Merge tag 'trace-v7.3-rc2' of git://git.kernel.org/pub/scm/linux/kernel/git/trace/linux-tracegrafted

Pull tracing fixes from Steven Rostedt:

 - Don't destroy user event fields when removal fails

   User event fields are destroyed before the event is removed from
   visibility. But that can fail leaving the still visible event with no
   fields. Move the destroying of the fields to after the event is
   successfully removed from visibility.

 - Initialize function graph state is fork before calling
   copy_exec_state()

   For non-CLONE_VM forks, copy_exec_state() allocates a new
   task_exec_state. If that allocation fails, ftrace_graph_exit_task()
   will free the tasks ret_stack pointer. Since that pointer is still
   using the parent's ret_stack, it mistakenly frees the parent's
   pointer too.

   Call ftrace_graph_init() on the task first which will NULL out the
   new tasks's ret_stack and if the copy fails, it will not free
   anything.

 - Remove FGRAPH_MAX_INDEX

   The macro FGRAPH_MAX_INDEX was added but never used. Remove it.

 - Save ent_size in function graph printing of nested functions

   The function graph tracer needs to look at the next event to see if
   the next event is the return of the current function entry. If it is,
   it prints a single line:

	ktime_get();

   Otherwise it prints it like a nested function:

	tick_nohz_irq_exit() {
	    ktime_get();
	    kcpustat_irq_exit();
	}

   In order to look at the next event, it must save the current event so
   that it has the information to print from it. It saves the event in
   the iterator descriptor called "ent". What it doesn't save is the
   ent_size of the event which is now used to know if the function graph
   arguments are to be printed. The peek doesn't save the size so the
   size used happens to be that of the size of the last event that was
   seen.

   Save the entry event size in the iterator descriptor so that the
   correct size is used.

 - Fix several errors with freeing data in the histogram code

   The histogram code had a lot of leaked or or incorrect accounting
   when failures happen. Correct them.

 - Fix histogram regression of .percent and .graph modifiers

   Up until 6.3 histogram values could have "percent" or "graph"
   modifiers that changed how they were printed. But a change that added
   restricting histograms values from being strings, stack traces and
   other modifiers inadvertently prevented them from using the percent
   and graph modifiers, which were legal use cases for values.

   Put back the percent and graph modifiers.

 - Fix various typos in the comments

 - Set the trace_clock before initializing a histogram with clock
   argument

   The histogram API allows the user to specific which trace clock to
   use via a "clock=" string. The histogram is set up first before the
   clock is checked. If the passed in clock is not valid, it exits
   without fully fixing up the histogram leaving it on the list and a
   use-after-free can trigger.

   Update the clock argument first and if it fails then exit gracefully
   before the histogram trigger is placed on any lists.

 - Restore :mod: trailer after parsing in ftrace_set_clr_event

   The function ftrace_set_clr_event() modifies the parse string and
   needs to put it back to what was passed in. It searches for ":mod:"
   via a strsep() but fails to put back the first ':' in the string.

   Add back the ':' in the passed in string.

 - Take trace_array reference when opening a tracer options file

   The options files are dynamically created and some tracers add their
   own options. When a tracer adds their own list of options, the
   trace_array holding them has an array to hold the list of options for
   each tracer. This array increases in size via a krealloc(), and the
   new entry gets a newly allocated array to hold the options of the new
   tracer being added.

   The element in each entry of the tracer's option array holds a
   pointer back to the trace_array, a pointer to the tracer it is
   associated to, a pointer to the flags of the option.

   The issue is that these arrays are freed when the trace_array is
   freed when its instance it represents is removed from the instances
   directory. There's a race that an open of one of these options files
   can happen when the instance is being removed.

   Add a new helper function to be called by the open function of the
   options file to iterate all existing trace_arrays under a lock and
   find the one that has the given option element in one of it's tracer
   arrays. If found, then update the associated trace_array's reference
   counter to keep it from being freed. If not found, have the open call
   return -ENODEV.

 - Disable interrupts when acquiring the lock in rb_wake_up_waiters()

   The function rb_wake_up_waiters() assumes it will be called in
   interrupt context and does not disable irqs when taking
   cpu_buffer->reader_lock, which can be called in hard interrupt
   context. The issue is in PREEMPT_RT, this function is called in
   thread context leaving this lock open to a deadlock.

   Take the lock with interrupts disabled.

 - Use rcu_assign_pointer() for tmp_ops filter hash

   The tmp_ops used in update_ftrace_direct_mod() assigns its
   filter_hash field directly, but that field is annotated as __rcu and
   sparse complains. Assign it with rcu_assign_pointer()

 - Fix use-after-free in enable_trigger_private_data_free()

   The trace_event_call is accessed through the event_trigger_data's
   trace_event_file pointer to put the trace_event_call on freeing. The
   issue is that the trace_event_file data may have been freed already
   causing a use-after-free. Add a field to the event_trigger_data that
   points directly to the trace_event_call so that it can decrement its
   reference directly without needing to go through the
   trace_event_file.

 - Fix accounting of buffer data remote headers

   trace_buffer_desc_size() and trace_remote_alloc_buffer() undercount
   the number of pages is needed for the asked for size as it doesn't
   take into account the meta data on each page. Add a helper function
   to do the calculation properly and use that in these functions.

 - Catch nr_page_va overflow in ring_buffer_desc sizing

   The number of pages per remote ring buffer is capped by
   ring_buffer_desc::nr_page_va (32 bits). A buffer_size large enough to
   overflow that field would silently allocate a descriptor smaller than
   what was asked for.

 - Do not resize the subbuf order if any per_cpu buffer is disabled

   The mmapping of ring buffers disables resizing the subbuffers, but it
   is done per-cpu whereas the subbuf size change is done for all the
   per_cpu buffers under the buffer->mutex. It could change the size of
   some while the mapping is happening on others. Have the resize of the
   subbuf order check all the per_cpu buffers under the lock to see if
   any of them is disabled before starting and causing an inconsistency
   between buffers that are being mapped.

