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authorChristian Brauner <brauner@kernel.org>2026-08-03 23:36:38 +0200
committerChristian Brauner <brauner@kernel.org>2026-08-03 23:36:38 +0200
commit68aabd01ddd26ced458a9e5716a640eaf8e4b7a6 (patch)
tree9b5def7de1374c3a5707f3a3631a9ab32f0f9e8d
parente98067e72ff81b054bda8135e1c348d13f54e553 (diff)
parenta0ff406303591e714ca82a3fede627f9f7c13231 (diff)
Merge patch series "binfmt_misc: bound the interpreters an entry can pre-open"
Christian Brauner <brauner@kernel.org> says: An 'F' entry opens its interpreter at registration and every exec runs a clone of it. A 'B' entry does the same for each interpreter it binds. That file stays open for as long as the entry lives. So it pins the file, its inode, the mount it came from and that mount's superblock. An entry binds at most 100 interpreters, but nothing caps the entries. binfmt_misc is container mountable so all of this is reachable by unprivileged users. While the pins go away when the instance is unmounted, it's still weird for an unprivileged namespace to be allowed to do this. And the fix is simple. Charge each binding to the user namespace and uid that makes it against a new UCOUNT_BINFMT_MISC_INTERPRETERS and refuse with -ENOSPC when the limit is hit. A per-instance cap won't do. Instances are keyed on the user namespace, so whatever constant I pick gets multiplied by however many namespaces the caller cares to create. inc_ucount() charges the namespace and every one of its ancestors, and a namespace can only ever raise its own limit, so nesting buys nothing. The knob is /proc/sys/user/max_binfmt_misc_interpreters, per namespace like every other ucount. I left it at the max_threads/2 default that fork_init() hands a new ucount type. Nothing anyone runs today comes anywhere near that. Selftests for all of it, including that a nested namespace can't buy itself budget. * patches from https://patch.msgid.link/20260803-work-binfmt_misc-interplimit-v1-0-4a2435500bd9@kernel.org: binfmt_misc: document the pre-opened interpreter limit selftests/exec: test the pre-opened interpreter limit binfmt_misc: correctly account pre-opened interpreters Link: https://patch.msgid.link/20260803-work-binfmt_misc-interplimit-v1-0-4a2435500bd9@kernel.org Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>
-rw-r--r--Documentation/admin-guide/binfmt-misc.rst12
-rw-r--r--fs/binfmt_misc.c16
-rw-r--r--include/linux/binfmt_misc.h3
-rw-r--r--include/linux/user_namespace.h3
-rw-r--r--kernel/ucount.c6
-rw-r--r--tools/testing/selftests/exec/.gitignore1
-rw-r--r--tools/testing/selftests/exec/Makefile6
-rw-r--r--tools/testing/selftests/exec/binfmt_misc_bpf.c70
-rw-r--r--tools/testing/selftests/exec/binfmt_misc_interplimit.c232
9 files changed, 343 insertions, 6 deletions
diff --git a/Documentation/admin-guide/binfmt-misc.rst b/Documentation/admin-guide/binfmt-misc.rst
index 622b5d8c8995..d26b63a27c25 100644
--- a/Documentation/admin-guide/binfmt-misc.rst
+++ b/Documentation/admin-guide/binfmt-misc.rst
@@ -128,6 +128,11 @@ There are some restrictions:
named by an absolute path. It is opened when the binary is executed, so
a relative one would be resolved against the working directory of
whoever runs the binary
+ - the amount of pre-opened interpreters by ``F``, or bound to a ``B`` entry
+ is limited by the ``/proc/sys/user/max_binfmt_misc_interpreters`` sysctl. A
+ registration past the limit is refused with ``-ENOSPC``. This limits an
+ unprivileged namespace pinning files. A nested namespace can raise only its
+ own limit and every ancestor is charged too
To use binfmt_misc you have to mount it first. You can mount it with
@@ -215,10 +220,9 @@ with the credentials the entry file was opened with, exactly the way ``F``
pre-opens a static entry's interpreter; the paths must be absolute. The
path is everything past the first space, so there is nothing it cannot
express, and no interpreter has to fit in a register string. An entry
-binds at most 100 interpreters; a write past that is refused with
-``-ENOSPC``. To bind a file that has no path of its own - already
-unlinked, a ``memfd``, or reachable only in another mount namespace -
-open it and write ``/proc/self/fd/N``.
