Merge branch 'fix_bpf_send_signal'

Yonghong Song says:

====================
Commit 8b401f9ed2 ("bpf: implement bpf_send_signal() helper")
introduced bpf_send_signal() helper and Commit 8482941f09
("bpf: Add bpf_send_signal_thread() helper") added bpf_send_signal_thread()
helper. Both helpers try to send a signel to current process or thread.

When bpf_prog is called with scheduler rq_lock held, a deadlock
could happen since bpf_send_signal() and bpf_send_signal_thread()
will call group_send_sig_info() which may ultimately want to acquire
rq_lock() again. This happens in 5.2 and 4.16 based kernels in our
production environment with perf_sw_event.

In a different scenario, the following is also possible in the last kernel:
  cpu 1:
     do_task_stat <- holding sighand->siglock
     ...
     task_sched_runtime <- trying to grab rq_lock

  cpu 2:
     __schedule <- holding rq_lock
     ...
     do_send_sig_info <- trying to grab sighand->siglock

Commit eac9153f2b ("bpf/stackmap: Fix deadlock with
rq_lock in bpf_get_stack()") has a similar issue with above
rq_lock() deadlock. This patch set addressed the issue
in a similar way. Patch #1 provided kernel solution and
Patch #2 added a selftest.

Changelogs:
  v2 -> v3:
    . simplify selftest send_signal_sched_switch().
      The previous version has mmap/munmap inherited
      from Song's reproducer. They are not necessary
      in this context.
  v1 -> v2:
    . previous fix using task_work in nmi() is incorrect.
      there is no nmi() involvement here. Using task_work
      in all cases might be a solution. But decided to
      use a similar fix as in Commit eac9153f2b.
====================

Signed-off-by: Alexei Starovoitov <ast@kernel.org>

kernel/trace/bpf_trace.c

@@ -732,7 +732,7 @@ static int bpf_send_signal_common(u32 sig, enum pid_type type)
 	if (unlikely(!nmi_uaccess_okay()))
 		return -EPERM;
 
-	if (in_nmi()) {
+	if (irqs_disabled()) {
 		/* Do an early check on signal validity. Otherwise,
 		 * the error is lost in deferred irq_work.
 		 */

tools/testing/selftests/bpf/prog_tests/send_signal_sched_switch.c

@@ -0,0 +1,60 @@
+// SPDX-License-Identifier: GPL-2.0
+#include <test_progs.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <sys/mman.h>
+#include <pthread.h>
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <fcntl.h>
+#include "test_send_signal_kern.skel.h"
+
+static void sigusr1_handler(int signum)
+{
+}
+
+#define THREAD_COUNT 100
+
+static void *worker(void *p)
+{
+	int i;
+
+	for ( i = 0; i < 1000; i++)
+		usleep(1);
+
+	return NULL;
+}
+
+void test_send_signal_sched_switch(void)
+{
+	struct test_send_signal_kern *skel;
+	pthread_t threads[THREAD_COUNT];
+	u32 duration = 0;
+	int i, err;
+
+	signal(SIGUSR1, sigusr1_handler);
+
+	skel = test_send_signal_kern__open_and_load();
+	if (CHECK(!skel, "skel_open_and_load", "skeleton open_and_load failed\n"))
+		return;
+
+	skel->bss->pid = getpid();
+	skel->bss->sig = SIGUSR1;
+
+	err = test_send_signal_kern__attach(skel);
+	if (CHECK(err, "skel_attach", "skeleton attach failed\n"))
+		goto destroy_skel;
+
+	for (i = 0; i < THREAD_COUNT; i++) {
+		err = pthread_create(threads + i, NULL, worker, NULL);
+		if (CHECK(err, "pthread_create", "Error creating thread, %s\n",
+			  strerror(errno)))
+			goto destroy_skel;
+	}
+
+	for (i = 0; i < THREAD_COUNT; i++)
+		pthread_join(threads[i], NULL);
+
+destroy_skel:
+	test_send_signal_kern__destroy(skel);
+}

tools/testing/selftests/bpf/progs/test_send_signal_kern.c

@@ -31,6 +31,12 @@ int send_signal_tp(void *ctx)
 	return bpf_send_signal_test(ctx);
 }
 
+SEC("tracepoint/sched/sched_switch")
+int send_signal_tp_sched(void *ctx)
+{
+	return bpf_send_signal_test(ctx);
+}
+
 SEC("perf_event")
 int send_signal_perf(void *ctx)
 {