* tag 'trace-v7.3-rc2' of git://git.kernel.org/pub/scm/linux/kernel/git/trace/linux-trace: (25 commits)
  ring-buffer: Check resize_disabled before publishing the new subbuf order
  tracing/remotes: Catch nr_page_va overflow in ring_buffer_desc sizing
  tracing/remotes: Account for ring buffer page header in size calculation
  tracing: Don't dereference trace_event_file in deferred trigger free
  ftrace: Use rcu_assign_pointer() for tmp_ops filter hash
  ring-buffer: Acquire the lock with irqsave in rb_wake_up_waiters()
  tracing: Take trace_array reference when opening a tracer options file
  tracing: Fix ring_buffer_read_page_size() kernel-doc
  tracing: Restore :mod: trailer after parsing in ftrace_set_clr_event()
  tracing: Fix memory corruption from a "STACKTRACE" histogram key
  tracing: Fix memory corruption from the histogram stacktrace modifier
  tracing: Undo the registration when enabling the histogram trigger fails
  tracing: Take the reference before publishing the named histogram trigger
  tracing: Set the trace clock before registering the histogram trigger
  tracing: Fix typo "preceeded" in comment
  tracing: Fix typo "availabe" in comment
  tracing: Let histogram values keep the percent and graph modifiers
  tracing: Keep the entry count when the histogram stats allocation fails
  tracing: Free histogram the field rejected for a bad modifier
  tracing: Free histogram the var ref when its initialization fails
  ...