+binds at most 100 interpreters, and each one is charged against
+``max_binfmt_misc_interpreters`` like any other binding. A write past either
+limit is refused with ``-ENOSPC``.
The ``load`` program then selects one per exec by name with the
``bpf_binprm_select_interp()`` kfunc, and every exec runs a clone of the
diff --git a/fs/binfmt_misc.c b/fs/binfmt_misc.c
index ad8c4f64bf10..a3aa42fd5761 100644
--- a/fs/binfmt_misc.c
+++ b/fs/binfmt_misc.c
@@ -289,6 +289,7 @@ static void entry_put_interpreters(struct binfmt_misc_entry *e)
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);
}
}
@@ -307,7 +308,8 @@ static void entry_put_interpreters(struct binfmt_misc_entry *e)
* 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, a negative errno on failure
+ * 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,
@@ -315,22 +317,32 @@ static int entry_attach_interpreter(struct binfmt_misc_entry *e,
{
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)
+ 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);
diff --git a/include/linux/binfmt_misc.h b/include/linux/binfmt_misc.h
index 072e4b3dd78d..8045b10dd3e5 100644
--- a/include/linux/binfmt_misc.h
+++ b/include/linux/binfmt_misc.h
@@ -7,6 +7,7 @@
struct bpf_prog;
struct file;
struct linux_binprm;
+struct ucounts;
struct user_namespace;
#define BINFMT_MISC_OPS_NAME_MAX 16
@@ -21,6 +22,7 @@ struct user_namespace;
* struct binfmt_misc_interp - an interpreter an entry was registered with
* @list: link in the entry's list, in registration order
* @file: the file, opened at registration and never resolved again
+ * @ucounts: the UCOUNT_BINFMT_MISC_INTERPRETERS charge the binding took
* @path: the path it was registered under, used as the name the interpreter
* runs under; stored after @name in the same allocation
* @name: the name the load program selects it by; empty for the fixed
@@ -33,6 +35,7 @@ struct user_namespace;
struct binfmt_misc_interp {
struct list_head list;
struct file *file;
+ struct ucounts *ucounts;
const char *path;
char name[];
};
diff --git a/include/linux/user_namespace.h b/include/linux/user_namespace.h
index 9c3be157397e..e38d9e60569f 100644
--- a/include/linux/user_namespace.h
+++ b/include/linux/user_namespace.h
@@ -58,6 +58,9 @@ enum ucount_type {
UCOUNT_FANOTIFY_GROUPS,
UCOUNT_FANOTIFY_MARKS,
#endif
+#if IS_ENABLED(CONFIG_BINFMT_MISC)
+ UCOUNT_BINFMT_MISC_INTERPRETERS,
+#endif
UCOUNT_COUNTS,
};
diff --git a/kernel/ucount.c b/kernel/ucount.c
index d6dc3e859f12..ec8b1445e287 100644
--- a/kernel/ucount.c
+++ b/kernel/ucount.c
@@ -4,6 +4,7 @@
#include <linux/sysctl.h>
#include <linux/slab.h>
#include <linux/cred.h>
+#include <linux/export.h>
#include <linux/hash.h>
#include <linux/kmemleak.h>
#include <linux/user_namespace.h>
@@ -89,6 +90,9 @@ static const struct ctl_table user_table[] = {
UCOUNT_ENTRY("max_fanotify_groups"),
UCOUNT_ENTRY("max_fanotify_marks"),
#endif
+#if IS_ENABLED(CONFIG_BINFMT_MISC)
+ UCOUNT_ENTRY("max_binfmt_misc_interpreters"),
+#endif
};
#endif /* CONFIG_SYSCTL */
@@ -233,6 +237,7 @@ fail:
put_ucounts(ucounts);
return NULL;
}
+EXPORT_SYMBOL_FOR_MODULES(inc_ucount, "binfmt_misc");
void dec_ucount(struct ucounts *ucounts, enum ucount_type type)
{
@@ -243,6 +248,7 @@ void dec_ucount(struct ucounts *ucounts, enum ucount_type type)
}
put_ucounts(ucounts);
}
