[PATCH RESEND v2 51/51] selftests/proc: check that a faulting readlink() doesn't hold up removal
From: Christian Brauner
Date: Wed Nov 04 2026 - 15:41:39 EST
Add the symlink case:
- readlink() /proc/asound/<id> of a dummy card into a buffer backed by
userfaultfd
- change the card's id through sysfs while the reader faults, which
removes the symlink with snd_card_mutex and info_mutex held
- expect that to finish without waiting for the reader
Skipped without snd-dummy.
Signed-off-by: Christian Brauner (Amutable) <brauner@xxxxxxxxxx>
---
tools/testing/selftests/proc/.gitignore | 1 +
tools/testing/selftests/proc/Makefile | 1 +
tools/testing/selftests/proc/config | 5 +
.../selftests/proc/proc-remove-slow-readlink.c | 191 +++++++++++++++++++++
4 files changed, 198 insertions(+)
diff --git a/tools/testing/selftests/proc/.gitignore b/tools/testing/selftests/proc/.gitignore
index 7b8133492782..3fa89f46b9c7 100644
--- a/tools/testing/selftests/proc/.gitignore
+++ b/tools/testing/selftests/proc/.gitignore
@@ -22,6 +22,7 @@
/proc-uptime-002
/proc-pidns
/proc-remove-slow-sysctl
+/proc-remove-slow-readlink
/read
/self
/setns-dcache
diff --git a/tools/testing/selftests/proc/Makefile b/tools/testing/selftests/proc/Makefile
index bce6beca7274..df02699a3587 100644
--- a/tools/testing/selftests/proc/Makefile
+++ b/tools/testing/selftests/proc/Makefile
@@ -33,5 +33,6 @@ TEST_GEN_PROGS += proc-fsconfig-hidepid
TEST_GEN_PROGS += proc-sysctl-readdir-churn
TEST_GEN_PROGS += proc-pidns
TEST_GEN_PROGS += proc-remove-slow-sysctl
+TEST_GEN_PROGS += proc-remove-slow-readlink
include ../lib.mk
diff --git a/tools/testing/selftests/proc/config b/tools/testing/selftests/proc/config
index 052e365af5a9..1504a761b39b 100644
--- a/tools/testing/selftests/proc/config
+++ b/tools/testing/selftests/proc/config
@@ -3,3 +3,8 @@ CONFIG_NET_NS=y
CONFIG_IPV6=y
CONFIG_TUN=y
CONFIG_USERFAULTFD=y
+CONFIG_SOUND=y
+CONFIG_SND=y
+CONFIG_SND_DUMMY=m
+CONFIG_MODULES=y
+CONFIG_MODULE_UNLOAD=y
diff --git a/tools/testing/selftests/proc/proc-remove-slow-readlink.c b/tools/testing/selftests/proc/proc-remove-slow-readlink.c
new file mode 100644
index 000000000000..3290225daecd
--- /dev/null
+++ b/tools/testing/selftests/proc/proc-remove-slow-readlink.c
@@ -0,0 +1,191 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * A readlink() that faults while copying out the link must not hold up
+ * removal of the /proc symlink. Changing a sound card's id removes
+ * /proc/asound/<id> with snd_card_mutex and info_mutex held.
