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/* SPDX-License-Identifier: GPL-2.0 */
/*
* Hold a readdir of a directory in the page fault of its buffer and see
* whether a create and a lookup in that directory wait for it. For a
* directory that is permanently empty they must not.
*/
#ifndef __SELFTESTS_READDIR_HOLD_H
#define __SELFTESTS_READDIR_HOLD_H
#include <errno.h>
#include <fcntl.h>
#include <poll.h>
#include <pthread.h>
#include <stdbool.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <linux/userfaultfd.h>
#include <sys/ioctl.h>
#include <sys/mman.h>
#include <sys/stat.h>
#include <sys/syscall.h>
#define HOLD_FAULT_MS 5000 /* for the reader to reach the fault */
#define HOLD_QUEUE_MS 2000 /* for the create to queue up behind it */
#define HOLD_LOOKUP_MS 5000 /* for the lookup to come back */
struct readdir_hold {
int uffd;
int taskdir; /* /proc/self/task, from before any namespace change */
long page_size;
char *page; /* faults until released */
int dfd;
pid_t creator_tid;
int created;
int done[2]; /* the finder writes a byte when it is back */
};
/*
* The fault happens in the kernel, so the userfaultfd needs CAP_SYS_PTRACE
* in the initial user namespace or vm.unprivileged_userfaultfd. Call this
* before entering a user namespace.
*/
static inline int readdir_hold_init(struct readdir_hold *h)
{
struct uffdio_api api = { .api = UFFD_API };
h->page = MAP_FAILED;
h->page_size = sysconf(_SC_PAGESIZE);
h->taskdir = open("/proc/self/task", O_RDONLY | O_DIRECTORY | O_CLOEXEC);
h->uffd = syscall(__NR_userfaultfd, O_CLOEXEC | O_NONBLOCK);
if (h->uffd < 0 || h->taskdir < 0 || ioctl(h->uffd, UFFDIO_API, &api)) {
if (h->uffd >= 0)
close(h->uffd);
if (h->taskdir >= 0)
close(h->taskdir);
h->uffd = h->taskdir = -1;
return -1;
}
return 0;
}
static inline void readdir_hold_destroy(struct readdir_hold *h)
{
if (h->uffd >= 0)
close(h->uffd);
if (h->taskdir >= 0)
close(h->taskdir);
h->uffd = h->taskdir = -1;
}
static inline void *readdir_hold_reader(void *arg)
{
struct readdir_hold *h = arg;
/* the first byte written to the buffer faults until released */
syscall(__NR_getdents64, h->dfd, h->page, h->page_size);
return NULL;
}
static inline void *readdir_hold_creator(void *arg)
{
struct readdir_hold *h = arg;
h->creator_tid = syscall(__NR_gettid);
/* takes the directory lock exclusive before it fails */
mkdirat(h->dfd, "x", 0755);
__atomic_store_n(&h->created, 1, __ATOMIC_RELEASE);
return NULL;
}
static inline void *readdir_hold_finder(void *arg)
{
struct readdir_hold *h = arg;
int fd;
/* a lookup that misses the dcache takes the lock shared */
fd = openat(h->dfd, "no_such_name", O_RDONLY | O_CLOEXEC);
if (fd >= 0)
close(fd);
if (write(h->done[1], "x", 1) != 1)
perror("readdir_hold: finder");
return NULL;
}
/* the creator is back, or waits in the kernel for the lock */
static inline bool readdir_hold_creator_settled(struct readdir_hold *h)
{
char path[32], buf[256], *p;
ssize_t n;
int fd;
if (__atomic_load_n(&h->created, __ATOMIC_ACQUIRE))
return true;
if (!h->creator_tid)
return false;
snprintf(path, sizeof(path), "%d/stat", h->creator_tid);
fd = openat(h->taskdir, path, O_RDONLY | O_CLOEXEC);
if (fd < 0)
return false;
n = read(fd, buf, sizeof(buf) - 1);
close(fd);
if (n <= 0)
return false;
buf[n] = 0;
/* "pid (comm) state ..." */
p = strrchr(buf, ')');
return p && p[1] == ' ' && p[2] == 'D';
}
static inline bool readdir_hold_faulted(struct readdir_hold *h)
{
struct pollfd pfd = { .fd = h->uffd, .events = POLLIN };
struct uffd_msg msg;
if (poll(&pfd, 1, HOLD_FAULT_MS) != 1)
return false;
if (read(h->uffd, &msg, sizeof(msg)) != sizeof(msg))
return false;
return msg.event == UFFD_EVENT_PAGEFAULT;
}
/* let the reader go on */
static inline void readdir_hold_release(struct readdir_hold *h)
{
struct uffdio_copy cp = {
.dst = (unsigned long)h->page,
.len = h->page_size,
};
void *zero;
zero = calloc(1, h->page_size);
if (!zero)
return;
cp.src = (unsigned long)zero;
if (ioctl(h->uffd, UFFDIO_COPY, &cp) && errno != EEXIST)
perror("readdir_hold: UFFDIO_COPY");
free(zero);
}
/*
* A readdir of @dfd that sticks in the fault of its buffer, then a create
* and a lookup in @dfd. Whether the lookup came back while the readdir was
* still stuck goes to @stalled. Returns -1 when that could not be found
* out.
*/
static inline int readdir_hold_check(struct readdir_hold *h, int dfd,
bool *stalled)
{
pthread_t reader, creator, finder;
struct uffdio_register reg = {};
struct pollfd pfd;
int ret = -1, i;
h->dfd = dfd;
h->created = 0;
h->creator_tid = 0;
h->page = mmap(NULL, h->page_size, PROT_READ | PROT_WRITE,
MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
if (h->page == MAP_FAILED)
return -1;
reg.range.start = (unsigned long)h->page;
reg.range.len = h->page_size;
reg.mode = UFFDIO_REGISTER_MODE_MISSING;
if (ioctl(h->uffd, UFFDIO_REGISTER, ®) || pipe2(h->done, O_CLOEXEC))
goto out_page;
if (pthread_create(&reader, NULL, readdir_hold_reader, h))
goto out_pipe;
if (!readdir_hold_faulted(h))
goto out_reader;
if (pthread_create(&creator, NULL, readdir_hold_creator, h))
goto out_reader;
for (i = 0; i < HOLD_QUEUE_MS / 10 && !readdir_hold_creator_settled(h); i++)
usleep(10000);
if (pthread_create(&finder, NULL, readdir_hold_finder, h))
goto out_creator;
pfd.fd = h->done[0];
pfd.events = POLLIN;
*stalled = poll(&pfd, 1, HOLD_LOOKUP_MS) != 1;
ret = 0;
readdir_hold_release(h);
pthread_join(finder, NULL);
out_creator:
if (ret)
readdir_hold_release(h);
pthread_join(creator, NULL);
out_reader:
if (ret)
readdir_hold_release(h);
pthread_join(reader, NULL);
out_pipe:
close(h->done[0]);
close(h->done[1]);
out_page:
munmap(h->page, h->page_size);
h->page = MAP_FAILED;
return ret;
}
#endif /* __SELFTESTS_READDIR_HOLD_H */
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