registers for kernel context switches (x86_64 ABI)
struct context {
  uint64 r15;
  uint64 r14;
  uint64 r13;
  uint64 r12;
  uint64 rbx;
  uint64 rbp;
  uint64 rip;
};

enum procstate { UNUSED, EMBRYO, SLEEPING, RUNNABLE, RUNNING, ZOMBIE };

// Per-process state
// A copy of the interesting fields of one process, taken under ptable.lock.
// procfs reads these rather than struct proc, so nothing outside proc.c needs
// the lock or the layout.
struct procinfo {
  int    pid, ppid, pgid, sid;
  uint   uid;
  int    state;             // enum procstate
  uint64 sz;
  int    killed;
  char   name[16];
  char   exe[64];           // full path, for /proc/<pid>/exe
};

struct proc {
  uint64 sz;                   // Size of process memory (bytes)     [+0x00]
  pml4e_t* pgdir;              // Page table (PML4 on x86_64)        [+0x08]
  char *kstack;                // Bottom of kernel stack              [+0x10]
  uint64 fs_base;              // User TLS base (IA32_FS_BASE MSR)   [+0x18] ← trapasm.S depends on this offset
  enum procstate state;        // Process state                       [+0x20]
  int pid;                     //                                     [+0x24]
  struct proc *parent;         //                                     [+0x28]
  struct trapframe *tf;        //                                     [+0x30]
  struct context *context;     //                                     [+0x38]
  void *chan;                  //                                     [+0x40]
  int killed;                  //                                     [+0x48]
  int _pad;                    //                                     [+0x4c]
  struct file *ofile[NOFILE];  // Open files                         [+0x50]
  struct inode *cwd;           //                                     follows
  char name[16];               //
  // The path exec() was given. `name` is only the last component and only 15
  // characters of it, which is what /proc/<pid>/stat wants; this is the whole
  // path, reported by /proc/<pid>/exe.
  //
  // Note it is exposed as a regular *file*, not the symlink Linux uses, so
  // `cat /proc/1/exe` works but readlink() does not. That is why `ps -o comm`
  // still prints "[init]": toybox readlink()s exe and brackets the name when
  // that fails. Default ps and top output is unaffected. Fixing it properly
  // means symlink support in procfs, which is worth doing with the rest of the
  // VFS work rather than on its own.
  char exe[64];

  // A kernel thread's entry point, or 0 for an ordinary process.
  //
  // Set by kthread_create and dispatched by forkret. A kernel thread has a
  // page directory with only the kernel mapped and never returns to user mode,
  // so it has no trapframe worth speaking of and no user stack -- it simply
  // runs `kfn` forever inside the scheduler's ordinary rotation, which is what
  // makes it able to sleep and to be preempted like anything else.
  void (*kfn)(void);

  uint64 signal_mask;
  uint64 pending_signals;
  int alarm_ticks;
  char ofile_flags[NOFILE];

  // Real signal delivery (added for SIGCHLD — see kernel/sys/syscall.c's
  // deliver_signals() and kernel/proc/proc.c's exit()). sig_handler[sig]/
  // sig_restorer[sig] come straight from the user's struct sigaction
  // (sys_rt_sigaction); 0 means no handler installed. saved_sigtf stashes
  // the pre-signal trapframe so sys_rt_sigreturn() can restore it; in_signal
  // prevents re-entering a handler while one is already running (we don't
  // support nested signal delivery).
  uint64 sig_handler[32];
  uint64 sig_restorer[32];
  struct trapframe saved_sigtf;
  int in_signal;

  // rt_sigsuspend() support: the pre-suspend mask to restore once the
  // handler that woke us finishes (sigsuspend swaps the mask atomically
  // for the duration of the wait -- POSIX). sigsusp_active is set while a
  // restore is owed; sys_rt_sigreturn() performs it.
  uint64 sigsusp_oldmask;
  int sigsusp_active;

  struct vma vma[NVMA];        // VMA tracking for mmap
  uint64 mmap_next;            // next free mmap VA (bump allocator, see sys_mmap)

  // ABI translation (Linux compat syscall number map)
  int *syscall_map;
  int map_size;

  // Current working directory path; maintained by sys_chdir / sys_fchdir.
  char cwd_path[256];

  // --- POSIX process identity -------------------------------------------
  // Appended at the end of the struct deliberately: trapasm.S hard-codes the
  // offsets of the fields at the top (see the _Static_assert on fs_base in
  // kernel/proc/proc.c), so nothing may be inserted above them.

  // Exit status, already encoded the way wait(2) reports it -- see
  // W_EXITCODE() below. Read by waitpid() before the slot is recycled.
  int xstate;

  // Signal that killed us, if any. Set by sigsend() alongside ->killed so the
  // eventual exit() can report *which* signal, instead of a blanket SIGKILL.
  int killsig;

  // Process group and session. Job control is defined entirely in terms of
  // these: a shell puts each pipeline in its own process group and signals the
  // group, not individual pids. Without them a shell's wait loop cannot work,
  // which is why mksh's job control never made progress.
  int pgid;
  int sid;

  // Real/effective/saved user and group ids. Nothing enforces permissions with
  // them yet (the on-disk format has no owner field until SchFS), but they
  // are tracked honestly so getuid()/setuid() stop lying to userland.
  uint uid, euid, suid;
  uint gid, egid, sgid;
};
#define FD_CLOEXEC 1

// wait(2) status encoding, as understood by musl's WIFEXITED/WEXITSTATUS/
// WIFSIGNALED/WTERMSIG macros:
//   normal exit      -> (code & 0xff) << 8
//   killed by signal -> sig & 0x7f
#define W_EXITCODE(code, sig)  ((((code) & 0xff) << 8) | ((sig) & 0x7f))