fs/binfmt_misc.c

// SPDX-License-Identifier: GPL-2.0-only
/*
 * binfmt_misc.c
 *
 * Copyright (C) 1997 Richard Günther
 *
 * binfmt_misc detects binaries via a magic or filename extension and invokes
 * a specified wrapper. See Documentation/admin-guide/binfmt-misc.rst for more details.
 */

#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt

#include <linux/array_size.h>
#include <linux/binfmt_misc.h>
#include <linux/binfmts.h>
#include <linux/bitops.h>
#include <linux/bits.h>
#include <linux/bug.h>
#include <linux/cleanup.h>
#include <linux/cred.h>
#include <linux/ctype.h>
#include <linux/file.h>
#include <linux/fs.h>
#include <linux/fs_context.h>
#include <linux/init.h>
#include <linux/kstrtox.h>
#include <linux/limits.h>
#include <linux/list.h>
#include <linux/magic.h>
#include <linux/module.h>
#include <linux/printk.h>
#include <linux/rculist.h>
#include <linux/refcount.h>
#include <linux/seq_file.h>
#include <linux/slab.h>
#include <linux/srcu.h>
#include <linux/string.h>
#include <linux/string_helpers.h>
#include <linux/uaccess.h>
#include <linux/user_namespace.h>

#include "internal.h"

/* Entry status and match type bit numbers. */
enum binfmt_misc_entry_bits {
	MISC_FMT_ENABLED_BIT	= 0,
	MISC_FMT_MAGIC_BIT	= 1,
	MISC_FMT_BPF_BIT	= 2,
};

/* Entry behavior flags, fixed at registration time. */
enum binfmt_misc_entry_flags {
	MISC_FMT_PRESERVE_ARGV0	= (1U << 31),
	MISC_FMT_OPEN_BINARY	= (1U << 30),
	MISC_FMT_CREDENTIALS	= (1U << 29),
	MISC_FMT_OPEN_FILE	= (1U << 28),
	MISC_FMT_TRANSPARENT	= (1U << 27),
	MISC_FMT_LOADER		= (1U << 26),
	MISC_FMT_DISABLED	= (1U << 25),
};

/* The flags that shape the invocation; a 'B' handler picks those per exec. */
#define MISC_FMT_INVOCATION_FLAGS (MISC_FMT_PRESERVE_ARGV0 |	\
				   MISC_FMT_OPEN_BINARY |	\
				   MISC_FMT_CREDENTIALS |	\
				   MISC_FMT_OPEN_FILE |		\
				   MISC_FMT_TRANSPARENT |	\
				   MISC_FMT_LOADER)

/**
 * struct binfmt_misc_flag - a flag character of the register string
 * @c: the character userspace writes and reads back
 * @flag: the entry flag it sets
 * @implies: entry flags it turns on in addition
 * @desc: what it does, for the registration debug output
 */
struct binfmt_misc_flag {
	char		c;
	unsigned long	flag;
	unsigned long	implies;
	const char	*desc;
};

static const struct binfmt_misc_flag misc_flags[] = {
	{ 'P', MISC_FMT_PRESERVE_ARGV0,	0,			"preserve argv0"		},
	{ 'O', MISC_FMT_OPEN_BINARY,	0,			"open binary"			},
	{ 'C', MISC_FMT_CREDENTIALS,	MISC_FMT_OPEN_BINARY,	"credentials from the binary"	},
	{ 'F', MISC_FMT_OPEN_FILE,	0,			"open interpreter file now"	},
	{ 'T', MISC_FMT_TRANSPARENT,	MISC_FMT_OPEN_BINARY,	"transparent"			},
	{ 'L', MISC_FMT_LOADER,		0,			"loader substitution"		},
	{ 'D', MISC_FMT_DISABLED,	0,			"register disabled"		},
};

/* Look up a flag character, NULL if @c is not one. */
static const struct binfmt_misc_flag *misc_flag_by_char(const char c)
{
	for (int i = 0; i < ARRAY_SIZE(misc_flags); i++)
		if (misc_flags[i].c == c)
			return &misc_flags[i];
	return NULL;
}

struct binfmt_misc_entry {
	struct hlist_node node;
	unsigned long flags;		/* type, status, etc. */
	int offset;			/* offset of magic */
	int size;			/* size of magic/mask */
	char *magic;			/* magic or filename extension */
	char *mask;			/* mask, NULL for exact match */
	const char *interpreter;	/* filename of interpreter */
	char *name;
	struct dentry *dentry;
	const struct binfmt_misc_ops *bpf_ops;	/* bpf-backed handler ('B') */
	const char *bpf_ops_name;
	struct list_head interps;	/* the interpreters it bound */
	refcount_t users;		/* sync removal with load_misc_binary() */
	struct rcu_head rcu;
	char buf[];			/* register string, fields point in here */
};

/*
 * Max length of the register string.  Determined by:
 *  - 7 delimiters
 *  - name:   ~50 bytes
 *  - type:   1 byte
 *  - offset: 3 bytes (has to be smaller than BINPRM_BUF_SIZE)
 *  - magic:  128 bytes (512 in escaped form)
 *  - mask:   128 bytes (512 in escaped form)
 *  - interp: ~50 bytes
 *  - flags:  5 bytes
 * Round that up a bit, and then back off to hold the internal data
 * (like struct binfmt_misc_entry).
 */
#define MAX_REGISTER_LENGTH 1920

/* Trailing delimiter pad so field parsing always terminates at a delimiter. */
#define MISC_DELIM_PAD 8

/* Protects the entry walk in load_misc_binary(), which may sleep in it. */
DEFINE_STATIC_SRCU_FAST(bm_entries_srcu);

/* Check if @e's magic matches @bprm's buffer, applying the mask if set. */
static bool entry_matches_magic(const struct binfmt_misc_entry *e,
				const struct linux_binprm *bprm)
{
	const char *s = bprm->buf + e->offset;
	int i;

	if (!e->mask)
		return !memcmp(s, e->magic, e->size);

	for (i = 0; i < e->size; i++)
		if ((s[i] ^ e->magic[i]) & e->mask[i])
			return false;
	return true;
}

/* Check if @e's registered extension matches @ext, NULL if there is none. */
static bool entry_matches_extension(const struct binfmt_misc_entry *e,
				    const char *ext)
{
	return ext && !strcmp(e->magic, ext);
}

/**
 * search_binfmt_handler - search for a binary handler for @bprm
 * @misc: handle to binfmt_misc instance
 * @bprm: binary for which we are looking for a handler
 *
 * Search for a binary type handler for @bprm in the list of registered binary
 * type handlers. A 'B' entry's match program decides whether the handler
 * applies; it may sleep to read the binary. The matched entry is returned
 * with a reference taken while the walk still held it; a dying entry -
 * unlinked with its last reference gone - cannot be matched and the walk
 * moves on.
 *
 * The caller must hold the bm_entries_srcu read lock, which allows an
 * entry's evaluation to sleep.
 *
 * Return: referenced binary type list entry on success, NULL on failure
 */
static struct binfmt_misc_entry *
search_binfmt_handler(struct binfmt_misc *misc, struct linux_binprm *bprm)
{
	char *dot = strrchr(bprm->interp, '.');
	const char *ext = dot ? dot + 1 : NULL;
	struct binfmt_misc_entry *e;

