SchOS/kernel/fs/devfs.c

// SPDX-License-Identifier: Apache-2.0
//
// devtmpfs: /dev, generated rather than stored.
//
// It used to be ~20 mknod() calls in cmd/schinit, which was wrong in two ways
// that are worth separating.
//
// *The cost.* Every node was a real inode on the root filesystem, and SchFS
// commits each mutation as its own transaction group -- a copy-on-write rewrite
// of the path to the root, then an uberblock behind a cache flush. Measured:
// 2600 ms of a first boot spent writing device nodes to a disk, before schinit
// printed its first line. On later boots the nodes already existed and the same
// code took 0 ms, so the cost was invisible unless you built a fresh image --
// which is exactly what anyone working on the system does every time.
//
// *The duplication, which matters more.* Major and minor numbers are the
// kernel's: they are what devsw[] in kernel/dev/cdev.c dispatches on. schinit
// held a second copy of that mapping and nothing checked the two agreed. A new
// character device meant editing a userspace program, and getting a minor wrong
// there produced a node that opened successfully and did nothing.
//
// So the table below sits next to the thing it describes, and /dev is a
// mount rather than a directory somebody populates. `mount("dev", "/dev",
// "devtmpfs", 0, 0)` is the whole of what init does now.
//
// The namespace is all this provides. Reads and writes never reach here --
// fileread/filewrite see T_DEV and dispatch through devsw themselves -- which
// is why there is no read op below that does anything.

#include "types.h"
#include "defs.h"
#include "param.h"
#include "stat.h"
#include "spinlock.h"
#include "sleeplock.h"
#include "proc.h"
#include "fs.h"
#include "file.h"
#include "errno.h"

// Inode numbering. Two directories and a flat table of nodes, so the number is
// either a directory id or an index into devnodes[] offset past them.
#define DEVFS_ROOT     1ULL
#define DEVFS_INPUT    2ULL          /* the /dev/input directory */
#define DEVFS_FILE0    16ULL         /* devnodes[0] is inode 16 */

// Which directory a node lives in. Only two exist and a third would want a real
// tree rather than a flag, which is the point at which this file should grow
// one instead of adding DEV_DIR_THIRD.
#define D_ROOT   0
#define D_INPUT  1

struct devnode {
  const char *name;
  short  major;
  short  minor;
  short  dir;
};

// The devices this kernel implements, in the order they appear in a listing.
//
// Minor numbers under major 1 follow Linux's mem-major layout where it costs
// nothing (null=2, zero=5, full=7, random=8, urandom=9), which is worth keeping
// because it is one less arbitrary mapping for anyone reading both.
//
// The majors are devsw[] indices and nothing else; see kernel/dev/cdev.c.
static const struct devnode devnodes[] = {
  { "console", 1, 1,  D_ROOT },
  { "null",    1, 2,  D_ROOT },
  { "keyboard",1, 3,  D_ROOT },
  { "zero",    1, 5,  D_ROOT },
  { "full",    1, 7,  D_ROOT },
  { "random",  1, 8,  D_ROOT },
  { "urandom", 1, 9,  D_ROOT },
  /* /dev/tty is the caller's *controlling* terminal, which is a different
     thing from the console and used to be the same thing only because there
     was one terminal. Minor 10 resolves through proc->ctty; see
     vt_for_minor() in kernel/core/console.c. */
  { "tty",     1, 10, D_ROOT },
  { "tty1",    1, 11, D_ROOT },
  { "tty2",    1, 12, D_ROOT },
  { "tty3",    1, 13, D_ROOT },
  { "tty4",    1, 14, D_ROOT },
  { "tty5",    1, 15, D_ROOT },
  { "tty6",    1, 16, D_ROOT },
  { "stdin",   1, 1,  D_ROOT },
  { "stdout",  1, 1,  D_ROOT },
  { "stderr",  1, 1,  D_ROOT },
  { "fb0",     2, 0,  D_ROOT },
  { "pci",     9, 0,  D_ROOT },
  { "mouse",   3, 1,  D_ROOT },

