[PATCH RFC 20/38] selftests/proc: check a /proc listing while entries come and go

[Christian Brauner]

List /proc/net/dev_snmp6 of a new network namespace while a tun device
comes and goes:

- with a buffer that holds the dots and "lo", add a device whose name
  sorts first, finish the listing: "lo" is listed once

- with that device added and a buffer that holds the dots and it,
  remove it, finish the listing: "lo" is still listed

- with that device added, stall getdents64() on a userfaultfd page
  while it writes the device's entry, remove the device, finish the
  listing: "lo" is still listed

All three fail without the fix, the first lists "lo" twice, the other
two not at all.

Signed-off-by: Christian Brauner (Amutable) <brauner@xxxxxxxxxx>
---
 tools/testing/selftests/proc/.gitignore           |   1 +
 tools/testing/selftests/proc/Makefile             |   1 +
 tools/testing/selftests/proc/proc-readdir-churn.c | 236 ++++++++++++++++++++++
 3 files changed, 238 insertions(+)

diff --git a/tools/testing/selftests/proc/.gitignore b/tools/testing/selftests/proc/.gitignore
index 9c9735570abf..8520c7878ea2 100644
--- a/tools/testing/selftests/proc/.gitignore
+++ b/tools/testing/selftests/proc/.gitignore
@@ -8,6 +8,7 @@
 /proc-maps-race
 /proc-multiple-procfs
 /proc-net-dev-lseek
+/proc-readdir-churn
 /proc-empty-vm
 /proc-pid-vm
 /proc-self-map-files-001
diff --git a/tools/testing/selftests/proc/Makefile b/tools/testing/selftests/proc/Makefile
index a7de2bb6d8be..e0c0d313f4d0 100644
--- a/tools/testing/selftests/proc/Makefile
+++ b/tools/testing/selftests/proc/Makefile
@@ -11,6 +11,7 @@ TEST_GEN_PROGS += proc-2-is-kthread
 TEST_GEN_PROGS += proc-loadavg-001
 TEST_GEN_PROGS += proc-maps-race
 TEST_GEN_PROGS += proc-net-dev-lseek
+TEST_GEN_PROGS += proc-readdir-churn
 TEST_GEN_PROGS += proc-empty-vm
 TEST_GEN_PROGS += proc-pid-vm
 TEST_GEN_PROGS += proc-self-map-files-001
diff --git a/tools/testing/selftests/proc/proc-readdir-churn.c b/tools/testing/selftests/proc/proc-readdir-churn.c
new file mode 100644
index 000000000000..223b8a6698ee
--- /dev/null
+++ b/tools/testing/selftests/proc/proc-readdir-churn.c
@@ -0,0 +1,236 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * A listing of a /proc directory that takes more than one getdents64()
+ * must not show an entry twice or skip one when an entry is added or
+ * removed in between, and a listing must go on after an entry that is
+ * removed while it is being emitted. /proc/net/dev_snmp6/<dev> of a tun
+ * device comes and goes with the device.
+ */
+#define _GNU_SOURCE
+#include <dirent.h>
+#include <errno.h>
+#include <fcntl.h>
+#include <limits.h>
+#include <linux/if.h>
+#include <linux/if_tun.h>
+#include <linux/userfaultfd.h>
+#include <poll.h>
+#include <pthread.h>
+#include <sched.h>
+#include <string.h>
+#include <sys/ioctl.h>
+#include <sys/mman.h>
+#include <sys/syscall.h>
+#include <unistd.h>
+
+#include "../kselftest_harness.h"
+
+#define SNMP6_DIR	"/proc/net/dev_snmp6"
+#define TUN_NAME	"a0"		/* sorts before "lo" */
+#define TUN_PATH	SNMP6_DIR "/" TUN_NAME
+#define SMALL_BUF	80		/* ".", ".." and one short name */
+#define NAMES		64
+
+static int tun_open(const char *name)
+{
+	struct ifreq ifr = {};
+	int fd;
+
+	fd = open("/dev/net/tun", O_RDWR | O_CLOEXEC);
+	if (fd < 0)
+		return -1;
+	strncpy(ifr.ifr_name, name, IFNAMSIZ - 1);
+	ifr.ifr_flags = IFF_TUN | IFF_NO_PI;
+	if (ioctl(fd, TUNSETIFF, &ifr)) {
+		close(fd);
+		return -1;
+	}
+	return fd;
+}
+
+/* one getdents64() into @buf, the names appended to @names */
+static int list(int fd, char *buf, size_t size, char names[][NAME_MAX + 1],
+		int *n)
+{
+	long ret, off;
+	int got = 0;
+
+	ret = syscall(__NR_getdents64, fd, buf, size);
+	if (ret < 0)
+		return -errno;
+	for (off = 0; off < ret;) {
+		struct dirent64 *d = (void *)(buf + off);
+
+		if (*n < NAMES)
+			strcpy(names[(*n)++], d->d_name);
+		got++;
+		off += d->d_reclen;
+	}
+	return got;
+}
+
+static int count(char names[][NAME_MAX + 1], int n, const char *name)
+{
+	int i, c = 0;
+
+	for (i = 0; i < n; i++)
+		if (!strcmp(names[i], name))
+			c++;
+	return c;
+}
+
+/* removes the device once getdents64() faults on its entry, then resolves */