	/* Walk all the registered handlers. */
	hlist_for_each_entry_rcu(e, &misc->entries, node,
				 srcu_read_lock_held(&bm_entries_srcu)) {
		/*
		 * Make sure this one is currently enabled. An entry enters
		 * the list at most once and only whole: its configuration is
		 * ordered before the rcu insertion that makes it visible
		 * here.
		 */
		if (!test_bit(MISC_FMT_ENABLED_BIT, &e->flags))
			continue;

		if (test_bit(MISC_FMT_BPF_BIT, &e->flags)) {
			if (!e->bpf_ops->match(bprm))
				continue;
		} else if (test_bit(MISC_FMT_MAGIC_BIT, &e->flags)) {
			if (!entry_matches_magic(e, bprm))
				continue;
		} else {
			if (!entry_matches_extension(e, ext))
				continue;
		}

		/* A dying entry cannot be matched, walk on. */
		if (refcount_inc_not_zero(&e->users))
			return e;
	}

	return NULL;
}

/**
 * get_binfmt_handler - try to find a binary type handler
 * @misc: handle to binfmt_misc instance
 * @bprm: binary for which we are looking for a handler
 *
 * Try to find a binfmt handler for the binary type. If one is found it is
 * returned with a reference protecting it against removal via
 * bm_{entry,status}_write().
 *
 * Return: binary type list entry on success, NULL on failure
 */
static struct binfmt_misc_entry *get_binfmt_handler(struct binfmt_misc *misc,
						    struct linux_binprm *bprm)
{
	guard(srcu_fast)(&bm_entries_srcu);
	return search_binfmt_handler(misc, bprm);
}

/**
 * binfmt_misc_find_interp - find a bound interpreter by name
 * @interps: the interpreters the matched entry was registered with
 * @name: the name to look for
 *
 * Return: the interpreter on success, NULL if @interps has none by that name
 */
const struct binfmt_misc_interp *
binfmt_misc_find_interp(const struct list_head *interps, const char *name)
{
	struct binfmt_misc_interp *interp;

	list_for_each_entry(interp, interps, list)
		if (!strcmp(interp->name, name))
			return interp;
	return NULL;
}

/* Undo the open_exec() a pre-opened interpreter file came from. */
static void close_interp_file(struct file *f)
{
	if (IS_ERR_OR_NULL(f))
		return;
	exe_file_allow_write_access(f);
	filp_close(f, NULL);
}

DEFINE_FREE(close_interp_file, struct file *, close_interp_file(_T))

/*
 * Open an interpreter @path for execution: now, in the writer's context,
 * and - since binfmt_misc mounts can be unprivileged - with @cred, the
 * credentials the control file being written was opened with, not the
 * writer's own.
 */
static struct file *open_interp_file(const struct cred *cred, const char *path)
{
	struct file *f;

	scoped_with_creds(cred)
		f = open_exec(path);
	if (IS_ERR(f))
		pr_notice("register: failed to install interpreter %s\n", path);
	return f;
}

/* Release the interpreters an entry was registered with. */
static void entry_put_interpreters(struct binfmt_misc_entry *e)
{
	struct binfmt_misc_interp *interp, *tmp;

	list_for_each_entry_safe(interp, tmp, &e->interps, list) {
		list_del(&interp->list);
		close_interp_file(interp->file);
		dec_ucount(interp->ucounts, UCOUNT_BINFMT_MISC_INTERPRETERS);
		kfree(interp);
	}
}

/**
 * entry_attach_interpreter - bind an opened interpreter to @e
 * @e: entry being configured
 * @name: name the load program will select it by; empty for the fixed
 *        interpreter of a static entry
 * @path: the path @f was opened from
 * @f: the interpreter, opened for execution
 *
 * Every exec runs a clone of @f, so the path decided which file is bound
 * and nothing else: it is not resolved again, in any namespace.
 *
 * The caller has to have validated @name and @path, established that @e
 * cannot be matched yet, and owns @f until this succeeds.
 *
 * Return: 0 on success, -ENOSPC if the entry is full or the binder is out of
 *         UCOUNT_BINFMT_MISC_INTERPRETERS budget, a negative errno on failure
 */
static int entry_attach_interpreter(struct binfmt_misc_entry *e,
				    const char *name, const char *path,
				    struct file *f)
{
	size_t nlen = strlen(name), plen = strlen(path);
	struct binfmt_misc_interp *interp;
	struct ucounts *ucounts;

	if (binfmt_misc_find_interp(&e->interps, name))
		return -EEXIST;
	if (list_count_nodes(&e->interps) >= BINFMT_MISC_INTERP_MAX)
		return -ENOSPC;

	/* The binding keeps a file open, so charge it to whoever binds it. */
	ucounts = inc_ucount(current_user_ns(), current_euid(),
			     UCOUNT_BINFMT_MISC_INTERPRETERS);
	if (!ucounts)
		return -ENOSPC;

	/* One allocation, both strings in it, like the entry's own buffer. */
	interp = kmalloc(struct_size(interp, name, nlen + plen + 2),
			 GFP_KERNEL_ACCOUNT);
	if (!interp) {
		dec_ucount(ucounts, UCOUNT_BINFMT_MISC_INTERPRETERS);
		return -ENOMEM;
	}

	interp->path = interp->name + nlen + 1;
	strscpy(interp->name, name, nlen + 1);
	strscpy(interp->name + nlen + 1, path, plen + 1);
	interp->file = f;
	interp->ucounts = ucounts;
	/* Publish the node: a lockless cat may be walking the list. */
	list_add_tail_rcu(&interp->list, &e->interps);
	pr_debug("register: interpreter: %s {%s}\n", name, path);
	return 0;
}

static void bm_entry_free_rcu(struct rcu_head *rcu)
{
	struct binfmt_misc_entry *e = container_of(rcu, struct binfmt_misc_entry, rcu);

	/* No walker that could sleep in the handler's programs is left. */
	if (e->bpf_ops)
		binfmt_misc_put_ops(e->bpf_ops);
	kfree(e);
}

/**
 * put_binfmt_handler - put binary handler entry
 * @e: entry to put
 *
 * Free entry syncing with load_misc_binary() and defer final free to
 * load_misc_binary() in case it is using the binary type handler we were
 * requested to remove. Also the teardown for a registration that fails
 * before add_entry() publishes the entry.
 */
static void put_binfmt_handler(struct binfmt_misc_entry *e)
{
	if (IS_ERR_OR_NULL(e))
		return;

	if (refcount_dec_and_test(&e->users)) {
		entry_put_interpreters(e);
		/* Walkers may still dereference this entry, even sleeping. */
		call_srcu(&bm_entries_srcu, &e->rcu, bm_entry_free_rcu);
	}
}

DEFINE_FREE(put_binfmt_handler, struct binfmt_misc_entry *, put_binfmt_handler(_T))

/* Drop everything a load program staged for this exec. */
static void drop_staged_selection(struct linux_binprm *bprm)
{
	kfree(bprm->bpf_interp);
	bprm->bpf_interp = NULL;
	kfree(bprm->bpf_interp_arg);
	bprm->bpf_interp_arg = NULL;
	if (bprm->bpf_interp_file) {
		fput(bprm->bpf_interp_file);
		bprm->bpf_interp_file = NULL;
	}
	bprm->bpf_flags = 0;
}

