// SPDX-License-Identifier: GPL-2.0-or-later
/* -*- linux-c -*- --------------------------------------------------------- *
 *
 * linux/fs/devpts/inode.c
 *
 *  Copyright 1998-2004 H. Peter Anvin -- All Rights Reserved
 *
 * ------------------------------------------------------------------------- */

#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt

#include <linux/module.h>
#include <linux/init.h>
#include <linux/fs.h>
#include <linux/fs_context.h>
#include <linux/fs_parser.h>
#include <linux/sched.h>
#include <linux/namei.h>
#include <linux/slab.h>
#include <linux/mount.h>
#include <linux/tty.h>
#include <linux/mutex.h>
#include <linux/magic.h>
#include <linux/idr.h>
#include <linux/devpts_fs.h>
#include <linux/fsnotify.h>
#include <linux/seq_file.h>

#define DEVPTS_DEFAULT_MODE 0600
/*
 * ptmx is a new node in /dev/pts and will be unused in legacy (single-
 * instance) mode. To prevent surprises in user space, set permissions of
 * ptmx to 0. Use 'chmod' or remount with '-o ptmxmode' to set meaningful
 * permissions.
 */
#define DEVPTS_DEFAULT_PTMX_MODE 0000
#define PTMX_MINOR      2

/*
 * sysctl support for setting limits on the number of Unix98 ptys allocated.
 * Otherwise one can eat up all kernel memory by opening /dev/ptmx repeatedly.
 */
static int pty_limit = NR_UNIX98_PTY_DEFAULT;
static int pty_reserve = NR_UNIX98_PTY_RESERVE;
static int pty_limit_min;
static int pty_limit_max = INT_MAX;
static atomic_t pty_count = ATOMIC_INIT(0);

static const struct ctl_table pty_table[] = {
        {
                .procname       = "max",
                .maxlen         = sizeof(int),
                .mode           = 0644,
                .data           = &pty_limit,
                .proc_handler   = proc_dointvec_minmax,
                .extra1         = &pty_limit_min,
                .extra2         = &pty_limit_max,
        }, {
                .procname       = "reserve",
                .maxlen         = sizeof(int),
                .mode           = 0644,
                .data           = &pty_reserve,
                .proc_handler   = proc_dointvec_minmax,
                .extra1         = &pty_limit_min,
                .extra2         = &pty_limit_max,
        }, {
                .procname       = "nr",
                .maxlen         = sizeof(int),
                .mode           = 0444,
                .data           = &pty_count,
                .proc_handler   = proc_dointvec,
        },
};

struct pts_mount_opts {
        int setuid;
        int setgid;
        kuid_t   uid;
        kgid_t   gid;
        umode_t mode;
        umode_t ptmxmode;
        int reserve;
        int max;
};

enum {
        Opt_uid, Opt_gid, Opt_mode, Opt_ptmxmode, Opt_newinstance,  Opt_max,
        Opt_err
};

static const struct fs_parameter_spec devpts_param_specs[] = {
        fsparam_gid     ("gid",         Opt_gid),
        fsparam_s32     ("max",         Opt_max),
        fsparam_u32oct  ("mode",        Opt_mode),
        fsparam_flag    ("newinstance", Opt_newinstance),
        fsparam_u32oct  ("ptmxmode",    Opt_ptmxmode),
        fsparam_uid     ("uid",         Opt_uid),
        {}
};

struct pts_fs_info {
        struct ida allocated_ptys;
        struct pts_mount_opts mount_opts;
        struct super_block *sb;
        struct inode *ptmx_inode; // borrowed
};

static inline struct pts_fs_info *DEVPTS_SB(struct super_block *sb)
{
        return sb->s_fs_info;
}

static int devpts_ptmx_path(struct path *path)
{
        struct super_block *sb;
        int err;

        /* Is a devpts filesystem at "pts" in the same directory? */
        err = path_pts(path);
        if (err)
                return err;

        /* Is the path the root of a devpts filesystem? */
        sb = path->mnt->mnt_sb;
        if ((sb->s_magic != DEVPTS_SUPER_MAGIC) ||
            (path->mnt->mnt_root != sb->s_root))
                return -ENODEV;

        return 0;
}

/*
 * Try to find a suitable devpts filesystem. We support the following
 * scenarios:
 * - The ptmx device node is located in the same directory as the devpts
 *   mount where the pts device nodes are located.
 *   This is e.g. the case when calling open on the /dev/pts/ptmx device
 *   node when the devpts filesystem is mounted at /dev/pts.
 * - The ptmx device node is located outside the devpts filesystem mount
 *   where the pts device nodes are located. For example, the ptmx device
 *   is a symlink, separate device node, or bind-mount.
 *   A supported scenario is bind-mounting /dev/pts/ptmx to /dev/ptmx and
 *   then calling open on /dev/ptmx. In this case a suitable pts
 *   subdirectory can be found in the common parent directory /dev of the
 *   devpts mount and the ptmx bind-mount, after resolving the /dev/ptmx
 *   bind-mount.
 *   If no suitable pts subdirectory can be found this function will fail.
 *   This is e.g. the case when bind-mounting /dev/pts/ptmx to /ptmx.
 */
struct vfsmount *devpts_mntget(struct file *filp, struct pts_fs_info *fsi)
{
        struct path path;
        int err = 0;

        path = filp->f_path;
        path_get(&path);