+struct stall {
+	int uffd;
+	int tun;
+	unsigned long addr;
+	unsigned long len;
+	int removed;
+};
+
+static void *stall_resolve(void *arg)
+{
+	struct stall *s = arg;
+	struct pollfd pfd = { .fd = s->uffd, .events = POLLIN };
+	struct uffdio_zeropage zero = {
+		.range = { .start = s->addr, .len = s->len },
+	};
+	struct uffd_msg msg;
+
+	if (poll(&pfd, 1, 10000) != 1)
+		return NULL;
+	if (read(s->uffd, &msg, sizeof(msg)) != sizeof(msg) ||
+	    msg.event != UFFD_EVENT_PAGEFAULT)
+		return NULL;
+	close(s->tun);
+	s->removed = access(TUN_PATH, F_OK) == -1;
+	ioctl(s->uffd, UFFDIO_ZEROPAGE, &zero);
+	return NULL;
+}
+
+FIXTURE(proc_readdir) {
+	int dir;
+	int n;
+	char names[NAMES][NAME_MAX + 1];
+	char buf[65536];
+};
+
+FIXTURE_SETUP(proc_readdir)
+{
+	if (unshare(CLONE_NEWNET))
+		SKIP(return, "unshare(CLONE_NEWNET): %s", strerror(errno));
+	if (access("/dev/net/tun", F_OK))
+		SKIP(return, "/dev/net/tun: %s", strerror(errno));
+	self->dir = open(SNMP6_DIR, O_RDONLY | O_DIRECTORY | O_CLOEXEC);
+	if (self->dir < 0)
+		SKIP(return, SNMP6_DIR ": %s", strerror(errno));
+	self->n = 0;
+}
+
+FIXTURE_TEARDOWN(proc_readdir)
+{
+	close(self->dir);
+}
+
+/* a device added in front of the position doesn't repeat an entry */
+TEST_F(proc_readdir, add_in_front)
+{
+	int tun;
+
+	/* ".", ".." and "lo" */
+	ASSERT_EQ(list(self->dir, self->buf, SMALL_BUF, self->names, &self->n), 3);
+	ASSERT_EQ(count(self->names, self->n, "lo"), 1);
+
+	tun = tun_open(TUN_NAME);
+	ASSERT_GE(tun, 0);
+	ASSERT_EQ(access(TUN_PATH, F_OK), 0);
+
+	while (list(self->dir, self->buf, sizeof(self->buf), self->names,
+		    &self->n) > 0)
+		;
+	EXPECT_EQ(count(self->names, self->n, "lo"), 1);
+	close(tun);
+}
+
+/* a device removed in front of the position doesn't skip an entry */
+TEST_F(proc_readdir, remove_in_front)
+{
+	int tun;
+
+	tun = tun_open(TUN_NAME);
+	ASSERT_GE(tun, 0);
+	ASSERT_EQ(access(TUN_PATH, F_OK), 0);
+
+	/* ".", ".." and the device, "lo" doesn't fit */
+	ASSERT_EQ(list(self->dir, self->buf, SMALL_BUF, self->names, &self->n), 3);
+	ASSERT_EQ(count(self->names, self->n, TUN_NAME), 1);
+
+	/* closing the device unregisters it and removes its entry */
+	close(tun);
+	ASSERT_EQ(access(TUN_PATH, F_OK), -1);
+
+	while (list(self->dir, self->buf, sizeof(self->buf), self->names,
+		    &self->n) > 0)
+		;
+	EXPECT_EQ(count(self->names, self->n, "lo"), 1);
+}
+
+/* the entry being emitted is removed meanwhile, the listing goes on */
+TEST_F(proc_readdir, remove_current)
+{
+	struct uffdio_api api = { .api = UFFD_API };
+	struct uffdio_register reg = {};
+	long page = sysconf(_SC_PAGESIZE);
+	struct stall s = {};
+	pthread_t thread;
+	char *map;
+	long ret, off;
+
+	s.uffd = syscall(__NR_userfaultfd, O_CLOEXEC | O_NONBLOCK);
+	if (s.uffd < 0)
+		SKIP(return, "userfaultfd: %s", strerror(errno));
+	ASSERT_EQ(ioctl(s.uffd, UFFDIO_API, &api), 0);
+
+	/* the first page is there, the second one faults */
+	map = mmap(NULL, 2 * page, PROT_READ | PROT_WRITE,
+		   MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
+	ASSERT_NE(map, MAP_FAILED);
+	map[0] = 0;
+	s.addr = (unsigned long)map + page;
+	s.len = page;
+	reg.range.start = s.addr;
+	reg.range.len = s.len;
+	reg.mode = UFFDIO_REGISTER_MODE_MISSING;
+	ASSERT_EQ(ioctl(s.uffd, UFFDIO_REGISTER, &reg), 0);
+
+	s.tun = tun_open(TUN_NAME);
+	ASSERT_GE(s.tun, 0);
+	ASSERT_EQ(access(TUN_PATH, F_OK), 0);
+	ASSERT_EQ(pthread_create(&thread, NULL, stall_resolve, &s), 0);
+
+	/*
+	 * ".." ends right before the second page, so writing the record of
+	 * the device stalls until it has been removed.
+	 */
+	ret = syscall(__NR_getdents64, self->dir, map + page - 48, 48 + page);
+	ASSERT_EQ(pthread_join(thread, NULL), 0);
+	ASSERT_TRUE(s.removed);
+	ASSERT_GT(ret, 0);
+	for (off = 0; off < ret;) {
+		struct dirent64 *d = (void *)(map + page - 48 + off);
+
+		if (self->n < NAMES)
+			strcpy(self->names[self->n++], d->d_name);
+		off += d->d_reclen;
+	}
+	ASSERT_EQ(count(self->names, self->n, TUN_NAME), 1);
+
+	while (list(self->dir, self->buf, sizeof(self->buf), self->names,
+		    &self->n) > 0)
+		;
+	EXPECT_EQ(count(self->names, self->n, "lo"), 1);
+	munmap(map, 2 * page);
+	close(s.uffd);
+}
+
+TEST_HARNESS_MAIN

-- 
2.53.0