/**
 * current_binfmt_misc - get the binfmt_misc instance of the caller's user namespace
 *
 * If a user namespace doesn't have its own binfmt_misc mount it uses the
 * handlers of its closest ancestor with one. This mimics the behavior of
 * pre-namespaced binfmt_misc where all registered handlers were available
 * to all users and user namespaces on the system. The init user namespace
 * instance is statically set up so the fallback is never reached in
 * practice.
 *
 * Return: the binfmt_misc instance of the caller's user namespace
 */
static struct binfmt_misc *current_binfmt_misc(void)
{
	const struct user_namespace *user_ns;
	struct binfmt_misc *misc;

	for (user_ns = current_user_ns(); user_ns; user_ns = user_ns->parent) {
		/* Pairs with smp_store_release() in bm_fill_super(). */
		misc = smp_load_acquire(&user_ns->binfmt_misc);
		if (misc)
			return misc;
	}

	return &init_binfmt_misc;
}

/**
 * entry_select_interpreter - get the interpreter for the matched @e
 * @e: matched binary type handler
 * @bprm: binary that is being executed
 *
 * A static entry carries its interpreter path, for a 'B' entry the
 * handler's load program selects it, either by path or by the name of one
 * of the interpreters the entry bound. The match is committed, so a failing
 * program fails the exec.
 *
 * Return: the interpreter on success, an ERR_PTR on failure
 */
static const char *entry_select_interpreter(const struct binfmt_misc_entry *e,
					    struct linux_binprm *bprm)
{
	int retval;

	/*
	 * Drop what a previous chain level staged before anything can pick it
	 * up. A static entry stages nothing but consumes a staged file just
	 * like a 'B' entry does.
	 */
	drop_staged_selection(bprm);

	if (!test_bit(MISC_FMT_BPF_BIT, &e->flags))
		return e->interpreter;

	/* The interpreters this entry lets the program choose from. */
	bprm->bpf_interps = &e->interps;
	retval = e->bpf_ops->load(bprm);
	bprm->bpf_interps = NULL;
	if (retval) {
		/* Keep a program-supplied error within errno range. */
		if (retval > 0 || retval < -MAX_ERRNO)
			retval = -ENOEXEC;
		goto drop_staged;
	}

	/* Selecting an interpreter is part of the contract. */
	if (!bprm->bpf_interp) {
		retval = -ENOEXEC;
		goto drop_staged;
	}

	return bprm->bpf_interp;

drop_staged:
	/* A failing load leaves nothing behind for later entries. */
	drop_staged_selection(bprm);
	return ERR_PTR(retval);
}

/**
 * entry_invocation_flags - the invocation flags in effect for this exec
 * @e: matched binary type handler
 * @bprm: binary that is being executed
 *
 * A static entry fixes its flags at registration, a 'B' entry's load program
 * picks them per exec with bpf_binprm_set_flags(). Translate the latter into
 * the former, implications included, so the dispatch has one set to act on.
 *
 * Return: the invocation flags for this exec
 */
static unsigned long entry_invocation_flags(const struct binfmt_misc_entry *e,
					    struct linux_binprm *bprm)
{
	unsigned long flags = 0;
	u64 bpf_flags;

	if (!test_bit(MISC_FMT_BPF_BIT, &e->flags))
		return e->flags;

	bpf_flags = bprm->bpf_flags;
	/* Clear so they can't accumulate into a nested interpreter level. */
	bprm->bpf_flags = 0;

	if (bpf_flags & BPF_BINPRM_PRESERVE_ARGV0)
		flags |= MISC_FMT_PRESERVE_ARGV0;
	if (bpf_flags & BPF_BINPRM_EXECFD)
		flags |= MISC_FMT_OPEN_BINARY;
	if (bpf_flags & BPF_BINPRM_CREDENTIALS)
		flags |= MISC_FMT_CREDENTIALS | MISC_FMT_OPEN_BINARY;
	if (bpf_flags & BPF_BINPRM_TRANSPARENT)
		flags |= MISC_FMT_TRANSPARENT | MISC_FMT_OPEN_BINARY;
	if (bpf_flags & BPF_BINPRM_LOADER)
		flags |= MISC_FMT_LOADER;

	return flags;
}

/**
 * entry_open_interpreter - open the entry's interpreter for execution
 * @e: matched binary type handler
 * @bprm: binary that is being executed
 * @interpreter: the interpreter selected for this exec
 *
 * An 'F' entry hands out a clone of the file it pre-opened at registration,
 * and so does a 'B' entry whose load program selected one of the
 * interpreters it bound. Any other entry opens the selected path.
 *
 * Return: the opened interpreter on success, an ERR_PTR on failure
 */
static struct file *entry_open_interpreter(const struct binfmt_misc_entry *e,
					   struct linux_binprm *bprm,
					   const char *interpreter)
{
	struct file *interp_file __free(fput) = NULL;
	struct binfmt_misc_interp *interp;
	struct file *bound;
	int retval;

	if (bprm->bpf_interp_file) {
		bound = bprm->bpf_interp_file;
	} else if (e->flags & MISC_FMT_OPEN_FILE) {
		/* An 'F' entry pre-opened exactly one interpreter. */
		interp = list_first_entry(&e->interps,
					  struct binfmt_misc_interp, list);
		bound = interp->file;
	} else {
		return open_exec(interpreter);
	}

	interp_file = file_clone_open(bound);
	if (IS_ERR(interp_file))
		return interp_file;

	retval = exe_file_deny_write_access(interp_file);
	if (retval)
		return ERR_PTR(retval);

	return no_free_ptr(interp_file);
}

/**
 * build_interp_argv - splice the interpreter invocation into the argv
 * @bprm: binary that is being executed
 * @interpreter: the interpreter selected for this exec
 * @flags: invocation flags in effect for this exec
 *
 * The interpreter becomes argv[0] and the binary its last argument, with an
 * optional staged argument in between. The caller's argv[0] is dropped
 * unless 'P' keeps it.
 *
 * Return: 0 on success, a negative error code on failure
 */
static int build_interp_argv(struct linux_binprm *bprm, const char *interpreter,
			     unsigned long flags)
{
	int retval;

	/* The interpreter has to be able to load the binary by path. */
	if (bprm->interp_flags & BINPRM_FLAGS_PATH_INACCESSIBLE)
		return -ENOENT;

	/* The entry's own choice - not one accumulated from an earlier level. */
	if (flags & MISC_FMT_PRESERVE_ARGV0) {
		bprm->interp_flags |= BINPRM_FLAGS_PRESERVE_ARGV0;
	} else {
		retval = remove_arg_zero(bprm);
		if (retval)
			return retval;
	}

	/* make the binary the last argument to the interpreter */
	retval = copy_string_kernel(bprm->interp, bprm);
	if (retval < 0)
		return retval;
	bprm->argc++;

	/*
	 * A single optional argument to the interpreter, inserted between it
	 * and the binary just like the argument of a #! interpreter line.
	 */
	if (bprm->bpf_interp_arg) {
		retval = copy_string_kernel(bprm->bpf_interp_arg, bprm);
		if (retval < 0)
			return retval;
		bprm->argc++;
		/* Consumed - don't let it leak into a nested interpreter's argv. */
		kfree(bprm->bpf_interp_arg);
		bprm->bpf_interp_arg = NULL;
	}