        /* Walk upward while the start point is a bind mount of
         * a single file.
         */
        while (path.mnt->mnt_root == path.dentry)
                if (follow_up(&path) == 0)
                        break;

        /* devpts_ptmx_path() finds a devpts fs or returns an error. */
        if ((path.mnt->mnt_sb->s_magic != DEVPTS_SUPER_MAGIC) ||
            (DEVPTS_SB(path.mnt->mnt_sb) != fsi))
                err = devpts_ptmx_path(&path);
        dput(path.dentry);
        if (!err) {
                if (DEVPTS_SB(path.mnt->mnt_sb) == fsi)
                        return path.mnt;

                err = -ENODEV;
        }

        mntput(path.mnt);
        return ERR_PTR(err);
}

struct pts_fs_info *devpts_acquire(struct file *filp)
{
        struct pts_fs_info *result;
        struct path path;
        struct super_block *sb;

        path = filp->f_path;
        path_get(&path);

        /* Has the devpts filesystem already been found? */
        if (path.mnt->mnt_sb->s_magic != DEVPTS_SUPER_MAGIC) {
                int err;

                err = devpts_ptmx_path(&path);
                if (err) {
                        result = ERR_PTR(err);
                        goto out;
                }
        }

        /*
         * pty code needs to hold extra references in case of last /dev/tty close
         */
        sb = path.mnt->mnt_sb;
        atomic_inc(&sb->s_active);
        result = DEVPTS_SB(sb);

out:
        path_put(&path);
        return result;
}

void devpts_release(struct pts_fs_info *fsi)
{
        deactivate_super(fsi->sb);
}

/*
 * devpts_parse_param - Parse mount parameters
 */
static int devpts_parse_param(struct fs_context *fc, struct fs_parameter *param)
{
        struct pts_fs_info *fsi = fc->s_fs_info;
        struct pts_mount_opts *opts = &fsi->mount_opts;
        struct fs_parse_result result;
        int opt;

        opt = fs_parse(fc, devpts_param_specs, param, &result);
        if (opt < 0)
                return opt;

        switch (opt) {
        case Opt_uid:
                opts->uid = result.uid;
                opts->setuid = 1;
                break;
        case Opt_gid:
                opts->gid = result.gid;
                opts->setgid = 1;
                break;
        case Opt_mode:
                opts->mode = result.uint_32 & S_IALLUGO;
                break;
        case Opt_ptmxmode:
                opts->ptmxmode = result.uint_32 & S_IALLUGO;
                break;
        case Opt_newinstance:
                break;
        case Opt_max:
                if (result.uint_32 > NR_UNIX98_PTY_MAX)
                        return invalf(fc, "max out of range");
                opts->max = result.uint_32;
                break;
        }

        return 0;
}

static int mknod_ptmx(struct super_block *sb, struct fs_context *fc)
{
        int mode;
        struct dentry *dentry;
        struct inode *inode;
        struct dentry *root = sb->s_root;
        struct pts_fs_info *fsi = DEVPTS_SB(sb);
        struct pts_mount_opts *opts = &fsi->mount_opts;
        kuid_t ptmx_uid = current_fsuid();
        kgid_t ptmx_gid = current_fsgid();

        dentry = simple_start_creating(root, "ptmx");
        if (IS_ERR(dentry)) {
                pr_err("Unable to alloc dentry for ptmx node\n");
                return PTR_ERR(dentry);
        }

        /*
         * Create a new 'ptmx' node in this mount of devpts.
         */
        inode = new_inode(sb);
        if (!inode) {
                simple_done_creating(dentry);
                pr_err("Unable to alloc inode for ptmx node\n");
                return -ENOMEM;
        }

        inode->i_ino = 2;
        simple_inode_init_ts(inode);

        mode = S_IFCHR|opts->ptmxmode;
        init_special_inode(inode, mode, MKDEV(TTYAUX_MAJOR, 2));
        inode->i_uid = ptmx_uid;
        inode->i_gid = ptmx_gid;
        fsi->ptmx_inode = inode;

        d_make_persistent(dentry, inode);

        simple_done_creating(dentry);

        return 0;
}

static void update_ptmx_mode(struct pts_fs_info *fsi)
{
        fsi->ptmx_inode->i_mode = S_IFCHR|fsi->mount_opts.ptmxmode;
}

static int devpts_reconfigure(struct fs_context *fc)
{
        struct pts_fs_info *fsi = DEVPTS_SB(fc->root->d_sb);
        struct pts_fs_info *new = fc->s_fs_info;