8 files changed, 1122 insertions, 0 deletions

diff --git a/samples/seccomp/.gitignore b/samples/seccomp/.gitignore
new file mode 100644
index 000000000..a6df0da77
--- /dev/null
+++ b/samples/seccomp/.gitignore
@@ -0,0 +1,5 @@
+# SPDX-License-Identifier: GPL-2.0-only
+/bpf-direct
+/bpf-fancy
+/dropper
+/user-trap
diff --git a/samples/seccomp/Makefile b/samples/seccomp/Makefile
new file mode 100644
index 000000000..c85ae0ed8
--- /dev/null
+++ b/samples/seccomp/Makefile
@@ -0,0 +1,6 @@
+# SPDX-License-Identifier: GPL-2.0
+userprogs-always-y += bpf-fancy dropper bpf-direct user-trap
+
+bpf-fancy-objs := bpf-fancy.o bpf-helper.o
+
+userccflags += -I usr/include
diff --git a/samples/seccomp/bpf-direct.c b/samples/seccomp/bpf-direct.c
new file mode 100644
index 000000000..c09e4a17a
--- /dev/null
+++ b/samples/seccomp/bpf-direct.c
@@ -0,0 +1,191 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Seccomp filter example for x86 (32-bit and 64-bit) with BPF macros
+ *
+ * Copyright (c) 2012 The Chromium OS Authors <chromium-os-dev@chromium.org>
+ * Author: Will Drewry <wad@chromium.org>
+ *
+ * The code may be used by anyone for any purpose,
+ * and can serve as a starting point for developing
+ * applications using prctl(PR_SET_SECCOMP, 2, ...).
+ */
+#if defined(__i386__) || defined(__x86_64__)
+#define SUPPORTED_ARCH 1
+#endif
+
+#if defined(SUPPORTED_ARCH)
+#define __USE_GNU 1
+#define _GNU_SOURCE 1
+
+#include <linux/types.h>
+#include <linux/filter.h>
+#include <linux/seccomp.h>
+#include <linux/unistd.h>
+#include <signal.h>
+#include <stdio.h>
+#include <stddef.h>
+#include <string.h>
+#include <sys/prctl.h>
+#include <unistd.h>
+
+#define syscall_arg(_n) (offsetof(struct seccomp_data, args[_n]))
+#define syscall_nr (offsetof(struct seccomp_data, nr))
+
+#if defined(__i386__)
+#define REG_RESULT	REG_EAX
+#define REG_SYSCALL	REG_EAX
+#define REG_ARG0	REG_EBX
+#define REG_ARG1	REG_ECX
+#define REG_ARG2	REG_EDX
+#define REG_ARG3	REG_ESI
+#define REG_ARG4	REG_EDI
+#define REG_ARG5	REG_EBP
+#elif defined(__x86_64__)
+#define REG_RESULT	REG_RAX
+#define REG_SYSCALL	REG_RAX
+#define REG_ARG0	REG_RDI
+#define REG_ARG1	REG_RSI
+#define REG_ARG2	REG_RDX
+#define REG_ARG3	REG_R10
+#define REG_ARG4	REG_R8
+#define REG_ARG5	REG_R9
+#endif
+
+#ifndef PR_SET_NO_NEW_PRIVS
+#define PR_SET_NO_NEW_PRIVS 38
+#endif
+
+#ifndef SYS_SECCOMP
+#define SYS_SECCOMP 1
+#endif
+
+static void emulator(int nr, siginfo_t *info, void *void_context)
+{
+	ucontext_t *ctx = (ucontext_t *)(void_context);
+	int syscall;
+	char *buf;
+	ssize_t bytes;
+	size_t len;
+	if (info->si_code != SYS_SECCOMP)
+		return;
+	if (!ctx)
+		return;
+	syscall = ctx->uc_mcontext.gregs[REG_SYSCALL];
+	buf = (char *) ctx->uc_mcontext.gregs[REG_ARG1];
+	len = (size_t) ctx->uc_mcontext.gregs[REG_ARG2];
+
+	if (syscall != __NR_write)
+		return;
+	if (ctx->uc_mcontext.gregs[REG_ARG0] != STDERR_FILENO)
+		return;
+	/* Redirect stderr messages to stdout. Doesn't handle EINTR, etc */
+	ctx->uc_mcontext.gregs[REG_RESULT] = -1;
+	if (write(STDOUT_FILENO, "[ERR] ", 6) > 0) {
+		bytes = write(STDOUT_FILENO, buf, len);
+		ctx->uc_mcontext.gregs[REG_RESULT] = bytes;
+	}
+	return;
+}
+
+static int install_emulator(void)
+{
+	struct sigaction act;
+	sigset_t mask;
+	memset(&act, 0, sizeof(act));
+	sigemptyset(&mask);
+	sigaddset(&mask, SIGSYS);
+
+	act.sa_sigaction = &emulator;
+	act.sa_flags = SA_SIGINFO;
+	if (sigaction(SIGSYS, &act, NULL) < 0) {
+		perror("sigaction");
+		return -1;
+	}
+	if (sigprocmask(SIG_UNBLOCK, &mask, NULL)) {
+		perror("sigprocmask");
+		return -1;
+	}
+	return 0;
+}
+
+static int install_filter(void)
+{
+	struct sock_filter filter[] = {
+		/* Grab the system call number */
+		BPF_STMT(BPF_LD+BPF_W+BPF_ABS, syscall_nr),
+		/* Jump table for the allowed syscalls */
+		BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, __NR_rt_sigreturn, 0, 1),
+		BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_ALLOW),
+#ifdef __NR_sigreturn
+		BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, __NR_sigreturn, 0, 1),
+		BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_ALLOW),
+#endif
+		BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, __NR_exit_group, 0, 1),
+		BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_ALLOW),
+		BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, __NR_exit, 0, 1),
+		BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_ALLOW),
+		BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, __NR_read, 1, 0),
+		BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, __NR_write, 3, 2),
+
+		/* Check that read is only using stdin. */
+		BPF_STMT(BPF_LD+BPF_W+BPF_ABS, syscall_arg(0)),
+		BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, STDIN_FILENO, 4, 0),
+		BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_KILL),
+
+		/* Check that write is only using stdout */
+		BPF_STMT(BPF_LD+BPF_W+BPF_ABS, syscall_arg(0)),
+		BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, STDOUT_FILENO, 1, 0),
+		/* Trap attempts to write to stderr */
+		BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, STDERR_FILENO, 1, 2),
+
+		BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_ALLOW),
+		BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_TRAP),
+		BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_KILL),