	/* add the interp as argv[0] */
	retval = copy_string_kernel(interpreter, bprm);
	if (retval < 0)
		return retval;
	bprm->argc++;

	return 0;
}

/*
 * the loader itself
 */
static int load_misc_binary(struct linux_binprm *bprm)
{
	struct binfmt_misc_entry *fmt __free(put_binfmt_handler) = NULL;
	const char *interpreter;
	struct file *interp_file;
	struct binfmt_misc *misc;
	unsigned long flags;
	int retval;

	/* Only binfmt_misc stages one and exec_binprm() clears it per round. */
	WARN_ON_ONCE(bprm->loader);

	misc = current_binfmt_misc();
	if (!READ_ONCE(misc->enabled))
		return -ENOEXEC;

	fmt = get_binfmt_handler(misc, bprm);
	if (!fmt)
		return -ENOEXEC;

	interpreter = entry_select_interpreter(fmt, bprm);
	if (IS_ERR(interpreter))
		return PTR_ERR(interpreter);

	flags = entry_invocation_flags(fmt, bprm);

	/* No argv is built for a staged argument to land in. */
	if ((flags & (MISC_FMT_LOADER | MISC_FMT_TRANSPARENT)) &&
	    bprm->bpf_interp_arg)
		return -EINVAL;

	/*
	 * Stash the interpreter for binfmt_elf to consume in place of the
	 * binary's PT_INTERP and decline the match, so the search continues
	 * to the real format in the same round.
	 */
	if (flags & MISC_FMT_LOADER) {
		interp_file = entry_open_interpreter(fmt, bprm, interpreter);
		if (IS_ERR(interp_file)) {
			retval = PTR_ERR(interp_file);
			/* Declining here would run the binary's own PT_INTERP. */
			return retval == -ENOEXEC ? -EACCES : retval;
		}

		bprm->loader = interp_file;
		return -ENOEXEC;
	}

	if (!(flags & MISC_FMT_TRANSPARENT)) {
		retval = build_interp_argv(bprm, interpreter, flags);
		if (retval)
			return retval;
	}

	/* Update interp for the next round; sched_prepare_exec reports it. */
	retval = bprm_change_interp(interpreter, bprm);
	if (retval < 0)
		return retval;

	interp_file = entry_open_interpreter(fmt, bprm, interpreter);
	if (IS_ERR(interp_file))
		return PTR_ERR(interp_file);

	/* Raise only past the last failure, or an -ENOEXEC decline leaks it. */
	if (flags & MISC_FMT_TRANSPARENT)
		bprm->interp_flags |= BINPRM_FLAGS_TRANSPARENT_INTERP;

	bprm->interpreter = interp_file;
	if (flags & MISC_FMT_OPEN_BINARY)
		bprm->have_execfd = 1;
	if (flags & MISC_FMT_CREDENTIALS)
		bprm->execfd_creds = 1;
	return 0;
}

/* Command parsers */

/*
 * Scan the argument starting at @s up to the delimiter @del, recognising
 * the \x escape. Terminates the argument with a NUL and returns a pointer
 * past it or NULL on a malformed escape.
 */
static char *scanarg(char *s, char del)
{
	char c;

	while ((c = *s++) != del) {
		if (c == '\\' && *s == 'x') {
			s++;
			if (!isxdigit(*s++))
				return NULL;
			if (!isxdigit(*s++))
				return NULL;
		}
	}
	s[-1] = '\0';
	return s;
}

/* Parse the 'flags' field, stopping at the first character that is not one. */
static char *check_special_flags(char *p, struct binfmt_misc_entry *e)
{
	for (;; p++) {
		const struct binfmt_misc_flag *f = misc_flag_by_char(*p);

		if (!f)
			return p;
		pr_debug("register: flag: %c (%s)\n", f->c, f->desc);
		e->flags |= f->flag | f->implies;
	}
}

/* Parse the 'offset', 'magic' and 'mask' fields of an 'M' entry. */
static char *parse_magic_fields(struct binfmt_misc_entry *e, char *p, char del)
{
	char *s;

	/* Parse the 'offset' field. */
	s = strchr(p, del);
	if (!s)
		return NULL;
	*s = '\0';
	if (p != s) {
		if (kstrtoint(p, 10, &e->offset) || e->offset < 0)
			return NULL;
	}
	p = s + 1;
	pr_debug("register: offset: %#x\n", e->offset);

	/* Parse the 'magic' field. */
	e->magic = p;
	p = scanarg(p, del);
	if (!p || !e->magic[0])
		return NULL;
	print_hex_dump_debug(
		KBUILD_MODNAME ": register: magic[raw]: ",
		DUMP_PREFIX_NONE, 16, 1, e->magic, p - e->magic, true);

	/* Parse the 'mask' field. */
	e->mask = p;
	p = scanarg(p, del);
	if (!p)
		return NULL;
	if (!e->mask[0]) {
		e->mask = NULL;
		pr_debug("register:  mask[raw]: none\n");
	} else {
		print_hex_dump_debug(
			KBUILD_MODNAME ": register:  mask[raw]: ",
			DUMP_PREFIX_NONE, 16, 1, e->mask, p - e->mask, true);
	}

	/*
	 * Decode the magic & mask fields. Note: while we might have accepted
	 * embedded NUL bytes from above, the unescape helpers will stop at
	 * the first one they encounter.
	 */
	e->size = string_unescape_inplace(e->magic, UNESCAPE_HEX);
	if (e->mask && string_unescape_inplace(e->mask, UNESCAPE_HEX) != e->size)
		return NULL;
	if (e->size > BINPRM_BUF_SIZE || BINPRM_BUF_SIZE - e->size < e->offset)
		return NULL;
	pr_debug("register: magic/mask length: %i\n", e->size);
	print_hex_dump_debug(
		KBUILD_MODNAME ": register: magic[decoded]: ",
		DUMP_PREFIX_NONE, 16, 1, e->magic, e->size, true);
	if (e->mask)
		print_hex_dump_debug(
			KBUILD_MODNAME ": register:  mask[decoded]: ",
			DUMP_PREFIX_NONE, 16, 1, e->mask, e->size, true);
	return p;
}

/* Parse the 'magic' field of an 'E' entry: the filename extension. */
static char *parse_extension_fields(struct binfmt_misc_entry *e, char *p,
				    char del)
{
	/* Skip the 'offset' field. */
	p = strchr(p, del);
	if (!p)
		return NULL;
	*p++ = '\0';

	/* Parse the 'magic' field. */
	e->magic = p;
	p = strchr(p, del);
	if (!p)
		return NULL;
	*p++ = '\0';
	if (!e->magic[0] || strchr(e->magic, '/'))
		return NULL;
	pr_debug("register: extension: {%s}\n", e->magic);

	/* Skip the 'mask' field. */
	p = strchr(p, del);
	if (!p)
		return NULL;
	*p++ = '\0';
	return p;
}

/*
 * Parse the fields of a 'B' entry: the 'offset', 'magic' and 'mask' fields
 * must be empty. The handler name is carried in the 'interpreter' field.
 */
static char *parse_bpf_fields(struct binfmt_misc_entry *e, char *p, char del)
{
	/* The 'offset' field must be empty. */
	if (*p++ != del)
		return NULL;

	/* The 'magic' field must be empty. */
	if (*p++ != del)
		return NULL;