        /* Apply the revised options.  We don't want to change ->reserve.
         * Ideally, we'd update each option conditionally on it having been
         * explicitly changed, but the default is to reset everything so that
         * would break UAPI...
         */
        fsi->mount_opts.setuid          = new->mount_opts.setuid;
        fsi->mount_opts.setgid          = new->mount_opts.setgid;
        fsi->mount_opts.uid             = new->mount_opts.uid;
        fsi->mount_opts.gid             = new->mount_opts.gid;
        fsi->mount_opts.mode            = new->mount_opts.mode;
        fsi->mount_opts.ptmxmode        = new->mount_opts.ptmxmode;
        fsi->mount_opts.max             = new->mount_opts.max;

        /*
         * parse_mount_options() restores options to default values
         * before parsing and may have changed ptmxmode. So, update the
         * mode in the inode too. Bogus options don't fail the remount,
         * so do this even on error return.
         */
        update_ptmx_mode(fsi);

        return 0;
}

static int devpts_show_options(struct seq_file *seq, struct dentry *root)
{
        struct pts_fs_info *fsi = DEVPTS_SB(root->d_sb);
        struct pts_mount_opts *opts = &fsi->mount_opts;

        if (opts->setuid)
                seq_printf(seq, ",uid=%u",
                           from_kuid_munged(&init_user_ns, opts->uid));
        if (opts->setgid)
                seq_printf(seq, ",gid=%u",
                           from_kgid_munged(&init_user_ns, opts->gid));
        seq_printf(seq, ",mode=%03o", opts->mode);
        seq_printf(seq, ",ptmxmode=%03o", opts->ptmxmode);
        if (opts->max < NR_UNIX98_PTY_MAX)
                seq_printf(seq, ",max=%d", opts->max);

        return 0;
}

static const struct super_operations devpts_sops = {
        .statfs         = simple_statfs,
        .show_options   = devpts_show_options,
};

static int devpts_fill_super(struct super_block *s, struct fs_context *fc)
{
        struct pts_fs_info *fsi = DEVPTS_SB(s);
        struct inode *inode;

        s->s_iflags &= ~SB_I_NODEV;
        s->s_blocksize = 1024;
        s->s_blocksize_bits = 10;
        s->s_magic = DEVPTS_SUPER_MAGIC;
        s->s_op = &devpts_sops;
        s->s_d_flags = DCACHE_DONTCACHE;
        s->s_time_gran = 1;
        fsi->sb = s;

        inode = new_inode(s);
        if (!inode)
                return -ENOMEM;
        inode->i_ino = 1;
        simple_inode_init_ts(inode);
        inode->i_mode = S_IFDIR | S_IRUGO | S_IXUGO | S_IWUSR;
        inode->i_op = &simple_dir_inode_operations;
        inode->i_fop = &simple_dir_operations;
        set_nlink(inode, 2);

        s->s_root = d_make_root(inode);
        if (!s->s_root) {
                pr_err("get root dentry failed\n");
                return -ENOMEM;
        }

        return mknod_ptmx(s, fc);
}

/*
 * devpts_get_tree()
 *
 *     Mount a new (private) instance of devpts.  PTYs created in this
 *     instance are independent of the PTYs in other devpts instances.
 */
static int devpts_get_tree(struct fs_context *fc)
{
        return get_tree_nodev(fc, devpts_fill_super);
}

static void devpts_free_fc(struct fs_context *fc)
{
        kfree(fc->s_fs_info);
}

static const struct fs_context_operations devpts_context_ops = {
        .free           = devpts_free_fc,
        .parse_param    = devpts_parse_param,
        .get_tree       = devpts_get_tree,
        .reconfigure    = devpts_reconfigure,
};