+	};
+	struct sock_fprog prog = {
+		.len = (unsigned short)(sizeof(filter)/sizeof(filter[0])),
+		.filter = filter,
+	};
+
+	if (prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0)) {
+		perror("prctl(NO_NEW_PRIVS)");
+		return 1;
+	}
+
+
+	if (prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &prog)) {
+		perror("prctl");
+		return 1;
+	}
+	return 0;
+}
+
+#define payload(_c) (_c), sizeof((_c))
+int main(int argc, char **argv)
+{
+	char buf[4096];
+	ssize_t bytes = 0;
+	if (install_emulator())
+		return 1;
+	if (install_filter())
+		return 1;
+	syscall(__NR_write, STDOUT_FILENO,
+		payload("OHAI! WHAT IS YOUR NAME? "));
+	bytes = syscall(__NR_read, STDIN_FILENO, buf, sizeof(buf));
+	syscall(__NR_write, STDOUT_FILENO, payload("HELLO, "));
+	syscall(__NR_write, STDOUT_FILENO, buf, bytes);
+	syscall(__NR_write, STDERR_FILENO,
+		payload("Error message going to STDERR\n"));
+	return 0;
+}
+#else	/* SUPPORTED_ARCH */
+/*
+ * This sample is x86-only.  Since kernel samples are compiled with the
+ * host toolchain, a non-x86 host will result in using only the main()
+ * below.
+ */
+int main(void)
+{
+	return 1;
+}
+#endif	/* SUPPORTED_ARCH */
diff --git a/samples/seccomp/bpf-fancy.c b/samples/seccomp/bpf-fancy.c
new file mode 100644
index 000000000..1ccb43502
--- /dev/null
+++ b/samples/seccomp/bpf-fancy.c
@@ -0,0 +1,105 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Seccomp BPF example using a macro-based generator.
+ *
+ * Copyright (c) 2012 The Chromium OS Authors <chromium-os-dev@chromium.org>
+ * Author: Will Drewry <wad@chromium.org>
+ *
+ * The code may be used by anyone for any purpose,
+ * and can serve as a starting point for developing
+ * applications using prctl(PR_ATTACH_SECCOMP_FILTER).
+ */
+
+#include <linux/filter.h>
+#include <linux/seccomp.h>
+#include <linux/unistd.h>
+#include <stdio.h>
+#include <string.h>
+#include <sys/prctl.h>
+#include <unistd.h>
+
+#include "bpf-helper.h"
+
+#ifndef PR_SET_NO_NEW_PRIVS
+#define PR_SET_NO_NEW_PRIVS 38
+#endif
+
+int main(int argc, char **argv)
+{
+	struct bpf_labels l = {
+		.count = 0,
+	};
+	static const char msg1[] = "Please type something: ";
+	static const char msg2[] = "You typed: ";
+	char buf[256];
+	struct sock_filter filter[] = {
+		/* TODO: LOAD_SYSCALL_NR(arch) and enforce an arch */
+		LOAD_SYSCALL_NR,
+		SYSCALL(__NR_exit, ALLOW),
+		SYSCALL(__NR_exit_group, ALLOW),
+		SYSCALL(__NR_write, JUMP(&l, write_fd)),
+		SYSCALL(__NR_read, JUMP(&l, read)),
+		DENY,  /* Don't passthrough into a label */
+
+		LABEL(&l, read),
+		ARG(0),
+		JNE(STDIN_FILENO, DENY),
+		ARG(1),
+		JNE((unsigned long)buf, DENY),
+		ARG(2),
+		JGE(sizeof(buf), DENY),
+		ALLOW,
+
+		LABEL(&l, write_fd),
+		ARG(0),
+		JEQ(STDOUT_FILENO, JUMP(&l, write_buf)),
+		JEQ(STDERR_FILENO, JUMP(&l, write_buf)),
+		DENY,
+
+		LABEL(&l, write_buf),
+		ARG(1),
+		JEQ((unsigned long)msg1, JUMP(&l, msg1_len)),
+		JEQ((unsigned long)msg2, JUMP(&l, msg2_len)),
+		JEQ((unsigned long)buf, JUMP(&l, buf_len)),
+		DENY,
+
+		LABEL(&l, msg1_len),
+		ARG(2),
+		JLT(sizeof(msg1), ALLOW),
+		DENY,
+
+		LABEL(&l, msg2_len),
+		ARG(2),
+		JLT(sizeof(msg2), ALLOW),
+		DENY,
+
+		LABEL(&l, buf_len),
+		ARG(2),
+		JLT(sizeof(buf), ALLOW),
+		DENY,
+	};
+	struct sock_fprog prog = {
+		.filter = filter,
+		.len = (unsigned short)(sizeof(filter)/sizeof(filter[0])),
+	};
+	ssize_t bytes;
+	bpf_resolve_jumps(&l, filter, sizeof(filter)/sizeof(*filter));
+
+	if (prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0)) {
+		perror("prctl(NO_NEW_PRIVS)");
+		return 1;
+	}
+
+	if (prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &prog)) {
+		perror("prctl(SECCOMP)");
+		return 1;
+	}
+	syscall(__NR_write, STDOUT_FILENO, msg1, strlen(msg1));
+	bytes = syscall(__NR_read, STDIN_FILENO, buf, sizeof(buf)-1);
+	bytes = (bytes > 0 ? bytes : 0);
+	syscall(__NR_write, STDERR_FILENO, msg2, strlen(msg2));
+	syscall(__NR_write, STDERR_FILENO, buf, bytes);
+	/* Now get killed */
+	syscall(__NR_write, STDERR_FILENO, msg2, strlen(msg2)+2);
+	return 0;
+}
diff --git a/samples/seccomp/bpf-helper.c b/samples/seccomp/bpf-helper.c
new file mode 100644
index 000000000..ae260d77a
--- /dev/null
+++ b/samples/seccomp/bpf-helper.c
@@ -0,0 +1,96 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Seccomp BPF helper functions
+ *
+ * Copyright (c) 2012 The Chromium OS Authors <chromium-os-dev@chromium.org>
+ * Author: Will Drewry <wad@chromium.org>
+ *
+ * The code may be used by anyone for any purpose,
+ * and can serve as a starting point for developing
+ * applications using prctl(PR_ATTACH_SECCOMP_FILTER).
+ */
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+
+#include "bpf-helper.h"
+
+int bpf_resolve_jumps(struct bpf_labels *labels,
+		      struct sock_filter *filter, size_t count)
+{
+	size_t i;
+
+	if (count < 1 || count > BPF_MAXINSNS)
+		return -1;
+	/*
+	* Walk it once, backwards, to build the label table and do fixups.
+	* Since backward jumps are disallowed by BPF, this is easy.
+	*/
+	for (i = 0; i < count; ++i) {
+		size_t offset = count - i - 1;
+		struct sock_filter *instr = &filter[offset];
+		if (instr->code != (BPF_JMP+BPF_JA))
+			continue;
+		switch ((instr->jt<<8)|instr->jf) {
+		case (JUMP_JT<<8)|JUMP_JF:
+			if (labels->labels[instr->k].location == 0xffffffff) {
