// SPDX-License-Identifier: GPL-2.0
/*
* memfd GUP test-case
* This tests memfd interactions with get_user_pages(). We require the
* fuse_mnt.c program to provide a fake direct-IO FUSE mount-point for us. This
* file-system delays _all_ reads by 1s and forces direct-IO. This means, any
* read() on files in that file-system will pin the receive-buffer pages for at
* least 1s via get_user_pages().
*
* We use this trick to race ADD_SEALS against a write on a memfd object. The
* ADD_SEALS must fail if the memfd pages are still pinned. Note that we use
* the read() syscall with our memory-mapped memfd object as receive buffer to
* force the kernel to write into our memfd object.
*/
#define _GNU_SOURCE
#define __EXPORTED_HEADERS__
#include <errno.h>
#include <inttypes.h>
#include <limits.h>
#include <linux/falloc.h>
#include <linux/fcntl.h>
#include <linux/memfd.h>
#include <sched.h>
#include <stdio.h>
#include <stdlib.h>
#include <signal.h>
#include <string.h>
#include <sys/mman.h>
#include <sys/stat.h>
#include <sys/syscall.h>
#include <sys/wait.h>
#include <unistd.h>
#include "common.h"
#define MFD_DEF_SIZE 8192
#define STACK_SIZE 65536
static size_t mfd_def_size = MFD_DEF_SIZE;
static int mfd_assert_new(const char *name, loff_t sz, unsigned int flags)
{
int r, fd;
fd = sys_memfd_create(name, flags);
if (fd < 0) {
printf("memfd_create(\"%s\", %u) failed: %m\n",
name, flags);
abort();
}
r = ftruncate(fd, sz);
if (r < 0) {
printf("ftruncate(%llu) failed: %m\n", (unsigned long long)sz);
abort();
}
return fd;
}
static __u64 mfd_assert_get_seals(int fd)
{
long r;
r = fcntl(fd, F_GET_SEALS);
if (r < 0) {
printf("GET_SEALS(%d) failed: %m\n", fd);
abort();
}
return r;
}
static void mfd_assert_has_seals(int fd, __u64 seals)
{
__u64 s;
s = mfd_assert_get_seals(fd);
if (s != seals) {
printf("%llu != %llu = GET_SEALS(%d)\n",
(unsigned long long)seals, (unsigned long long)s, fd);
abort();
}
}
static void mfd_assert_add_seals(int fd, __u64 seals)
{
long r;
__u64 s;
s = mfd_assert_get_seals(fd);
r = fcntl(fd, F_ADD_SEALS, seals);
if (r < 0) {
printf("ADD_SEALS(%d, %llu -> %llu) failed: %m\n",
fd, (unsigned long long)s, (unsigned long long)seals);
abort();
}
}
static int mfd_busy_add_seals(int fd, __u64 seals)
{
long r;
__u64 s;
r = fcntl(fd, F_GET_SEALS);
if (r < 0)
s = 0;
else
s = r;
r = fcntl(fd, F_ADD_SEALS, seals);
if (r < 0 && errno != EBUSY) {
printf("ADD_SEALS(%d, %llu -> %llu) didn't fail as expected with EBUSY: %m\n",
fd, (unsigned long long)s, (unsigned long long)seals);
abort();
}
return r;
}
static void *mfd_assert_mmap_shared(int fd)
{
void *p;
p = mmap(NULL,
mfd_def_size,
PROT_READ | PROT_WRITE,
MAP_SHARED,
fd,
0);
if (p == MAP_FAILED) {
printf("mmap() failed: %m\n");
abort();
}
return p;
}
static void *mfd_assert_mmap_private(int fd)
{
void *p;
p = mmap(NULL,
mfd_def_size,
PROT_READ | PROT_WRITE,
MAP_PRIVATE,
fd,
0);
if (p == MAP_FAILED) {
printf("mmap() failed: %m\n");
abort();
}
return p;
}
static int global_mfd = -1;
static void *global_p = NULL;
static int sealing_thread_fn(void *arg)
{
int sig, r;
/*
* This thread first waits 200ms so any pending operation in the parent
* is correctly started. After that, it tries to seal @global_mfd as
* SEAL_WRITE. This _must_ fail as the parent thread has a read() into
* that memory mapped object still ongoing.
* We then wait one more second and try sealing again. This time it
* must succeed as there shouldn't be anyone else pinning the pages.