+EXPORT_SYMBOL_FOR_MODULES(dec_ucount, "binfmt_misc");
long inc_rlimit_ucounts(struct ucounts *ucounts, enum rlimit_type type, long v)
{
diff --git a/tools/testing/selftests/exec/.gitignore b/tools/testing/selftests/exec/.gitignore
index fbbb1600ddb9..e42ecd4c908d 100644
--- a/tools/testing/selftests/exec/.gitignore
+++ b/tools/testing/selftests/exec/.gitignore
@@ -20,6 +20,7 @@ xxxxxxxx*
pipe
S_I*.test
binfmt_misc_bpf
+binfmt_misc_interplimit
binfmt_bpf_interp
binfmt_bpf_app
binfmt_misc_transparent
diff --git a/tools/testing/selftests/exec/Makefile b/tools/testing/selftests/exec/Makefile
index 410c93606a0c..b640af8f02b5 100644
--- a/tools/testing/selftests/exec/Makefile
+++ b/tools/testing/selftests/exec/Makefile
@@ -25,6 +25,10 @@ TEST_GEN_PROGS += check-exec
# or an 'F' entry can pin the instance that owns it. Unprivileged, no bpf.
TEST_GEN_PROGS += binfmt_misc_selfpin
+# The interpreters an 'F' or 'B' entry pre-opens are charged against
+# UCOUNT_BINFMT_MISC_INTERPRETERS. Unprivileged, no bpf.
+TEST_GEN_PROGS += binfmt_misc_interplimit
+
# 'D' (register disabled) binfmt_misc test: an entry that exists but does
# not dispatch until it is enabled. Static magic entry, no bpf toolchain.
TEST_GEN_PROGS += binfmt_misc_disabled
@@ -104,6 +108,8 @@ $(OUTPUT)/script-noexec.inc: $(CHECK_EXEC_SAMPLES)/script-noexec.inc
# CFLAGS for every program in this directory.
$(OUTPUT)/binfmt_misc_selfpin: CFLAGS += $(TOOLS_INCLUDES)
$(OUTPUT)/binfmt_misc_selfpin: ../filesystems/utils.c
+$(OUTPUT)/binfmt_misc_interplimit: CFLAGS += $(TOOLS_INCLUDES)
+$(OUTPUT)/binfmt_misc_interplimit: ../filesystems/utils.c
# --- binfmt_misc bpf ('B') handler test ---------------------------------
# The struct_ops bpf objects are compiled against the running kernel's BTF.
diff --git a/tools/testing/selftests/exec/binfmt_misc_bpf.c b/tools/testing/selftests/exec/binfmt_misc_bpf.c
index 2c7b63075f1d..b2a4518901b0 100644
--- a/tools/testing/selftests/exec/binfmt_misc_bpf.c
+++ b/tools/testing/selftests/exec/binfmt_misc_bpf.c
@@ -38,6 +38,7 @@
#define _GNU_SOURCE
#include <elf.h>
#include <limits.h>
+#include <sched.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
@@ -65,6 +66,9 @@
#define BIND_RISCV_PATH "/tmp/binfmt_bind_riscv"
#define BIND_EXPECT "BIND_RAN "
#define BIND_MAX 100
+#define INTERP_LIMIT "/proc/sys/user/max_binfmt_misc_interpreters"
+/* Exit status of the binding child when it cannot set up a budget of its own. */
+#define BIND_NO_BUDGET 200
/* A minimal 64-bit little-endian ELF header, padded to the read size. */
static int create_fake_elf(const char *path, unsigned short machine)
@@ -378,6 +382,57 @@ static int entry_bind(const char *entry, const char *name, const char *path)
return entry_command(entry, cmd);
}
+/* Set the interpreter budget of this namespace. */
+static int write_interp_limit(const char *val)
+{
+ ssize_t n;
+ int fd;
+
+ fd = open(INTERP_LIMIT, O_WRONLY | O_CLOEXEC);
+ if (fd < 0)
+ return -1;
+ n = write(fd, val, strlen(val));
+ close(fd);
+ return n < 0 ? -1 : 0;
+}
+
+/*
+ * The errno a bind is refused with when the writer is a child that has spent
+ * the budget of a user namespace of its own, 0 if it succeeded and -1 if the
+ * child could not set itself up. The fd is opened here and inherited, so the
+ * interpreter is still opened with this process's credentials.