  // Raw disks. Created whether or not a drive is present: opening one with no
  // disk behind it fails with ENXIO, which is a better answer than a missing
  // path -- and with no node at all, `echo x > /dev/hdb` silently creates an
  // ordinary file of that name and appears to work.
  { "hda",     8, 0,  D_ROOT },
  { "hdb",     8, 1,  D_ROOT },

  // Raw input events, Linux evdev layout. The minor is the device index, so
  // event0 is the keyboard and event1 the mouse.
  { "event0",  7, 0,  D_INPUT },
  { "event1",  7, 1,  D_INPUT },
};

#define NDEVNODES ((int)(sizeof(devnodes) / sizeof(devnodes[0])))

static int
node_of(uint64 inum)
{
  if (inum < DEVFS_FILE0)
    return -1;
  uint64 i = inum - DEVFS_FILE0;
  return i < (uint64)NDEVNODES ? (int)i : -1;
}

static void
devfs_iload(struct inode *ip)
{
  int i = node_of(ip->inum);

  ip->major = ip->minor = 0;
  ip->size  = 0;

  if (ip->inum == DEVFS_ROOT || ip->inum == DEVFS_INPUT) {
    ip->type  = T_DIR;
    ip->nlink = 2;
  } else if (i >= 0) {
    ip->type  = T_DEV;
    ip->nlink = 1;
    ip->major = devnodes[i].major;
    ip->minor = devnodes[i].minor;
  } else {
    ip->type  = 0;                   /* no such node */
    ip->nlink = 0;
  }
  ip->valid = 1;
}

static void devfs_istore(struct inode *ip)               { (void)ip; }
static void devfs_ifree(struct inode *ip)                { (void)ip; }
static void devfs_truncate(struct inode *ip, uint64 n)   { (void)ip; (void)n; }

// Never reached for a device node: fileread and filewrite see T_DEV and go
// through devsw. Present because fs_ops has the slots, and answering -EIO is
// better than a null pointer if something ever does arrive here.
static int devfs_read(struct inode *ip, int u, uint64 d, uint64 o, uint n)
{ (void)ip; (void)u; (void)d; (void)o; (void)n; return -1; }
static int devfs_write(struct inode *ip, int u, uint64 s, uint64 o, uint n)
{ (void)ip; (void)u; (void)s; (void)o; (void)n; return -1; }

static int
streq(const char *a, const char *b)
{
  while (*a && *a == *b) { a++; b++; }
  return *a == *b;
}

static struct inode *
devfs_dirlookup(struct inode *dp, char *name, uint *poff)
{
  if (poff)
    *poff = 0;

  if (streq(name, ".") || (streq(name, "..") && dp->inum == DEVFS_ROOT))
    return iget(DEVFS_DEV, dp->inum);
  if (streq(name, ".."))
    return iget(DEVFS_DEV, DEVFS_ROOT);     /* out of /dev/input */

  if (dp->inum == DEVFS_ROOT && streq(name, "input"))
    return iget(DEVFS_DEV, DEVFS_INPUT);

  short want = dp->inum == DEVFS_INPUT ? D_INPUT : D_ROOT;
  for (int i = 0; i < NDEVNODES; i++)
    if (devnodes[i].dir == want && streq(name, devnodes[i].name))
      return iget(DEVFS_DEV, DEVFS_FILE0 + (uint64)i);

  return 0;
}

// A read-only filesystem, and these say so rather than pretending. mknod on
// /dev returning EROFS is the correct answer: the node either exists because
// the kernel has the device, or it does not exist because it has not.
static int devfs_dirlink(struct inode *dp, char *name, uint64 inum)
{ (void)dp; (void)name; (void)inum; return -EROFS; }
static int devfs_dirunlink(struct inode *dp, char *name, uint off)
{ (void)dp; (void)name; (void)off; return -EROFS; }
static int devfs_dirempty(struct inode *dp) { (void)dp; return 0; }
static struct inode *devfs_ialloc(uint dev, short type)
{ (void)dev; (void)type; return 0; }