+				fprintf(stderr, "Unresolved label: '%s'\n",
+					labels->labels[instr->k].label);
+				return 1;
+			}
+			instr->k = labels->labels[instr->k].location -
+				    (offset + 1);
+			instr->jt = 0;
+			instr->jf = 0;
+			continue;
+		case (LABEL_JT<<8)|LABEL_JF:
+			if (labels->labels[instr->k].location != 0xffffffff) {
+				fprintf(stderr, "Duplicate label use: '%s'\n",
+					labels->labels[instr->k].label);
+				return 1;
+			}
+			labels->labels[instr->k].location = offset;
+			instr->k = 0; /* fall through */
+			instr->jt = 0;
+			instr->jf = 0;
+			continue;
+		}
+	}
+	return 0;
+}
+
+/* Simple lookup table for labels. */
+__u32 seccomp_bpf_label(struct bpf_labels *labels, const char *label)
+{
+	struct __bpf_label *begin = labels->labels, *end;
+	int id;
+
+	if (labels->count == BPF_LABELS_MAX) {
+		fprintf(stderr, "Too many labels\n");
+		exit(1);
+	}
+	if (labels->count == 0) {
+		begin->label = label;
+		begin->location = 0xffffffff;
+		labels->count++;
+		return 0;
+	}
+	end = begin + labels->count;
+	for (id = 0; begin < end; ++begin, ++id) {
+		if (!strcmp(label, begin->label))
+			return id;
+	}
+	begin->label = label;
+	begin->location = 0xffffffff;
+	labels->count++;
+	return id;
+}
+
+void seccomp_bpf_print(struct sock_filter *filter, size_t count)
+{
+	struct sock_filter *end = filter + count;
+	for ( ; filter < end; ++filter)
+		printf("{ code=%u,jt=%u,jf=%u,k=%u },\n",
+			filter->code, filter->jt, filter->jf, filter->k);
+}
diff --git a/samples/seccomp/bpf-helper.h b/samples/seccomp/bpf-helper.h
new file mode 100644
index 000000000..417e48a4c
--- /dev/null
+++ b/samples/seccomp/bpf-helper.h
@@ -0,0 +1,263 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * Example wrapper around BPF macros.
+ *
+ * Copyright (c) 2012 The Chromium OS Authors <chromium-os-dev@chromium.org>
+ * Author: Will Drewry <wad@chromium.org>
+ *
+ * The code may be used by anyone for any purpose,
+ * and can serve as a starting point for developing
+ * applications using prctl(PR_SET_SECCOMP, 2, ...).
+ *
+ * No guarantees are provided with respect to the correctness
+ * or functionality of this code.
+ */
+#ifndef __BPF_HELPER_H__
+#define __BPF_HELPER_H__
+
+#include <asm/bitsperlong.h>	/* for __BITS_PER_LONG */
+#include <endian.h>
+#include <linux/filter.h>
+#include <linux/seccomp.h>	/* for seccomp_data */
+#include <linux/types.h>
+#include <linux/unistd.h>
+#include <stddef.h>
+
+#define BPF_LABELS_MAX 256
+struct bpf_labels {
+	int count;
+	struct __bpf_label {
+		const char *label;
+		__u32 location;
+	} labels[BPF_LABELS_MAX];
+};
+
+int bpf_resolve_jumps(struct bpf_labels *labels,
+		      struct sock_filter *filter, size_t count);
+__u32 seccomp_bpf_label(struct bpf_labels *labels, const char *label);
+void seccomp_bpf_print(struct sock_filter *filter, size_t count);
+
+#define JUMP_JT 0xff
+#define JUMP_JF 0xff
+#define LABEL_JT 0xfe
+#define LABEL_JF 0xfe
+
+#define ALLOW \
+	BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_ALLOW)
+#define DENY \
+	BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_KILL)
+#define JUMP(labels, label) \
+	BPF_JUMP(BPF_JMP+BPF_JA, FIND_LABEL((labels), (label)), \
+		 JUMP_JT, JUMP_JF)
+#define LABEL(labels, label) \
+	BPF_JUMP(BPF_JMP+BPF_JA, FIND_LABEL((labels), (label)), \
+		 LABEL_JT, LABEL_JF)
+#define SYSCALL(nr, jt) \
+	BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, (nr), 0, 1), \
+	jt
+
+/* Lame, but just an example */
+#define FIND_LABEL(labels, label) seccomp_bpf_label((labels), #label)
+
+#define EXPAND(...) __VA_ARGS__
+
+/* Ensure that we load the logically correct offset. */
+#if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
+#define LO_ARG(idx) offsetof(struct seccomp_data, args[(idx)])
+#elif __BYTE_ORDER__ == __ORDER_BIG_ENDIAN__
+#define LO_ARG(idx) offsetof(struct seccomp_data, args[(idx)]) + sizeof(__u32)
+#else
+#error "Unknown endianness"
+#endif
+
+/* Map all width-sensitive operations */
+#if __BITS_PER_LONG == 32
+
+#define JEQ(x, jt) JEQ32(x, EXPAND(jt))
+#define JNE(x, jt) JNE32(x, EXPAND(jt))
+#define JGT(x, jt) JGT32(x, EXPAND(jt))
+#define JLT(x, jt) JLT32(x, EXPAND(jt))
+#define JGE(x, jt) JGE32(x, EXPAND(jt))
+#define JLE(x, jt) JLE32(x, EXPAND(jt))
+#define JA(x, jt) JA32(x, EXPAND(jt))
+#define ARG(i) ARG_32(i)
+
+#elif __BITS_PER_LONG == 64
+
+/* Ensure that we load the logically correct offset. */
+#if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
+#define ENDIAN(_lo, _hi) _lo, _hi
+#define HI_ARG(idx) offsetof(struct seccomp_data, args[(idx)]) + sizeof(__u32)
+#elif __BYTE_ORDER__ == __ORDER_BIG_ENDIAN__
+#define ENDIAN(_lo, _hi) _hi, _lo
+#define HI_ARG(idx) offsetof(struct seccomp_data, args[(idx)])
+#endif
+
+union arg64 {
+	struct {
+		__u32 ENDIAN(lo32, hi32);
+	};
+	__u64 u64;
+};
+
+#define JEQ(x, jt) \
+	JEQ64(((union arg64){.u64 = (x)}).lo32, \
+	      ((union arg64){.u64 = (x)}).hi32, \
+	      EXPAND(jt))
+#define JGT(x, jt) \
+	JGT64(((union arg64){.u64 = (x)}).lo32, \
+	      ((union arg64){.u64 = (x)}).hi32, \
+	      EXPAND(jt))
+#define JGE(x, jt) \
+	JGE64(((union arg64){.u64 = (x)}).lo32, \
+	      ((union arg64){.u64 = (x)}).hi32, \
+	      EXPAND(jt))
+#define JNE(x, jt) \
+	JNE64(((union arg64){.u64 = (x)}).lo32, \