+ */
+static int bind_out_of_budget(const char *entry, const char *name,
+ const char *path)
+{
+ char cmd[PATH_MAX], file[PATH_MAX];
+ int fd, status, retval;
+ pid_t pid;
+
+ snprintf(file, sizeof(file), BINFMT_DIR "/%s", entry);
+ snprintf(cmd, sizeof(cmd), "+%s %s\n", name, path);
+
+ fd = open(file, O_WRONLY | O_CLOEXEC);
+ if (fd < 0)
+ return -1;
+
+ pid = fork();
+ if (pid == 0) {
+ ssize_t n;
+
+ /* A namespace of its own, with nothing left in it to spend. */
+ if (unshare(CLONE_NEWUSER) || write_interp_limit("0"))
+ _exit(BIND_NO_BUDGET);
+ n = write(fd, cmd, strlen(cmd));
+ _exit(n < 0 ? errno : 0);
+ }
+ close(fd);
+ if (pid < 0 || waitpid(pid, &status, 0) != pid || !WIFEXITED(status))
+ return -1;
+ retval = WEXITSTATUS(status);
+ return retval == BIND_NO_BUDGET ? -1 : retval;
+}
+
FIXTURE(bound_interp) {
char obj[PATH_MAX];
struct bpf_case c;
@@ -565,4 +620,19 @@ TEST_F(bound_interp, capped_bindings)
EXPECT_EQ(entry_bind("test_interp_bind", "over", BIND_FIRST), -ENOSPC);
}
+/* A binding pins a file: it is charged, and refused once the budget is out. */
+TEST_F(bound_interp, bindings_are_charged)
+{
+ int err = bind_out_of_budget("test_interp_bind", "third", BIND_FIRST);
+
+ if (err < 0)
+ SKIP(return, "no user namespaces or no " INTERP_LIMIT);
+
+ /* The charge follows the writer, not the entry file it writes to. */
+ EXPECT_EQ(err, ENOSPC);
+
+ /* The budget was the only thing in the way. */
+ EXPECT_EQ(entry_bind("test_interp_bind", "third", BIND_FIRST), 0);
+}
+
TEST_HARNESS_MAIN
diff --git a/tools/testing/selftests/exec/binfmt_misc_interplimit.c b/tools/testing/selftests/exec/binfmt_misc_interplimit.c
new file mode 100644
index 000000000000..bf611c551784
--- /dev/null
+++ b/tools/testing/selftests/exec/binfmt_misc_interplimit.c
@@ -0,0 +1,232 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * A pre-opened interpreter - what 'F' gives a static entry and what a 'B'
+ * entry binds - keeps a file open for as long as the entry lives, so it pins
+ * the mount it came from. It costs no file descriptor, and binfmt_misc is
+ * FS_USERNS_MOUNT, so an unprivileged user namespace can create them without
+ * bound. Check that UCOUNT_BINFMT_MISC_INTERPRETERS bounds it, that an entry
+ * that pre-opens nothing is not charged, that removing an entry gives the
+ * charge back, and that nesting a user namespace does not evade it.
+ *
+ * Runs unprivileged in a user namespace.