+ */
+#define _GNU_SOURCE
+#include <dirent.h>
+#include <errno.h>
+#include <fcntl.h>
+#include <limits.h>
+#include <linux/userfaultfd.h>
+#include <poll.h>
+#include <pthread.h>
+#include <stdio.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 CARD_ID "Dummy"
+#define NEW_ID "procslow"
+#define LINK_PATH "/proc/asound/" CARD_ID
+#define TIMEOUT_MS 5000
+
+struct reader {
+ char *buf;
+ size_t len;
+ ssize_t ret;
+};
+
+struct renamer {
+ const char *id_path;
+ const char *id;
+ int done;
+ int ret;
+};
+
+static void *readlink_thread(void *arg)
+{
+ struct reader *r = arg;
+
+ r->ret = readlink(LINK_PATH, r->buf, r->len);
+ return NULL;
+}
+
+/* a new id removes the card's old /proc/asound/<id> link */
+static int set_id(const char *id_path, const char *id)
+{
+ int fd, ret;
+
+ fd = open(id_path, O_WRONLY | O_CLOEXEC);
+ if (fd < 0)
+ return -errno;
+ ret = write(fd, id, strlen(id)) < 0 ? -errno : 0;
+ close(fd);
+ return ret;
+}
+
+static void *rename_thread(void *arg)
+{
+ struct renamer *r = arg;
+ ssize_t ret;
+
+ r->ret = set_id(r->id_path, r->id);
+ ret = write(r->done, "x", 1);
+ return (void *)ret;
+}
+
+/* the sysfs id file of the card with @id */
+static int find_card(const char *id, char *id_path, size_t size)
+{
+ struct dirent *d;
+ int ret = -1;
+ DIR *dir;
+
+ dir = opendir("/sys/class/sound");
+ if (!dir)
+ return -1;
+ while (ret && (d = readdir(dir))) {
+ char cur[32];
+ FILE *f;
+
+ if (strncmp(d->d_name, "card", 4))
+ continue;
+ snprintf(id_path, size, "/sys/class/sound/%s/id", d->d_name);
+ f = fopen(id_path, "r");
+ if (!f)
+ continue;
+ if (fgets(cur, sizeof(cur), f)) {
+ cur[strcspn(cur, "\n")] = '\0';
+ if (!strcmp(cur, id))
+ ret = 0;
+ }
+ fclose(f);
+ }
+ closedir(dir);
+ return ret;
+}
+
+FIXTURE(dummy) {
+ char id_path[PATH_MAX];
+ bool renamed;
+};
+
+FIXTURE_SETUP(dummy)
+{
+ self->renamed = false;
+ if (access(LINK_PATH, F_OK) && system("modprobe -q snd-dummy"))
+ SKIP(return, "no snd-dummy");
+ if (access(LINK_PATH, F_OK))
+ SKIP(return, LINK_PATH ": %s", strerror(errno));
+ if (find_card(CARD_ID, self->id_path, sizeof(self->id_path)))
+ SKIP(return, "no sysfs id file for " CARD_ID);
+ if (access(self->id_path, W_OK))
+ SKIP(return, "%s: %s", self->id_path, strerror(errno));
+}
+
+FIXTURE_TEARDOWN(dummy)
+{
+ if (self->renamed && set_id(self->id_path, CARD_ID))
+ TH_LOG("couldn't name the card " CARD_ID " again");
+}
+
+TEST_F(dummy, remove_while_readlink_faults)
+{
+ struct uffdio_api api = { .api = UFFD_API };
+ struct uffdio_register reg = {};
+ struct uffdio_zeropage zero = {};
+ long psz = sysconf(_SC_PAGESIZE);
+ struct renamer rn = {};
+ struct reader r = {};
+ struct uffd_msg msg;
+ struct pollfd pfd;
+ pthread_t rt, nt;
+ int uffd, done[2];
+ char *buf;
+
+ 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);
+
+ buf = mmap(NULL, 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, ®), 0);
+
+ r.buf = buf;
+ r.len = psz;
+ ASSERT_EQ(pthread_create(&rt, NULL, readlink_thread, &r), 0);
+
+ /* The reader now sleeps in the copy of the link 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);
+
+ /* Renaming the card removes the link that is being read. */
+ ASSERT_EQ(pipe2(done, O_CLOEXEC), 0);
+ rn.id_path = self->id_path;
+ rn.id = NEW_ID;
+ rn.done = done[1];
+ self->renamed = true;
+ ASSERT_EQ(pthread_create(&nt, NULL, rename_thread, &rn), 0);
+ pfd = (struct pollfd){ .fd = done[0], .events = POLLIN };
+ EXPECT_EQ(poll(&pfd, 1, TIMEOUT_MS), 1) {
+ TH_LOG("removing " LINK_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(nt, NULL), 0);
+ EXPECT_EQ(rn.ret, 0);
+
+ /* The link was read before it went away. */
+ EXPECT_GT(r.ret, 0);
+ EXPECT_EQ(readlink(LINK_PATH, buf, psz), -1);
+ EXPECT_EQ(errno, ENOENT);
+}
+
+TEST_HARNESS_MAIN
--
2.53.0