+	      ((union arg64){.u64 = (x)}).hi32, \
+	      EXPAND(jt))
+#define JLT(x, jt) \
+	JLT64(((union arg64){.u64 = (x)}).lo32, \
+	      ((union arg64){.u64 = (x)}).hi32, \
+	      EXPAND(jt))
+#define JLE(x, jt) \
+	JLE64(((union arg64){.u64 = (x)}).lo32, \
+	      ((union arg64){.u64 = (x)}).hi32, \
+	      EXPAND(jt))
+
+#define JA(x, jt) \
+	JA64(((union arg64){.u64 = (x)}).lo32, \
+	       ((union arg64){.u64 = (x)}).hi32, \
+	       EXPAND(jt))
+#define ARG(i) ARG_64(i)
+
+#else
+#error __BITS_PER_LONG value unusable.
+#endif
+
+/* Loads the arg into A */
+#define ARG_32(idx) \
+	BPF_STMT(BPF_LD+BPF_W+BPF_ABS, LO_ARG(idx))
+
+/* Loads lo into M[0] and hi into M[1] and A */
+#define ARG_64(idx) \
+	BPF_STMT(BPF_LD+BPF_W+BPF_ABS, LO_ARG(idx)), \
+	BPF_STMT(BPF_ST, 0), /* lo -> M[0] */ \
+	BPF_STMT(BPF_LD+BPF_W+BPF_ABS, HI_ARG(idx)), \
+	BPF_STMT(BPF_ST, 1) /* hi -> M[1] */
+
+#define JEQ32(value, jt) \
+	BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, (value), 0, 1), \
+	jt
+
+#define JNE32(value, jt) \
+	BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, (value), 1, 0), \
+	jt
+
+#define JA32(value, jt) \
+	BPF_JUMP(BPF_JMP+BPF_JSET+BPF_K, (value), 0, 1), \
+	jt
+
+#define JGE32(value, jt) \
+	BPF_JUMP(BPF_JMP+BPF_JGE+BPF_K, (value), 0, 1), \
+	jt
+
+#define JGT32(value, jt) \
+	BPF_JUMP(BPF_JMP+BPF_JGT+BPF_K, (value), 0, 1), \
+	jt
+
+#define JLE32(value, jt) \
+	BPF_JUMP(BPF_JMP+BPF_JGT+BPF_K, (value), 1, 0), \
+	jt
+
+#define JLT32(value, jt) \
+	BPF_JUMP(BPF_JMP+BPF_JGE+BPF_K, (value), 1, 0), \
+	jt
+
+/*
+ * All the JXX64 checks assume lo is saved in M[0] and hi is saved in both
+ * A and M[1]. This invariant is kept by restoring A if necessary.
+ */
+#define JEQ64(lo, hi, jt) \
+	/* if (hi != arg.hi) goto NOMATCH; */ \
+	BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, (hi), 0, 5), \
+	BPF_STMT(BPF_LD+BPF_MEM, 0), /* swap in lo */ \
+	/* if (lo != arg.lo) goto NOMATCH; */ \
+	BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, (lo), 0, 2), \
+	BPF_STMT(BPF_LD+BPF_MEM, 1), \
+	jt, \
+	BPF_STMT(BPF_LD+BPF_MEM, 1)
+
+#define JNE64(lo, hi, jt) \
+	/* if (hi != arg.hi) goto MATCH; */ \
+	BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, (hi), 0, 3), \
+	BPF_STMT(BPF_LD+BPF_MEM, 0), \
+	/* if (lo != arg.lo) goto MATCH; */ \
+	BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, (lo), 2, 0), \
+	BPF_STMT(BPF_LD+BPF_MEM, 1), \
+	jt, \
+	BPF_STMT(BPF_LD+BPF_MEM, 1)
+
+#define JA64(lo, hi, jt) \
+	/* if (hi & arg.hi) goto MATCH; */ \
+	BPF_JUMP(BPF_JMP+BPF_JSET+BPF_K, (hi), 3, 0), \
+	BPF_STMT(BPF_LD+BPF_MEM, 0), \
+	/* if (lo & arg.lo) goto MATCH; */ \
+	BPF_JUMP(BPF_JMP+BPF_JSET+BPF_K, (lo), 0, 2), \
+	BPF_STMT(BPF_LD+BPF_MEM, 1), \
+	jt, \
+	BPF_STMT(BPF_LD+BPF_MEM, 1)
+
+#define JGE64(lo, hi, jt) \
+	/* if (hi > arg.hi) goto MATCH; */ \
+	BPF_JUMP(BPF_JMP+BPF_JGT+BPF_K, (hi), 4, 0), \
+	/* if (hi != arg.hi) goto NOMATCH; */ \
+	BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, (hi), 0, 5), \
+	BPF_STMT(BPF_LD+BPF_MEM, 0), \
+	/* if (lo >= arg.lo) goto MATCH; */ \
+	BPF_JUMP(BPF_JMP+BPF_JGE+BPF_K, (lo), 0, 2), \
+	BPF_STMT(BPF_LD+BPF_MEM, 1), \
+	jt, \
+	BPF_STMT(BPF_LD+BPF_MEM, 1)
+
+#define JGT64(lo, hi, jt) \
+	/* if (hi > arg.hi) goto MATCH; */ \
+	BPF_JUMP(BPF_JMP+BPF_JGT+BPF_K, (hi), 4, 0), \
+	/* if (hi != arg.hi) goto NOMATCH; */ \
+	BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, (hi), 0, 5), \
+	BPF_STMT(BPF_LD+BPF_MEM, 0), \
+	/* if (lo > arg.lo) goto MATCH; */ \
+	BPF_JUMP(BPF_JMP+BPF_JGT+BPF_K, (lo), 0, 2), \
+	BPF_STMT(BPF_LD+BPF_MEM, 1), \
+	jt, \
+	BPF_STMT(BPF_LD+BPF_MEM, 1)
+
+#define JLE64(lo, hi, jt) \
+	/* if (hi < arg.hi) goto MATCH; */ \
+	BPF_JUMP(BPF_JMP+BPF_JGE+BPF_K, (hi), 0, 4), \
+	/* if (hi != arg.hi) goto NOMATCH; */ \
+	BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, (hi), 0, 5), \
+	BPF_STMT(BPF_LD+BPF_MEM, 0), \
+	/* if (lo <= arg.lo) goto MATCH; */ \
+	BPF_JUMP(BPF_JMP+BPF_JGT+BPF_K, (lo), 2, 0), \
+	BPF_STMT(BPF_LD+BPF_MEM, 1), \
+	jt, \
+	BPF_STMT(BPF_LD+BPF_MEM, 1)
+
+#define JLT64(lo, hi, jt) \
+	/* if (hi < arg.hi) goto MATCH; */ \
+	BPF_JUMP(BPF_JMP+BPF_JGE+BPF_K, (hi), 0, 4), \
+	/* if (hi != arg.hi) goto NOMATCH; */ \
+	BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, (hi), 0, 5), \
+	BPF_STMT(BPF_LD+BPF_MEM, 0), \
+	/* if (lo < arg.lo) goto MATCH; */ \
+	BPF_JUMP(BPF_JMP+BPF_JGE+BPF_K, (lo), 2, 0), \
+	BPF_STMT(BPF_LD+BPF_MEM, 1), \
+	jt, \
+	BPF_STMT(BPF_LD+BPF_MEM, 1)
+
+#define LOAD_SYSCALL_NR \
+	BPF_STMT(BPF_LD+BPF_W+BPF_ABS, \
+		 offsetof(struct seccomp_data, nr))
+
+#endif  /* __BPF_HELPER_H__ */
diff --git a/samples/seccomp/dropper.c b/samples/seccomp/dropper.c
new file mode 100644
index 000000000..4bca4b70f
--- /dev/null
+++ b/samples/seccomp/dropper.c
@@ -0,0 +1,77 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Naive system call dropper built on seccomp_filter.
+ *
+ * Copyright (c) 2012 The Chromium OS Authors <chromium-os-dev@chromium.org>
+ * Author: Will Drewry <wad@chromium.org>
+ *
+ * The code may be used by anyone for any purpose,
+ * and can serve as a starting point for developing
+ * applications using prctl(PR_SET_SECCOMP, 2, ...).
+ *
+ * When run, returns the specified errno for the specified
+ * system call number against the given architecture.
+ *
+ */
+
+#include <errno.h>