	/* The 'mask' field must be empty. */
	if (*p++ != del)
		return NULL;

	return p;
}

/*
 * This registers a new binary format, it recognises the syntax
 * ':name:type:offset:magic:mask:interpreter:flags'
 * where the ':' is the IFS, that can be chosen with the first char
 */
static struct binfmt_misc_entry *create_entry(const char __user *buffer,
					      size_t count)
{
	struct binfmt_misc_entry *e __free(kfree) = NULL;
	char *buf, *p;
	char del;

	pr_debug("register: received %zu bytes\n", count);

	/* some sanity checks */
	if ((count < 11) || (count > MAX_REGISTER_LENGTH))
		return ERR_PTR(-EINVAL);

	e = kmalloc(struct_size(e, buf, count + MISC_DELIM_PAD),
		    GFP_KERNEL_ACCOUNT);
	if (!e)
		return ERR_PTR(-ENOMEM);

	p = buf = e->buf;

	memset(e, 0, sizeof(*e));
	INIT_LIST_HEAD(&e->interps);
	if (copy_from_user(buf, buffer, count))
		return ERR_PTR(-EFAULT);

	del = *p++;	/* delimiter */

	pr_debug("register: delim: %#x {%c}\n", del, del);

	/* A flag-char delimiter runs the flag scan off the buffer. */
	if (misc_flag_by_char(del))
		return ERR_PTR(-EINVAL);

	/* Pad the buffer with the delim to simplify parsing below. */
	memset(buf + count, del, MISC_DELIM_PAD);

	/* Parse the 'name' field. */
	e->name = p;
	p = strchr(p, del);
	if (!p)
		return ERR_PTR(-EINVAL);
	*p++ = '\0';
	if (!e->name[0] ||
	    !strcmp(e->name, ".") ||
	    !strcmp(e->name, "..") ||
	    strchr(e->name, '/'))
		return ERR_PTR(-EINVAL);

	pr_debug("register: name: {%s}\n", e->name);

	/* Parse the 'type' field. */
	switch (*p++) {
	case 'E':
		pr_debug("register: type: E (extension)\n");
		e->flags = BIT(MISC_FMT_ENABLED_BIT);
		break;
	case 'M':
		pr_debug("register: type: M (magic)\n");
		e->flags = BIT(MISC_FMT_ENABLED_BIT) | BIT(MISC_FMT_MAGIC_BIT);
		break;
	case 'B':
		pr_debug("register: type: B (bpf)\n");
		if (!IS_ENABLED(CONFIG_BINFMT_MISC_BPF))
			return ERR_PTR(-EINVAL);
		e->flags = BIT(MISC_FMT_ENABLED_BIT) | BIT(MISC_FMT_BPF_BIT);
		break;
	default:
		return ERR_PTR(-EINVAL);
	}
	if (*p++ != del)
		return ERR_PTR(-EINVAL);

	if (test_bit(MISC_FMT_BPF_BIT, &e->flags))
		p = parse_bpf_fields(e, p, del);
	else if (test_bit(MISC_FMT_MAGIC_BIT, &e->flags))
		p = parse_magic_fields(e, p, del);
	else
		p = parse_extension_fields(e, p, del);
	if (!p)
		return ERR_PTR(-EINVAL);

	/* Parse the 'interpreter' field. */
	e->interpreter = p;
	p = strchr(p, del);
	if (!p)
		return ERR_PTR(-EINVAL);
	*p++ = '\0';
	if (test_bit(MISC_FMT_BPF_BIT, &e->flags)) {
		/* The 'interpreter' field carries the handler name. */
		e->bpf_ops_name = e->interpreter;
		e->interpreter = NULL;
		if (!e->bpf_ops_name[0])
			return ERR_PTR(-EINVAL);
		pr_debug("register: bpf handler: {%s}\n", e->bpf_ops_name);
	} else if (!e->interpreter[0]) {
		return ERR_PTR(-EINVAL);
	} else {
		pr_debug("register: interpreter: {%s}\n", e->interpreter);
	}

	/* Parse the 'flags' field. */
	p = check_special_flags(p, e);

	/*
	 * A bpf handler decides the invocation flags per exec with
	 * bpf_binprm_set_flags() rather than fixing them at registration, and
	 * the interpreters it binds pre-open what 'F' would have, so a 'B'
	 * entry carries no invocation flags.
	 */
	if (test_bit(MISC_FMT_BPF_BIT, &e->flags) &&
	    (e->flags & MISC_FMT_INVOCATION_FLAGS))
		return ERR_PTR(-EINVAL);

	/*
	 * 'D' is a directive for this registration rather than a lasting
	 * property, so consume it: the entry is created disabled and stays
	 * out of the search list until '1' is written to its entry file.
	 * Staying out is what leaves it open to being given interpreters;
	 * the first enable publishes it, for good.
	 */
	if (e->flags & MISC_FMT_DISABLED) {
		e->flags &= ~MISC_FMT_DISABLED;
		clear_bit(MISC_FMT_ENABLED_BIT, &e->flags);
	}

	/* Transparency preserves the whole argv, argv[0] included. */
	if ((e->flags & MISC_FMT_TRANSPARENT) &&
	    (e->flags & MISC_FMT_PRESERVE_ARGV0))
		return ERR_PTR(-EINVAL);

	/* A native exec splices no argv, passes no execfd and needs no creds. */
	if ((e->flags & MISC_FMT_LOADER) &&
	    (e->flags & (MISC_FMT_TRANSPARENT | MISC_FMT_PRESERVE_ARGV0 |
			 MISC_FMT_CREDENTIALS | MISC_FMT_OPEN_BINARY)))
		return ERR_PTR(-EINVAL);

	if (*p == '\n')
		p++;
	if (p != buf + count)
		return ERR_PTR(-EINVAL);

	/* Non-F opens the interp at exec against the caller's cwd; require absolute. */
	if ((e->flags & (MISC_FMT_LOADER | MISC_FMT_CREDENTIALS)) &&
	    !(e->flags & MISC_FMT_OPEN_FILE) &&
	    e->interpreter[0] != '/')
		return ERR_PTR(-EINVAL);

	/* Born holding one reference; put_binfmt_handler() is the teardown. */
	refcount_set(&e->users, 1);
	return no_free_ptr(e);
}

/* Commands accepted by the /status and /<entry> files. */
enum bm_command {
	BM_CMD_IGNORE,	/* empty write */
	BM_CMD_DISABLE,	/* "0" */
	BM_CMD_ENABLE,	/* "1" */
	BM_CMD_REMOVE,	/* "-1" */
};

/* Longest of the commands above, "-1\n". */
#define MAX_COMMAND_LENGTH 3

/*
 * Parse what userspace wrote to /status or an entry file: '1' enables,
 * '0' disables and '-1' removes the entry or all entries.
 */
static int parse_command(const char *s, size_t count)
{
	if (count > MAX_COMMAND_LENGTH)
		return -EINVAL;
	if (!count)
		return BM_CMD_IGNORE;
	if (s[count - 1] == '\n')
		count--;
	if (count == 1 && s[0] == '0')
		return BM_CMD_DISABLE;
	if (count == 1 && s[0] == '1')
		return BM_CMD_ENABLE;
	if (count == 2 && s[0] == '-' && s[1] == '1')
		return BM_CMD_REMOVE;
	return -EINVAL;
}