+ */
+#define _GNU_SOURCE
+#include <errno.h>
+#include <fcntl.h>
+#include <limits.h>
+#include <stdio.h>
+#include <string.h>
+#include <sys/mount.h>
+#include <sys/stat.h>
+#include <unistd.h>
+
+#include "../filesystems/utils.h"
+#include "kselftest_harness.h"
+
+#define MNT "/tmp/binfmt_interplimit"
+#define NESTED_MNT "/tmp/binfmt_interplimit_nested"
+#define LIMIT_SYSCTL "/proc/sys/user/max_binfmt_misc_interpreters"
+
+#define MAGIC "\\xde\\xad"
+/* Not on the instance, and unlike /bin/true it always exists. */
+#define INTERP "/proc/self/exe"
+
+/* Small enough to fill by hand, big enough that a refund is visible. */
+#define LIMIT 4
+
+/* What UCOUNT_ENTRY() lets a namespace raise its own limit to. */
+#define LIMIT_MAX "2147483647"
+
+static int ensure_dir(const char *path)
+{
+ if (mkdir(path, 0755) && errno != EEXIST)
+ return -1;
+ return 0;
+}
+
+/* Write @val to @path, preserving write(2)'s errno for the caller. */
+static int write_keep_errno(const char *path, const char *val)
+{
+ int fd, saved;
+ ssize_t n;
+
+ fd = open(path, O_WRONLY | O_CLOEXEC);
+ if (fd < 0)
+ return -1;
+ n = write(fd, val, strlen(val));
+ saved = errno;
+ close(fd);
+ errno = saved;
+ return n < 0 ? -1 : 0;
+}
+
+static int set_limit(const char *val)
+{
+ return write_keep_errno(LIMIT_SYSCTL, val);
+}
+
+static int register_at(const char *mnt, const char *rule)
+{
+ char path[PATH_MAX];
+
+ snprintf(path, sizeof(path), "%s/register", mnt);
+ return write_keep_errno(path, rule);
+}
+
+/* An 'F' entry: one interpreter pre-opened at registration, one charge. */
+static int register_fixed(const char *mnt, const char *name)
+{
+ char rule[PATH_MAX];
+
+ snprintf(rule, sizeof(rule), ":%s:M::" MAGIC "::" INTERP ":F", name);
+ return register_at(mnt, rule);
+}
+
+/* The same entry without 'F': the interpreter is opened per exec instead. */
+static int register_plain(const char *mnt, const char *name)
+{
+ char rule[PATH_MAX];
+
+ snprintf(rule, sizeof(rule), ":%s:M::" MAGIC "::" INTERP ":", name);
+ return register_at(mnt, rule);
+}
+
+static int remove_entry(const char *mnt, const char *name)
+{
+ char path[PATH_MAX];
+
+ snprintf(path, sizeof(path), "%s/%s", mnt, name);
+ return write_keep_errno(path, "-1\n");
+}
+
+static bool entry_exists(const char *mnt, const char *name)
+{
+ char path[PATH_MAX];
+
+ snprintf(path, sizeof(path), "%s/%s", mnt, name);
+ return access(path, F_OK) == 0;
+}
+
+/* Register @n 'F' entries, each with a name of its own. */
+static int fill_budget(const char *mnt, unsigned int n)
+{
+ char name[32];
+ unsigned int i;
+
+ for (i = 0; i < n; i++) {
+ snprintf(name, sizeof(name), "fixed%u", i);
+ if (register_fixed(mnt, name))
+ return -1;
+ }
+ return 0;
+}
+
+FIXTURE(interp_limit) {
+};
+
+FIXTURE_SETUP(interp_limit)
+{
+ /* setup_userns() exits rather than returns if this is not there. */
+ if (access("/proc/self/ns/user", F_OK))
