[PATCH RFC 27/27] selftests/proc: check that unloading configs doesn't wait for a reader

[Christian Brauner <brauner@kernel.org>]

Read /proc/config.gz into a buffer whose first page is backed by
userfaultfd and unload configs.ko while procfs copies what was read to
userspace:

- removing the entry doesn't wait for the reader

- the reader still gets the page that was read before the removal, and
  nothing more

- and the next read fails with -EIO

The test loads configs.ko if it isn't there, skips if configs is built
in or can't be unloaded, and loads it again afterwards. It needs
CONFIG_IKCONFIG=m. kselftest-merge keeps the =y that other fragments
ask for, so it skips there.

Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>
---
 tools/testing/selftests/proc/.gitignore            |   1 +
 tools/testing/selftests/proc/Makefile              |   1 +
 tools/testing/selftests/proc/config                |   4 +
 .../proc/proc-remove-slow-config-reader.c          | 157 +++++++++++++++++++++
 4 files changed, 163 insertions(+)

diff --git a/tools/testing/selftests/proc/.gitignore b/tools/testing/selftests/proc/.gitignore
index 51969f81afc5..ed3f28652bb7 100644
--- a/tools/testing/selftests/proc/.gitignore
+++ b/tools/testing/selftests/proc/.gitignore
@@ -25,6 +25,7 @@
 /proc-remove-slow-reader
 /proc-remove-slow-readlink
 /proc-remove-slow-writer
+/proc-remove-slow-config-reader
 /read
 /self
 /setns-dcache
diff --git a/tools/testing/selftests/proc/Makefile b/tools/testing/selftests/proc/Makefile
index aecec6b93173..adbb4b1e9e9e 100644
--- a/tools/testing/selftests/proc/Makefile
+++ b/tools/testing/selftests/proc/Makefile
@@ -36,5 +36,6 @@ TEST_GEN_PROGS += proc-remove-slow-sysctl
 TEST_GEN_PROGS += proc-remove-slow-reader
 TEST_GEN_PROGS += proc-remove-slow-readlink
 TEST_GEN_PROGS += proc-remove-slow-writer
+TEST_GEN_PROGS += proc-remove-slow-config-reader
 