/* Copy in a command from a file that takes nothing else, and parse it. */
static int read_command(const char __user *buffer, size_t count)
{
	char s[MAX_COMMAND_LENGTH + 1];

	if (count > sizeof(s) - 1)
		return -EINVAL;
	if (copy_from_user(s, buffer, count))
		return -EFAULT;
	return parse_command(s, count);
}

/* generic stuff */

/* The root directory's inode; its lock serializes configuring an instance. */
static struct inode *bm_root_inode(struct super_block *sb)
{
	return d_inode(sb->s_root);
}

static void bm_seq_hex(struct seq_file *m, const u8 *data, int size)
{
	for (int i = 0; i < size; i++)
		seq_printf(m, "%02x", data[i]);
}

static int bm_entry_show(struct seq_file *m, void *unused)
{
	struct binfmt_misc_entry *e = m->private;

	if (test_bit(MISC_FMT_ENABLED_BIT, &e->flags))
		seq_puts(m, "enabled\n");
	else
		seq_puts(m, "disabled\n");

	if (test_bit(MISC_FMT_BPF_BIT, &e->flags)) {
		struct binfmt_misc_interp *interp;

		seq_printf(m, "bpf %s\n", e->bpf_ops->name);
		/*
		 * A staged entry's set can still grow, so every binding is
		 * rcu-published. The open file pins the entry and with it
		 * every node, so rcu is for the tearing, not the lifetime.
		 */
		rcu_read_lock();
		list_for_each_entry_rcu(interp, &e->interps, list)
			seq_printf(m, "bpf-interpreter %s %s\n",
				   interp->name, interp->path);
		rcu_read_unlock();
	} else {
		seq_printf(m, "interpreter %s\n", e->interpreter);
	}

	/* print the special flags */
	seq_puts(m, "flags: ");
	for (int i = 0; i < ARRAY_SIZE(misc_flags); i++)
		if (e->flags & misc_flags[i].flag)
			seq_putc(m, misc_flags[i].c);
	seq_putc(m, '\n');

	if (test_bit(MISC_FMT_BPF_BIT, &e->flags)) {
		/* The program does the matching. */
	} else if (!test_bit(MISC_FMT_MAGIC_BIT, &e->flags)) {
		seq_printf(m, "extension .%s\n", e->magic);
	} else {
		seq_printf(m, "offset %i\nmagic ", e->offset);
		bm_seq_hex(m, e->magic, e->size);
		if (e->mask) {
			seq_puts(m, "\nmask ");
			bm_seq_hex(m, e->mask, e->size);
		}
		seq_putc(m, '\n');
	}
	return 0;
}

static struct inode *bm_get_inode(struct super_block *sb, umode_t mode)
{
	struct inode *inode = new_inode(sb);

	if (inode) {
		inode->i_ino = get_next_ino();
		inode->i_mode = mode;
		simple_inode_init_ts(inode);
	}
	return inode;
}

/**
 * i_binfmt_misc - retrieve struct binfmt_misc from a binfmt_misc inode
 * @inode: inode of the relevant binfmt_misc instance
 *
 * This helper retrieves struct binfmt_misc from a binfmt_misc inode. This can
 * be done without any memory barriers because we are guaranteed that
 * user_ns->binfmt_misc is fully initialized. It was fully initialized when the
 * binfmt_misc mount was first created.
 *
 * Return: struct binfmt_misc of the relevant binfmt_misc instance
 */
static struct binfmt_misc *i_binfmt_misc(struct inode *inode)
{
	return inode->i_sb->s_user_ns->binfmt_misc;
}

/**
 * bm_evict_inode - cleanup data associated with @inode
 * @inode: inode to which the data is attached
 *
 * Cleanup the binary type handler data associated with @inode if a binary type
 * entry is removed or the filesystem is unmounted and the super block is
 * shutdown.
 *
 * If the ->evict call was not caused by a super block shutdown but by
 * removing the entry via bm_{entry,status}_write() or unlink(2) the entry
 * will have already been removed from the list. We keep the hlist_unhashed()
 * check to make that explicit.
*/
static void bm_evict_inode(struct inode *inode)
{
	struct binfmt_misc_entry *e = inode->i_private;

	clear_inode(inode);

	if (e) {
		struct binfmt_misc *misc;

		misc = i_binfmt_misc(inode);
		spin_lock(&misc->entries_lock);
		if (!hlist_unhashed(&e->node))
			hlist_del_init_rcu(&e->node);
		spin_unlock(&misc->entries_lock);
		put_binfmt_handler(e);
	}
}

/**
 * unlink_binfmt_handler - unhash a binary type handler
 * @misc: handle to binfmt_misc instance
 * @e: binary type handler to unhash
 *
 * Adding and removing entries via bm_{entry,register,status}_write() and
 * unlink(2) happens under the exclusively held inode lock of the root
 * dentry keeping the list stable for writers. load_misc_binary() walks it
 * concurrently under SRCU. The entries_lock is only held around the actual
 * unlink to serialize against bm_evict_inode() which unlinks entries
 * during umount without holding the root inode lock.
 */
static void unlink_binfmt_handler(struct binfmt_misc *misc,
				  struct binfmt_misc_entry *e)
{
	spin_lock(&misc->entries_lock);
	hlist_del_init_rcu(&e->node);
	spin_unlock(&misc->entries_lock);
}

/**
 * remove_binfmt_handler - remove a binary type handler
 * @misc: handle to binfmt_misc instance
 * @e: binary type handler to remove
 *
 * Remove a binary type handler from the list of binary type handlers and
 * remove its associated dentry.
 */
static void remove_binfmt_handler(struct binfmt_misc *misc,
				  struct binfmt_misc_entry *e)
{
	unlink_binfmt_handler(misc, e);
	locked_recursive_removal(e->dentry, NULL);
}

/* Remove @e unless it was already removed. */
static void bm_remove_entry(struct binfmt_misc_entry *e, struct super_block *sb)
{
	struct inode *root = bm_root_inode(sb);

	inode_lock_nested(root, I_MUTEX_PARENT);
	/* A staged entry is not hashed; the dentry says if it was removed. */
	if (!d_unhashed(e->dentry))
		remove_binfmt_handler(i_binfmt_misc(root), e);
	inode_unlock(root);
}

/* Remove all entries of the binfmt_misc instance @misc belonging to @sb. */
static void bm_remove_all_entries(struct binfmt_misc *misc,
				  struct super_block *sb)
{
	struct inode *root = bm_root_inode(sb);
	struct dentry *child = NULL;

	inode_lock_nested(root, I_MUTEX_PARENT);
	/*
	 * Walk the directory rather than the search list: a staged entry
	 * is in the former but not yet in the latter. The control files
	 * carry no entry and stay.
	 */
	while ((child = find_next_child(sb->s_root, child))) {
		struct binfmt_misc_entry *e = d_inode(child)->i_private;

		if (e)
			remove_binfmt_handler(misc, e);
	}
	inode_unlock(root);
}

/**
 * bm_unlink - remove a binary type handler via unlink(2)
 * @dir: inode of the root directory
 * @dentry: entry file to remove
 *
 * Removing the entry file removes its binary type handler, exactly like
 * writing -1 to it does. The status and register control files can't be
 * removed. The VFS calls this with the root inode lock held which
 * serializes against the write based add and remove paths.
 */
static int bm_unlink(struct inode *dir, struct dentry *dentry)
{
	struct binfmt_misc_entry *e = d_inode(dentry)->i_private;

	if (!e)
		return -EPERM;

	unlink_binfmt_handler(i_binfmt_misc(dir), e);
	return simple_unlink(dir, dentry);
}

static const struct inode_operations bm_dir_inode_operations = {
	.lookup		= simple_lookup,
	.unlink		= bm_unlink,
};