struct linux_dirent64 {
  uint64 d_ino;
  int64  d_off;
  uint16 d_reclen;
  uchar  d_type;
  char   d_name[];
};

static int
emit(char *out, int cap, int *used, uint64 ino, uchar type, const char *name)
{
  int nl  = (int)strlen(name);
  int rec = (int)((sizeof(struct linux_dirent64) + nl + 1 + 7) & ~7);

  if (*used + rec > cap)
    return -1;                       /* caller stops and resumes next call */

  struct linux_dirent64 *d = (struct linux_dirent64 *)(out + *used);
  d->d_ino    = ino;
  d->d_off    = 0;
  d->d_reclen = (uint16)rec;
  d->d_type   = type;
  memmove(d->d_name, name, nl);
  memset(d->d_name + nl, 0, rec - (int)sizeof(*d) - nl);
  *used += rec;
  return 0;
}

// `off` is an ordinal over the entries of this directory: 0 is ".", 1 is "..",
// and the rest follow. An opaque cookie the way SchFS uses one buys nothing
// here, because the table is fixed and small enough to index directly.
static int
devfs_getdents(struct inode *dp, char *out, int cap, uint64 *poff)
{
  int used = 0;
  uint64 n = *poff;

  if (dp->inum != DEVFS_ROOT && dp->inum != DEVFS_INPUT)
    return -1;

  if (n == 0) {
    if (emit(out, cap, &used, dp->inum, 4 /* DT_DIR */, ".") < 0) return -1;
    n++;
  }
  if (n == 1) {
    uint64 up = dp->inum == DEVFS_ROOT ? DEVFS_ROOT : DEVFS_ROOT;
    if (emit(out, cap, &used, up, 4, "..") < 0) goto out;
    n++;
  }

  // /dev/input is a directory entry of the root, and it is emitted between the
  // dots and the nodes so that its ordinal is stable.
  if (dp->inum == DEVFS_ROOT && n == 2) {
    if (emit(out, cap, &used, DEVFS_INPUT, 4, "input") < 0) goto out;
    n++;
  }

  short want = dp->inum == DEVFS_INPUT ? D_INPUT : D_ROOT;
  uint64 base = dp->inum == DEVFS_ROOT ? 3 : 2;

  for (int i = 0; i < NDEVNODES; i++) {
    if (devnodes[i].dir != want)
      continue;
    if (n > base) { base++; continue; }         /* already emitted */
    if (emit(out, cap, &used, DEVFS_FILE0 + (uint64)i, 2 /* DT_CHR */,
             devnodes[i].name) < 0)
      goto out;
    n++;
    base++;
  }

out:
  *poff = n;
  return used;
}

const struct fs_ops devfs_ops = {
  .name      = "devtmpfs",
  .iload     = devfs_iload,
  .istore    = devfs_istore,
  .ifree     = devfs_ifree,
  .read      = devfs_read,
  .write     = devfs_write,
  .truncate  = devfs_truncate,
  .dirlookup = devfs_dirlookup,
  .dirlink   = devfs_dirlink,
  .dirunlink = devfs_dirunlink,
  .dirempty  = devfs_dirempty,
  .ialloc    = devfs_ialloc,
  .getdents  = devfs_getdents,
};

// Graft it over /dev. Called by init, which has made the directory.
void
devfs_mount(void)
{
  struct inode *dp = namei("/dev");

  if (dp == 0) {
    cprintf("devfs: /dev does not exist; not mounted\n");
    return;
  }
  if (mount_add(dp->dev, dp->inum, DEVFS_DEV, DEVFS_ROOT, "/dev", "dev",
                "devtmpfs") < 0)
    cprintf("devfs: /dev already has something mounted on it\n");
  iput(dp);
}