/*
 * Set up the filesystem mount context.
 */
static int devpts_init_fs_context(struct fs_context *fc)
{
        struct pts_fs_info *fsi;

        fsi = kzalloc_obj(struct pts_fs_info);
        if (!fsi)
                return -ENOMEM;

        ida_init(&fsi->allocated_ptys);
        fsi->mount_opts.uid     = GLOBAL_ROOT_UID;
        fsi->mount_opts.gid     = GLOBAL_ROOT_GID;
        fsi->mount_opts.mode    = DEVPTS_DEFAULT_MODE;
        fsi->mount_opts.ptmxmode = DEVPTS_DEFAULT_PTMX_MODE;
        fsi->mount_opts.max     = NR_UNIX98_PTY_MAX;

        if (fc->purpose == FS_CONTEXT_FOR_MOUNT &&
            current->nsproxy->mnt_ns == init_task.nsproxy->mnt_ns)
                fsi->mount_opts.reserve = true;

        fc->s_fs_info = fsi;
        fc->ops = &devpts_context_ops;
        return 0;
}

static void devpts_kill_sb(struct super_block *sb)
{
        struct pts_fs_info *fsi = DEVPTS_SB(sb);

        if (fsi)
                ida_destroy(&fsi->allocated_ptys);
        kfree(fsi);
        kill_anon_super(sb);
}

static struct file_system_type devpts_fs_type = {
        .name           = "devpts",
        .init_fs_context = devpts_init_fs_context,
        .parameters     = devpts_param_specs,
        .kill_sb        = devpts_kill_sb,
        .fs_flags       = FS_USERNS_MOUNT,
};

/*
 * The normal naming convention is simply /dev/pts/<number>; this conforms
 * to the System V naming convention
 */

int devpts_new_index(struct pts_fs_info *fsi)
{
        int index = -ENOSPC;

        if (atomic_inc_return(&pty_count) >= (pty_limit -
                          (fsi->mount_opts.reserve ? 0 : pty_reserve)))
                goto out;

        index = ida_alloc_max(&fsi->allocated_ptys, fsi->mount_opts.max - 1,
                        GFP_KERNEL);

out:
        if (index < 0)
                atomic_dec(&pty_count);
        return index;
}

void devpts_kill_index(struct pts_fs_info *fsi, int idx)
{
        ida_free(&fsi->allocated_ptys, idx);
        atomic_dec(&pty_count);
}

/**
 * devpts_pty_new -- create a new inode in /dev/pts/
 * @fsi: Filesystem info for this instance.
 * @index: used as a name of the node
 * @priv: what's given back by devpts_get_priv
 *
 * The dentry for the created inode is returned.
 * Remove it from /dev/pts/ with devpts_pty_kill().
 */
struct dentry *devpts_pty_new(struct pts_fs_info *fsi, int index, void *priv)
{
        struct dentry *dentry;
        struct super_block *sb = fsi->sb;
        struct inode *inode;
        struct dentry *root;
        struct pts_mount_opts *opts;
        char s[12];

        root = sb->s_root;
        opts = &fsi->mount_opts;

        inode = new_inode(sb);
        if (!inode)
                return ERR_PTR(-ENOMEM);

        inode->i_ino = index + 3;
        inode->i_uid = opts->setuid ? opts->uid : current_fsuid();
        inode->i_gid = opts->setgid ? opts->gid : current_fsgid();
        simple_inode_init_ts(inode);
        init_special_inode(inode, S_IFCHR|opts->mode, MKDEV(UNIX98_PTY_SLAVE_MAJOR, index));

        sprintf(s, "%d", index);

        dentry = d_alloc_name(root, s);
        if (!dentry) {
                iput(inode);
                return ERR_PTR(-ENOMEM);
        }
        dentry->d_fsdata = priv;
        d_make_persistent(dentry, inode);
        fsnotify_create(d_inode(root), dentry);
        dput(dentry);
        return dentry; // borrowed
}

/**
 * devpts_get_priv -- get private data for a slave
 * @dentry: dentry of the slave
 *
 * Returns whatever was passed as priv in devpts_pty_new for a given inode.
 */
void *devpts_get_priv(struct dentry *dentry)
{
        if (dentry->d_sb->s_magic != DEVPTS_SUPER_MAGIC)
                return NULL;
        return dentry->d_fsdata;
}

/**
 * devpts_pty_kill -- remove inode form /dev/pts/
 * @dentry: dentry of the slave to be removed
 *
 * This is an inverse operation of devpts_pty_new.
 */
void devpts_pty_kill(struct dentry *dentry)
{
        WARN_ON_ONCE(dentry->d_sb->s_magic != DEVPTS_SUPER_MAGIC);

        dentry->d_fsdata = NULL;
        drop_nlink(dentry->d_inode);
        d_drop(dentry);
        fsnotify_unlink(d_inode(dentry->d_parent), dentry);
        d_make_discardable(dentry);
}

static int __init init_devpts_fs(void)
{
        int err = register_filesystem(&devpts_fs_type);
        if (!err) {
                register_sysctl("kernel/pty", pty_table);
        }
        return err;
}
module_init(init_devpts_fs)