/* /<entry> */

static int bm_entry_open(struct inode *inode, struct file *file)
{
	int ret;

	ret = single_open(file, bm_entry_show, inode->i_private);
	if (ret)
		return ret;

	/* seq_open() clears FMODE_PWRITE, bm_entry_write() takes any offset */
	if (file->f_mode & FMODE_WRITE)
		file->f_mode |= FMODE_PWRITE;
	return 0;
}

/*
 * Longest '+<name> <path>' a write can spell, and with it the longest
 * command an entry file takes: the two delimiters and a newline on top of
 * the two names.
 */
#define MAX_BINDING_LENGTH (BINFMT_MISC_INTERP_NAME_MAX + PATH_MAX + 3)

/**
 * bm_entry_add_interp - bind another interpreter to a staged entry
 * @e: the entry
 * @file: the entry file being written to, for its credentials
 * @buf: the '+<name> <path>' command, parsed in place and owned by the caller
 * @count: its length
 *
 * A 'D' entry is registered outside the search list, which is what leaves
 * it open to being configured: it cannot be matched, so no exec can be
 * holding its interpreters and the set can still grow. Its first enable
 * publishes it and ends that. One interpreter per write, up to
 * BINFMT_MISC_INTERP_MAX of them, none of which has to fit in a register
 * string.
 *
 * Return: @count on success, a negative errno on failure
 */
static ssize_t bm_entry_add_interp(struct binfmt_misc_entry *e,
				   struct file *file, char *buf, size_t count)
{
	struct file *f __free(close_interp_file) = NULL;
	struct inode *root = bm_root_inode(file_inode(file)->i_sb);
	size_t nlen, plen;
	char *name, *path;
	int retval;

	/* Settled before the open: type is fixed, publication is permanent. */
	if (!test_bit(MISC_FMT_BPF_BIT, &e->flags))
		return -EINVAL;
	if (!hlist_unhashed_lockless(&e->node))
		return -EBUSY;

	/* '+<name> <path>': the path is everything past the first space. */
	name = buf + 1;
	path = strchr(name, ' ');
	if (!path)
		return -EINVAL;
	*path++ = '\0';

	plen = strlen(path);
	/* The command has to end at the write, like a register string. */
	if (path + plen != buf + count)
		return -EINVAL;
	if (plen && path[plen - 1] == '\n')
		path[--plen] = '\0';
	/* Resolved now, so a relative path would name the writer's cwd. */
	if (path[0] != '/')
		return -EINVAL;

	nlen = path - name - 1;
	if (!nlen || nlen > BINFMT_MISC_INTERP_NAME_MAX)
		return -EINVAL;
	/* The name prints between delimiters, so keep it a printable word. */
	for (const char *p = name; *p; p++)
		if (!isascii(*p) || !isgraph(*p))
			return -EINVAL;

	/* Opened before the lock: resolving it may walk this very filesystem. */
	f = open_interp_file(file->f_cred, path);
	if (IS_ERR(f))
		return PTR_ERR(f);

	inode_lock(root);
	if (d_unhashed(e->dentry))
		retval = -ENOENT;	/* removed while we were opening it */
	else if (!hlist_unhashed(&e->node))
		retval = -EBUSY;	/* published while we were opening it */
	else
		retval = entry_attach_interpreter(e, name, path, f);
	inode_unlock(root);
	if (retval)
		return retval;

	/* The file is owned by the entry now. */
	retain_and_null_ptr(f);
	return count;
}

static ssize_t bm_entry_write(struct file *file, const char __user *buffer,
				size_t count, loff_t *ppos)
{
	struct inode *inode = file_inode(file);
	struct binfmt_misc_entry *e = inode->i_private;
	char *buf __free(kfree) = NULL;
	int res;

	/* A binding is the longest command this file takes. */
	if (count > MAX_BINDING_LENGTH)
		return -E2BIG;

	buf = memdup_user_nul(buffer, count);
	if (IS_ERR(buf))
		return PTR_ERR(buf);

	/* '+<name> <path>' binds an interpreter, everything else toggles. */
	if (buf[0] == '+')
		return bm_entry_add_interp(e, file, buf, count);

	res = parse_command(buf, count);

	switch (res) {
	case BM_CMD_DISABLE:
		clear_bit(MISC_FMT_ENABLED_BIT, &e->flags);
		break;
	case BM_CMD_ENABLE: {
		struct inode *root = bm_root_inode(inode->i_sb);

		/*
		 * The first enable publishes a 'D' entry into the search
		 * list, whole. The lock keeps that ordered against a second
		 * enable, against removal - a removed entry has nothing left
		 * to publish - and against binding: what can be matched can
		 * no longer be configured.
		 */
		inode_lock(root);
		set_bit(MISC_FMT_ENABLED_BIT, &e->flags);
		if (hlist_unhashed(&e->node) && !d_unhashed(e->dentry)) {
			struct binfmt_misc *misc = i_binfmt_misc(inode);

			spin_lock(&misc->entries_lock);
			hlist_add_head_rcu(&e->node, &misc->entries);
			spin_unlock(&misc->entries_lock);
		}
		inode_unlock(root);
		break;
	}
	case BM_CMD_REMOVE:
		bm_remove_entry(e, inode->i_sb);
		break;
	default:
		return res;
	}

	return count;
}

static const struct file_operations bm_entry_operations = {
	.open		= bm_entry_open,
	.read		= seq_read,
	.write		= bm_entry_write,
	.llseek		= seq_lseek,
	.release	= single_release,
};

/* /register */

/* add to filesystem */
static int add_entry(struct binfmt_misc_entry *e, struct super_block *sb)
{
	struct dentry *dentry = simple_start_creating(sb->s_root, e->name);
	struct inode *inode;
	struct binfmt_misc *misc;

	if (IS_ERR(dentry))
		return PTR_ERR(dentry);

	inode = bm_get_inode(sb, S_IFREG | 0644);
	if (unlikely(!inode)) {
		simple_done_creating(dentry);
		return -ENOMEM;
	}

	e->dentry = dentry;
	inode->i_private = e;
	inode->i_fop = &bm_entry_operations;

	d_make_persistent(dentry, inode);
	/* A 'D' entry stays out of the search list until its first enable. */
	if (test_bit(MISC_FMT_ENABLED_BIT, &e->flags)) {
		misc = i_binfmt_misc(inode);
		spin_lock(&misc->entries_lock);
		hlist_add_head_rcu(&e->node, &misc->entries);
		spin_unlock(&misc->entries_lock);
	}
	simple_done_creating(dentry);
	return 0;
}

static ssize_t bm_register_write(struct file *file, const char __user *buffer,
			       size_t count, loff_t *ppos)
{
	struct binfmt_misc_entry *e __free(put_binfmt_handler) = NULL;
	struct super_block *sb = file_inode(file)->i_sb;
	int err;

	e = create_entry(buffer, count);
	if (IS_ERR(e))
		return PTR_ERR(e);

	if (test_bit(MISC_FMT_BPF_BIT, &e->flags)) {
		e->bpf_ops = binfmt_misc_get_ops(sb->s_user_ns, e->bpf_ops_name);
		if (!e->bpf_ops) {
			pr_notice("register: no bpf handler named %s\n",
				  e->bpf_ops_name);
			return -ENOENT;
		}
	}

	if (e->flags & MISC_FMT_OPEN_FILE) {
		struct file *f = open_interp_file(file->f_cred, e->interpreter);

		if (IS_ERR(f))
			return PTR_ERR(f);
		err = entry_attach_interpreter(e, "", e->interpreter, f);
		if (err) {
			close_interp_file(f);
			return err;
		}
	}

	err = add_entry(e, sb);
	if (err)
		return err;