 include ../lib.mk
diff --git a/tools/testing/selftests/proc/config b/tools/testing/selftests/proc/config
index c7c61a2d7927..8f672a8b49a8 100644
--- a/tools/testing/selftests/proc/config
+++ b/tools/testing/selftests/proc/config
@@ -8,3 +8,7 @@ CONFIG_SND_DUMMY=m
 CONFIG_SCSI=y
 CONFIG_SCSI_PROC_FS=y
 CONFIG_SCSI_DEBUG=m
+CONFIG_MODULES=y
+CONFIG_MODULE_UNLOAD=y
+CONFIG_IKCONFIG=m
+CONFIG_IKCONFIG_PROC=y
diff --git a/tools/testing/selftests/proc/proc-remove-slow-config-reader.c b/tools/testing/selftests/proc/proc-remove-slow-config-reader.c
new file mode 100644
index 000000000000..5f7bcb27b315
--- /dev/null
+++ b/tools/testing/selftests/proc/proc-remove-slow-config-reader.c
@@ -0,0 +1,157 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * A read that faults while procfs copies it to userspace must not hold up
+ * removal of the /proc entry. Unloading configs.ko removes /proc/config.gz,
+ * which isn't a seq_file.
+ */
+#define _GNU_SOURCE
+#include <errno.h>
+#include <fcntl.h>
+#include <linux/userfaultfd.h>
+#include <poll.h>
+#include <pthread.h>
+#include <stdlib.h>
+#include <string.h>
+#include <sys/ioctl.h>
+#include <sys/mman.h>
+#include <sys/syscall.h>
+#include <unistd.h>
+
+#include "../kselftest_harness.h"
+
+#define CONFIG_PATH	"/proc/config.gz"
+#define CONFIG_MODULE	"/sys/module/configs"
+#define READ_PAGES	2
+#define TIMEOUT_MS	5000
+
+struct reader {
+	int fd;
+	char *buf;
+	size_t len;
+	ssize_t ret;
+};
+
+struct unloader {
+	int done;
+	int ret;
+};
+
+static void *read_thread(void *arg)
+{
+	struct reader *r = arg;
+
+	r->ret = read(r->fd, r->buf, r->len);
+	return NULL;
+}
+
+static void *unload_thread(void *arg)
+{
+	struct unloader *u = arg;
+	ssize_t ret;
+
+	u->ret = syscall(__NR_delete_module, "configs", O_NONBLOCK) ? -errno : 0;
+	ret = write(u->done, "x", 1);
+	return (void *)ret;
+}
+
+FIXTURE(configs) {
+	bool loaded;
+};
+
+FIXTURE_SETUP(configs)
+{
+	self->loaded = false;
+	if (access(CONFIG_PATH, R_OK) && system("modprobe -q configs"))
+		SKIP(return, "no " CONFIG_PATH);
+	if (access(CONFIG_PATH, R_OK))
+		SKIP(return, CONFIG_PATH ": %s", strerror(errno));
+	if (access(CONFIG_MODULE, F_OK))
+		SKIP(return, "configs isn't a module");
+	self->loaded = true;
+}
+
+FIXTURE_TEARDOWN(configs)
+{
+	/* an ASSERT may have fired with the module unloaded */
+	if (self->loaded && access(CONFIG_PATH, F_OK) &&
+	    system("modprobe -q configs"))
+		TH_LOG("couldn't load configs again");
+}
+
+TEST_F(configs, remove_while_reader_faults)
+{
+	struct uffdio_api api = { .api = UFFD_API };
+	struct uffdio_register reg = {};
+	struct uffdio_zeropage zero = {};
+	long psz = sysconf(_SC_PAGESIZE);
+	struct unloader u = {};
+	struct reader r = {};
+	struct uffd_msg msg;
+	struct pollfd pfd;
+	pthread_t rt, ut;
+	int uffd, done[2];
+	char *buf, *ref;
+	ssize_t ref_len;
+
+	r.fd = open(CONFIG_PATH, O_RDONLY | O_CLOEXEC);
+	ASSERT_GE(r.fd, 0);
+
+	/* what the first page of a read looks like */
+	ref = malloc(psz);
+	ASSERT_NE(ref, NULL);
+	ref_len = pread(r.fd, ref, psz, 0);
+	ASSERT_GT(ref_len, 0);
+
+	uffd = syscall(__NR_userfaultfd, O_CLOEXEC | O_NONBLOCK);
+	if (uffd < 0)
+		SKIP(return, "userfaultfd: %s", strerror(errno));
+	ASSERT_EQ(ioctl(uffd, UFFDIO_API, &api), 0);
+
+	/* Only the first page of the buffer goes through userfaultfd. */
+	buf = mmap(NULL, READ_PAGES * psz, PROT_READ | PROT_WRITE,
+		   MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
+	ASSERT_NE(buf, MAP_FAILED);
+	reg.range.start = (unsigned long)buf;
+	reg.range.len = psz;
+	reg.mode = UFFDIO_REGISTER_MODE_MISSING;
+	ASSERT_EQ(ioctl(uffd, UFFDIO_REGISTER, &reg), 0);
+
+	r.buf = buf;
+	r.len = READ_PAGES * psz;
+	ASSERT_EQ(pthread_create(&rt, NULL, read_thread, &r), 0);
+
+	/* The reader now sleeps in the copy to userspace. */
+	pfd = (struct pollfd){ .fd = uffd, .events = POLLIN };
+	ASSERT_EQ(poll(&pfd, 1, TIMEOUT_MS), 1);
+	ASSERT_EQ(read(uffd, &msg, sizeof(msg)), sizeof(msg));
+	ASSERT_EQ(msg.event, UFFD_EVENT_PAGEFAULT);
+
+	/* Unloading configs.ko removes the file that is being read. */
+	ASSERT_EQ(pipe2(done, O_CLOEXEC), 0);
+	u.done = done[1];
+	ASSERT_EQ(pthread_create(&ut, NULL, unload_thread, &u), 0);
+	pfd = (struct pollfd){ .fd = done[0], .events = POLLIN };
+	EXPECT_EQ(poll(&pfd, 1, TIMEOUT_MS), 1) {
+		TH_LOG("removing " CONFIG_PATH " waited for the reader");
+	}
+
+	zero.range.start = (unsigned long)buf;
+	zero.range.len = psz;
+	ASSERT_EQ(ioctl(uffd, UFFDIO_ZEROPAGE, &zero), 0);
+	ASSERT_EQ(pthread_join(rt, NULL), 0);
+	ASSERT_EQ(pthread_join(ut, NULL), 0);
+	if (u.ret)
+		SKIP(return, "unloading configs: %s", strerror(-u.ret));
+
+	/*
+	 * The page that was read before the removal still arrives and the
+	 * next piece isn't filled anymore, then nothing.
+	 */
+	EXPECT_EQ(r.ret, ref_len);
+	EXPECT_EQ(memcmp(buf, ref, ref_len), 0);
+	EXPECT_EQ(read(r.fd, buf, psz), -1);
+	EXPECT_EQ(errno, EIO);
+	free(ref);
+}
+
+TEST_HARNESS_MAIN

-- 
2.53.0