+#include <linux/audit.h>
+#include <linux/filter.h>
+#include <linux/seccomp.h>
+#include <linux/unistd.h>
+#include <stdio.h>
+#include <stddef.h>
+#include <stdlib.h>
+#include <sys/prctl.h>
+#include <unistd.h>
+
+static int install_filter(int arch, int nr, int error)
+{
+	struct sock_filter filter[] = {
+		BPF_STMT(BPF_LD+BPF_W+BPF_ABS,
+			 (offsetof(struct seccomp_data, arch))),
+		BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, arch, 0, 3),
+		BPF_STMT(BPF_LD+BPF_W+BPF_ABS,
+			 (offsetof(struct seccomp_data, nr))),
+		BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, nr, 0, 1),
+		BPF_STMT(BPF_RET+BPF_K,
+			 SECCOMP_RET_ERRNO|(error & SECCOMP_RET_DATA)),
+		BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_ALLOW),
+	};
+	struct sock_fprog prog = {
+		.len = (unsigned short)(sizeof(filter)/sizeof(filter[0])),
+		.filter = filter,
+	};
+	if (error == -1) {
+		struct sock_filter kill = BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_KILL);
+		filter[4] = kill;
+	}
+	if (prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0)) {
+		perror("prctl(NO_NEW_PRIVS)");
+		return 1;
+	}
+	if (prctl(PR_SET_SECCOMP, 2, &prog)) {
+		perror("prctl(PR_SET_SECCOMP)");
+		return 1;
+	}
+	return 0;
+}
+
+int main(int argc, char **argv)
+{
+	if (argc < 5) {
+		fprintf(stderr, "Usage:\n"
+			"dropper <arch> <syscall_nr> <errno> <prog> [<args>]\n"
+			"Hint:	AUDIT_ARCH_I386: 0x%X\n"
+			"	AUDIT_ARCH_X86_64: 0x%X\n"
+			"	errno == -1 means SECCOMP_RET_KILL\n"
+			"\n", AUDIT_ARCH_I386, AUDIT_ARCH_X86_64);
+		return 1;
+	}
+	if (install_filter(strtol(argv[1], NULL, 0), strtol(argv[2], NULL, 0),
+			   strtol(argv[3], NULL, 0)))
+		return 1;
+	execv(argv[4], &argv[4]);
+	printf("Failed to execv\n");
+	return 255;
+}
diff --git a/samples/seccomp/user-trap.c b/samples/seccomp/user-trap.c
new file mode 100644
index 000000000..a23fec357
--- /dev/null
+++ b/samples/seccomp/user-trap.c
@@ -0,0 +1,379 @@
+#include <signal.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <unistd.h>
+#include <errno.h>
+#include <fcntl.h>
+#include <string.h>
+#include <stddef.h>
+#include <sys/sysmacros.h>
+#include <sys/types.h>
+#include <sys/wait.h>
+#include <sys/socket.h>
+#include <sys/stat.h>
+#include <sys/mman.h>
+#include <sys/syscall.h>
+#include <sys/user.h>
+#include <sys/ioctl.h>
+#include <sys/ptrace.h>
+#include <sys/mount.h>
+#include <linux/limits.h>
+#include <linux/filter.h>
+#include <linux/seccomp.h>
+
+#define ARRAY_SIZE(x) (sizeof(x) / sizeof(*(x)))
+
+static int seccomp(unsigned int op, unsigned int flags, void *args)
+{
+	errno = 0;
+	return syscall(__NR_seccomp, op, flags, args);
+}
+
+static int send_fd(int sock, int fd)
+{
+	struct msghdr msg = {};
+	struct cmsghdr *cmsg;
+	int *fd_ptr;
+	char buf[CMSG_SPACE(sizeof(int))] = {0}, c = 'c';
+	struct iovec io = {
+		.iov_base = &c,
+		.iov_len = 1,
+	};
+
+	msg.msg_iov = &io;
+	msg.msg_iovlen = 1;
+	msg.msg_control = buf;
+	msg.msg_controllen = sizeof(buf);
+	cmsg = CMSG_FIRSTHDR(&msg);
+	cmsg->cmsg_level = SOL_SOCKET;
+	cmsg->cmsg_type = SCM_RIGHTS;
+	cmsg->cmsg_len = CMSG_LEN(sizeof(int));
+	fd_ptr = (int *)CMSG_DATA(cmsg);
+	*fd_ptr = fd;
+	msg.msg_controllen = cmsg->cmsg_len;
+
+	if (sendmsg(sock, &msg, 0) < 0) {
+		perror("sendmsg");
+		return -1;
+	}
+
+	return 0;
+}
+
+static int recv_fd(int sock)
+{
+	struct msghdr msg = {};
+	struct cmsghdr *cmsg;
+	int *fd_ptr;
+	char buf[CMSG_SPACE(sizeof(int))] = {0}, c = 'c';
+	struct iovec io = {
+		.iov_base = &c,
+		.iov_len = 1,
+	};
+
+	msg.msg_iov = &io;
+	msg.msg_iovlen = 1;
+	msg.msg_control = buf;
+	msg.msg_controllen = sizeof(buf);
+
+	if (recvmsg(sock, &msg, 0) < 0) {
+		perror("recvmsg");
+		return -1;
+	}
+
+	cmsg = CMSG_FIRSTHDR(&msg);
+	fd_ptr = (int *)CMSG_DATA(cmsg);
+
+	return *fd_ptr;
+}
+
+static int user_trap_syscall(int nr, unsigned int flags)
+{
+	struct sock_filter filter[] = {
+		BPF_STMT(BPF_LD+BPF_W+BPF_ABS,
+			offsetof(struct seccomp_data, nr)),
+		BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, nr, 0, 1),
+		BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_USER_NOTIF),
+		BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_ALLOW),
+	};
+
+	struct sock_fprog prog = {
+		.len = (unsigned short)ARRAY_SIZE(filter),
+		.filter = filter,
+	};
+
+	return seccomp(SECCOMP_SET_MODE_FILTER, flags, &prog);
+}
+
+static int handle_req(struct seccomp_notif *req,
+		      struct seccomp_notif_resp *resp, int listener)
+{
+	char path[PATH_MAX], source[PATH_MAX], target[PATH_MAX];
+	int ret = -1, mem;
+
+	resp->id = req->id;
+	resp->error = -EPERM;
+	resp->val = 0;
+
+	if (req->data.nr != __NR_mount) {
+		fprintf(stderr, "huh? trapped something besides mount? %d\n", req->data.nr);
+		return -1;
+	}
+
+	/* Only allow bind mounts. */
+	if (!(req->data.args[3] & MS_BIND))
+		return 0;
+
+	/*
+	 * Ok, let's read the task's memory to see where they wanted their
+	 * mount to go.
+	 */
+	snprintf(path, sizeof(path), "/proc/%d/mem", req->pid);
+	mem = open(path, O_RDONLY);
+	if (mem < 0) {
+		perror("open mem");
+		return -1;
+	}
+
+	/*
+	 * Now we avoid a TOCTOU: we referred to a pid by its pid, but since
+	 * the pid that made the syscall may have died, we need to confirm that
+	 * the pid is still valid after we open its /proc/pid/mem file. We can