	/* The entry is owned by its inode now. */
	retain_and_null_ptr(e);
	return count;
}

static const struct file_operations bm_register_operations = {
	.write		= bm_register_write,
	.llseek		= noop_llseek,
};

/* /status */

static ssize_t
bm_status_read(struct file *file, char __user *buf, size_t nbytes, loff_t *ppos)
{
	struct binfmt_misc *misc;
	const char *s;

	misc = i_binfmt_misc(file_inode(file));
	s = READ_ONCE(misc->enabled) ? "enabled\n" : "disabled\n";
	return simple_read_from_buffer(buf, nbytes, ppos, s, strlen(s));
}

static ssize_t bm_status_write(struct file *file, const char __user *buffer,
		size_t count, loff_t *ppos)
{
	struct binfmt_misc *misc;
	int res = read_command(buffer, count);

	misc = i_binfmt_misc(file_inode(file));
	switch (res) {
	case BM_CMD_DISABLE:
		WRITE_ONCE(misc->enabled, false);
		break;
	case BM_CMD_ENABLE:
		WRITE_ONCE(misc->enabled, true);
		break;
	case BM_CMD_REMOVE:
		bm_remove_all_entries(misc, file_inode(file)->i_sb);
		break;
	default:
		return res;
	}

	return count;
}

static const struct file_operations bm_status_operations = {
	.read		= bm_status_read,
	.write		= bm_status_write,
	.llseek		= default_llseek,
};

/* Superblock handling */

static const struct super_operations bm_super_ops = {
	.statfs		= simple_statfs,
	.evict_inode	= bm_evict_inode,
};

static int bm_fill_super(struct super_block *sb, struct fs_context *fc)
{
	int err;
	struct user_namespace *user_ns = sb->s_user_ns;
	struct binfmt_misc *misc;
	static const struct tree_descr bm_files[] = {
		[2] = {"status", &bm_status_operations, S_IWUSR|S_IRUGO},
		[3] = {"register", &bm_register_operations, S_IWUSR},
		/* last one */ {""}
	};

	/* The fscontext fd may have been passed to another user namespace. */
	if (user_ns != current_user_ns())
		return -EINVAL;

	/* Never exec off this instance and never let anything stack on it. */
	sb->s_iflags |= SB_I_NOEXEC | SB_I_NODEV;
	sb->s_stack_depth = FILESYSTEM_MAX_STACK_DEPTH;

	/*
	 * Lazily allocate a new binfmt_misc instance for this namespace, i.e.
	 * do it here during the first mount of binfmt_misc. We don't need to
	 * waste memory for every user namespace allocation. It's likely much
	 * more common to not mount a separate binfmt_misc instance than it is
	 * to mount one.
	 *
	 * While multiple superblocks can exist they are keyed by userns in
	 * s_fs_info for binfmt_misc. Hence, the vfs guarantees that
	 * bm_fill_super() is called exactly once whenever a binfmt_misc
	 * superblock for a userns is created. This in turn lets us conclude
	 * that when a binfmt_misc superblock is created for the first time for
	 * a userns there's no one racing us. Therefore we don't need any
	 * barriers when we dereference binfmt_misc.
	 */
	misc = user_ns->binfmt_misc;
	if (!misc) {
		/*
		 * If it turns out that most user namespaces actually want to
		 * register their own binary type handler and therefore all
		 * create their own separate binfmt_misc mounts we should
		 * consider turning this into a kmem cache.
		 */
		misc = kzalloc_obj(struct binfmt_misc);
		if (!misc)
			return -ENOMEM;

		INIT_HLIST_HEAD(&misc->entries);
		spin_lock_init(&misc->entries_lock);

		/* Pairs with smp_load_acquire() in current_binfmt_misc(). */
		smp_store_release(&user_ns->binfmt_misc, misc);
	}

	/*
	 * When the binfmt_misc superblock for this userns is shutdown
	 * ->enabled might have been set to false and we don't reinitialize
	 * ->enabled again during shutdown as someone might already be mounting
	 * binfmt_misc again. It also would be pointless since by then we know
	 * that the binary type list for this binfmt_misc mount is empty making
	 * load_misc_binary() return -ENOEXEC independent of whether ->enabled
	 * is true. Instead, if someone mounts binfmt_misc for the first time or
	 * again we simply reset ->enabled to true.
	 */
	WRITE_ONCE(misc->enabled, true);

	err = simple_fill_super(sb, BINFMTFS_MAGIC, bm_files);
	if (err)
		return err;

	sb->s_op = &bm_super_ops;
	d_inode(sb->s_root)->i_op = &bm_dir_inode_operations;
	return 0;
}

static void bm_free(struct fs_context *fc)
{
	if (fc->s_fs_info)
		put_user_ns(fc->s_fs_info);
}

static int bm_get_tree(struct fs_context *fc)
{
	return get_tree_keyed(fc, bm_fill_super, get_user_ns(fc->user_ns));
}

static const struct fs_context_operations bm_context_ops = {
	.free		= bm_free,
	.get_tree	= bm_get_tree,
};

static void bm_kill_sb(struct super_block *sb)
{
	struct user_namespace *user_ns = sb->s_fs_info;

	kill_anon_super(sb);
	put_user_ns(user_ns);
}

static int bm_init_fs_context(struct fs_context *fc)
{
	fc->ops = &bm_context_ops;
	return 0;
}

static struct linux_binfmt misc_format = {
	.module = THIS_MODULE,
	.load_binary = load_misc_binary,
};

static struct file_system_type bm_fs_type = {
	.owner		= THIS_MODULE,
	.name		= "binfmt_misc",
	.init_fs_context = bm_init_fs_context,
	.fs_flags	= FS_USERNS_MOUNT,
	.kill_sb	= bm_kill_sb,
};
MODULE_ALIAS_FS("binfmt_misc");

static int __init init_misc_binfmt(void)
{
	int err = register_filesystem(&bm_fs_type);
	if (!err)
		insert_binfmt(&misc_format);
	return err;
}

static void __exit exit_misc_binfmt(void)
{
	unregister_binfmt(&misc_format);
	unregister_filesystem(&bm_fs_type);
	/* Flush pending bm_entry_free_rcu() callbacks before the text goes. */
	srcu_barrier(&bm_entries_srcu);
}

core_initcall(init_misc_binfmt);
module_exit(exit_misc_binfmt);
MODULE_DESCRIPTION("Kernel support for miscellaneous binaries");
MODULE_LICENSE("GPL");