*/
/* wait 200ms for FUSE-request to be active */
usleep(200000);
/* unmount mapping before sealing to avoid i_mmap_writable failures */
munmap(global_p, mfd_def_size);
/* Try sealing the global file; expect EBUSY or success. Current
* kernels will never succeed, but in the future, kernels might
* implement page-replacements or other fancy ways to avoid racing
* writes. */
r = mfd_busy_add_seals(global_mfd, F_SEAL_WRITE);
if (r >= 0) {
printf("HURRAY! This kernel fixed GUP races!\n");
} else {
/* wait 1s more so the FUSE-request is done */
sleep(1);
/* try sealing the global file again */
mfd_assert_add_seals(global_mfd, F_SEAL_WRITE);
}
return 0;
}
static pid_t spawn_sealing_thread(void)
{
uint8_t *stack;
pid_t pid;
stack = malloc(STACK_SIZE);
if (!stack) {
printf("malloc(STACK_SIZE) failed: %m\n");
abort();
}
pid = clone(sealing_thread_fn,
stack + STACK_SIZE,
SIGCHLD | CLONE_FILES | CLONE_FS | CLONE_VM,
NULL);
if (pid < 0) {
printf("clone() failed: %m\n");
abort();
}
return pid;
}
static void join_sealing_thread(pid_t pid)
{
waitpid(pid, NULL, 0);
}
int main(int argc, char **argv)
{
char *zero;
int fd, mfd, r;
void *p;
int was_sealed;
pid_t pid;
if (argc < 2) {
printf("error: please pass path to file in fuse_mnt mount-point\n");
abort();
}
if (argc >= 3) {
if (!strcmp(argv[2], "hugetlbfs")) {
unsigned long hpage_size = default_huge_page_size();
if (!hpage_size) {
printf("Unable to determine huge page size\n");
abort();
}
hugetlbfs_test = 1;
mfd_def_size = hpage_size * 2;
} else {
printf("Unknown option: %s\n", argv[2]);
abort();
}
}
zero = calloc(sizeof(*zero), mfd_def_size);
/* open FUSE memfd file for GUP testing */
printf("opening: %s\n", argv[1]);
fd = open(argv[1], O_RDONLY | O_CLOEXEC);
if (fd < 0) {
printf("cannot open(\"%s\"): %m\n", argv[1]);
abort();
}
/* create new memfd-object */
mfd = mfd_assert_new("kern_memfd_fuse",
mfd_def_size,
MFD_CLOEXEC | MFD_ALLOW_SEALING);
/* mmap memfd-object for writing */
p = mfd_assert_mmap_shared(mfd);
/* pass mfd+mapping to a separate sealing-thread which tries to seal
* the memfd objects with SEAL_WRITE while we write into it */
global_mfd = mfd;
global_p = p;
pid = spawn_sealing_thread();
/* Use read() on the FUSE file to read into our memory-mapped memfd
* object. This races the other thread which tries to seal the
* memfd-object.
* If @fd is on the memfd-fake-FUSE-FS, the read() is delayed by 1s.
* This guarantees that the receive-buffer is pinned for 1s until the
* data is written into it. The racing ADD_SEALS should thus fail as
* the pages are still pinned. */
r = read(fd, p, mfd_def_size);
if (r < 0) {
printf("read() failed: %m\n");
abort();
} else if (!r) {
printf("unexpected EOF on read()\n");
abort();
}
was_sealed = mfd_assert_get_seals(mfd) & F_SEAL_WRITE;
/* Wait for sealing-thread to finish and verify that it
* successfully sealed the file after the second try. */
join_sealing_thread(pid);
mfd_assert_has_seals(mfd, F_SEAL_WRITE);
/* *IF* the memfd-object was sealed at the time our read() returned,
* then the kernel did a page-replacement or canceled the read() (or
* whatever magic it did..). In that case, the memfd object is still
* all zero.
* In case the memfd-object was *not* sealed, the read() was successfull
* and the memfd object must *not* be all zero.
* Note that in real scenarios, there might be a mixture of both, but
* in this test-cases, we have explicit 200ms delays which should be
* enough to avoid any in-flight writes. */
p = mfd_assert_mmap_private(mfd);
if (was_sealed && memcmp(p, zero, mfd_def_size)) {
printf("memfd sealed during read() but data not discarded\n");
abort();
} else if (!was_sealed && !memcmp(p, zero, mfd_def_size)) {
printf("memfd sealed after read() but data discarded\n");
abort();
}
close(mfd);
close(fd);
printf("fuse: DONE\n");
free(zero);
return 0;
}