+	 * ask the listener fd this as follows.
+	 *
+	 * Note that this check should occur *after* any task-specific
+	 * resources are opened, to make sure that the task has not died and
+	 * we're not wrongly reading someone else's state in order to make
+	 * decisions.
+	 */
+	if (ioctl(listener, SECCOMP_IOCTL_NOTIF_ID_VALID, &req->id) < 0) {
+		fprintf(stderr, "task died before we could map its memory\n");
+		goto out;
+	}
+
+	/*
+	 * Phew, we've got the right /proc/pid/mem. Now we can read it. Note
+	 * that to avoid another TOCTOU, we should read all of the pointer args
+	 * before we decide to allow the syscall.
+	 */
+	if (lseek(mem, req->data.args[0], SEEK_SET) < 0) {
+		perror("seek");
+		goto out;
+	}
+
+	ret = read(mem, source, sizeof(source));
+	if (ret < 0) {
+		perror("read");
+		goto out;
+	}
+
+	if (lseek(mem, req->data.args[1], SEEK_SET) < 0) {
+		perror("seek");
+		goto out;
+	}
+
+	ret = read(mem, target, sizeof(target));
+	if (ret < 0) {
+		perror("read");
+		goto out;
+	}
+
+	/*
+	 * Our policy is to only allow bind mounts inside /tmp. This isn't very
+	 * interesting, because we could do unprivlieged bind mounts with user
+	 * namespaces already, but you get the idea.
+	 */
+	if (!strncmp(source, "/tmp/", 5) && !strncmp(target, "/tmp/", 5)) {
+		if (mount(source, target, NULL, req->data.args[3], NULL) < 0) {
+			ret = -1;
+			perror("actual mount");
+			goto out;
+		}
+		resp->error = 0;
+	}
+
+	/* Even if we didn't allow it because of policy, generating the
+	 * response was be a success, because we want to tell the worker EPERM.
+	 */
+	ret = 0;
+
+out:
+	close(mem);
+	return ret;
+}
+
+int main(void)
+{
+	int sk_pair[2], ret = 1, status, listener;
+	pid_t worker = 0 , tracer = 0;
+
+	if (socketpair(PF_LOCAL, SOCK_SEQPACKET, 0, sk_pair) < 0) {
+		perror("socketpair");
+		return 1;
+	}
+
+	worker = fork();
+	if (worker < 0) {
+		perror("fork");
+		goto close_pair;
+	}
+
+	if (worker == 0) {
+		listener = user_trap_syscall(__NR_mount,
+					     SECCOMP_FILTER_FLAG_NEW_LISTENER);
+		if (listener < 0) {
+			perror("seccomp");
+			exit(1);
+		}
+
+		/*
+		 * Drop privileges. We definitely can't mount as uid 1000.
+		 */
+		if (setuid(1000) < 0) {
+			perror("setuid");
+			exit(1);
+		}
+
+		/*
+		 * Send the listener to the parent; also serves as
+		 * synchronization.
+		 */
+		if (send_fd(sk_pair[1], listener) < 0)
+			exit(1);
+		close(listener);
+
+		if (mkdir("/tmp/foo", 0755) < 0) {
+			perror("mkdir");
+			exit(1);
+		}
+
+		/*
+		 * Try a bad mount just for grins.
+		 */
+		if (mount("/dev/sda", "/tmp/foo", NULL, 0, NULL) != -1) {
+			fprintf(stderr, "huh? mounted /dev/sda?\n");
+			exit(1);
+		}
+
+		if (errno != EPERM) {
+			perror("bad error from mount");
+			exit(1);
+		}
+
+		/*
+		 * Ok, we expect this one to succeed.
+		 */
+		if (mount("/tmp/foo", "/tmp/foo", NULL, MS_BIND, NULL) < 0) {
+			perror("mount");
+			exit(1);
+		}
+
+		exit(0);
+	}
+
+	/*
+	 * Get the listener from the child.
+	 */
+	listener = recv_fd(sk_pair[0]);
+	if (listener < 0)
+		goto out_kill;
+
+	/*
+	 * Fork a task to handle the requests. This isn't strictly necessary,
+	 * but it makes the particular writing of this sample easier, since we
+	 * can just wait ofr the tracee to exit and kill the tracer.
+	 */
+	tracer = fork();
+	if (tracer < 0) {
+		perror("fork");
+		goto out_kill;
+	}
+
+	if (tracer == 0) {
+		struct seccomp_notif *req;
+		struct seccomp_notif_resp *resp;
+		struct seccomp_notif_sizes sizes;
+
+		if (seccomp(SECCOMP_GET_NOTIF_SIZES, 0, &sizes) < 0) {
+			perror("seccomp(GET_NOTIF_SIZES)");
+			goto out_close;
+		}
+
+		req = malloc(sizes.seccomp_notif);
+		if (!req)
+			goto out_close;
+
+		resp = malloc(sizes.seccomp_notif_resp);
+		if (!resp)
+			goto out_req;
+		memset(resp, 0, sizes.seccomp_notif_resp);
+
+		while (1) {
+			memset(req, 0, sizes.seccomp_notif);
+			if (ioctl(listener, SECCOMP_IOCTL_NOTIF_RECV, req)) {
+				perror("ioctl recv");
+				goto out_resp;
+			}
+
+			if (handle_req(req, resp, listener) < 0)
+				goto out_resp;
+
+			/*
+			 * ENOENT here means that the task may have gotten a
+			 * signal and restarted the syscall. It's up to the
+			 * handler to decide what to do in this case, but for
+			 * the sample code, we just ignore it. Probably
+			 * something better should happen, like undoing the
+			 * mount, or keeping track of the args to make sure we
+			 * don't do it again.
+			 */
+			if (ioctl(listener, SECCOMP_IOCTL_NOTIF_SEND, resp) < 0 &&
+			    errno != ENOENT) {
+				perror("ioctl send");
+				goto out_resp;
+			}
+		}
+out_resp:
+		free(resp);
+out_req:
+		free(req);
+out_close:
+		close(listener);
+		exit(1);
+	}
+
+	close(listener);
+
+	if (waitpid(worker, &status, 0) != worker) {
+		perror("waitpid");
+		goto out_kill;
+	}
+
+	if (umount2("/tmp/foo", MNT_DETACH) < 0 && errno != EINVAL) {
+		perror("umount2");
+		goto out_kill;
+	}
+
+	if (remove("/tmp/foo") < 0 && errno != ENOENT) {
+		perror("remove");
+		exit(1);
+	}
+
+	if (!WIFEXITED(status) || WEXITSTATUS(status)) {
+		fprintf(stderr, "worker exited nonzero\n");
+		goto out_kill;
+	}
+
+	ret = 0;
+
+out_kill:
+	if (tracer > 0)
+		kill(tracer, SIGKILL);
+	if (worker > 0)
+		kill(worker, SIGKILL);
+
+close_pair:
+	close(sk_pair[0]);
+	close(sk_pair[1]);
+	return ret;
+}