+ SKIP(return, "kernel without user namespaces");
+ ASSERT_EQ(setup_userns(), 0);
+
+ /* CAP_SYS_RESOURCE in this namespace is what makes it writable. */
+ if (set_limit(LIMIT_MAX)) {
+ if (errno == ENOENT)
+ SKIP(return, "kernel without " LIMIT_SYSCTL);
+ SKIP(return, "cannot set the limit: %s", strerror(errno));
+ }
+
+ ASSERT_EQ(ensure_dir(MNT), 0);
+ if (mount("binfmt_misc", MNT, "binfmt_misc", 0, NULL)) {
+ int saved = errno;
+
+ /* Teardown doesn't run when setup skips, so clean up here. */
+ rmdir(MNT);
+ SKIP(return, "no binfmt_misc: %s", strerror(saved));
+ }
+}
+
+FIXTURE_TEARDOWN(interp_limit)
+{
+ /* The namespaces go with the process; just don't litter /tmp. */
+ umount2(NESTED_MNT, MNT_DETACH);
+ umount2(MNT, MNT_DETACH);
+ rmdir(NESTED_MNT);
+ rmdir(MNT);
+}
+
+/* Every pre-opened interpreter is charged, and the budget is a hard stop. */
+TEST_F(interp_limit, fixed_interpreters_are_charged)
+{
+ char buf[32];
+
+ snprintf(buf, sizeof(buf), "%u", LIMIT);
+ ASSERT_EQ(set_limit(buf), 0);
+
+ ASSERT_EQ(fill_budget(MNT, LIMIT), 0);
+
+ EXPECT_NE(register_fixed(MNT, "over"), 0);
+ EXPECT_EQ(errno, ENOSPC);
+
+ /* A refused registration leaves nothing behind. */
+ EXPECT_FALSE(entry_exists(MNT, "over"));
+}
+
+/* An entry that pre-opens nothing pins nothing, so it is not charged. */
+TEST_F(interp_limit, plain_entries_are_not_charged)
+{
+ ASSERT_EQ(set_limit("0"), 0);
+
+ EXPECT_EQ(register_plain(MNT, "plain"), 0);
+ EXPECT_TRUE(entry_exists(MNT, "plain"));
+
+ /* ... while the same entry with 'F' has nothing to spend. */
+ EXPECT_NE(register_fixed(MNT, "fixed"), 0);
+ EXPECT_EQ(errno, ENOSPC);
+}
+
+/* Removing an entry closes its interpreters and gives the charge back. */
+TEST_F(interp_limit, removal_refunds_the_charge)
+{
+ char buf[32];
+
+ snprintf(buf, sizeof(buf), "%u", LIMIT);
+ ASSERT_EQ(set_limit(buf), 0);
+
+ ASSERT_EQ(fill_budget(MNT, LIMIT), 0);
+ ASSERT_NE(register_fixed(MNT, "over"), 0);
+
+ ASSERT_EQ(remove_entry(MNT, "fixed0"), 0);
+ EXPECT_EQ(register_fixed(MNT, "over"), 0);
+}
+
+/*
+ * The charge walks the ancestors, so a namespace cannot buy itself budget by
+ * nesting: it may raise only its own limit, and the parent it was created
+ * from is charged for every binding made below it.
+ */
+TEST_F(interp_limit, nesting_does_not_evade_it)
+{
+ char buf[32];
+
+ snprintf(buf, sizeof(buf), "%u", LIMIT);
+ ASSERT_EQ(set_limit(buf), 0);
+ ASSERT_EQ(fill_budget(MNT, LIMIT), 0);
+
+ ASSERT_EQ(setup_userns(), 0);
+ ASSERT_EQ(set_limit(LIMIT_MAX), 0);
+
+ ASSERT_EQ(ensure_dir(NESTED_MNT), 0);
+ ASSERT_EQ(mount("binfmt_misc", NESTED_MNT, "binfmt_misc", 0, NULL), 0);
+
+ /* A fresh instance with an unlimited budget of its own, and yet: */
+ EXPECT_NE(register_fixed(NESTED_MNT, "nested"), 0);
+ EXPECT_EQ(errno, ENOSPC);
+
+ /* The nested instance works for anything that pins no file. */
+ EXPECT_EQ(register_plain(NESTED_MNT, "nested_plain"), 0);
+}
+
+TEST_HARNESS_MAIN