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SubjectRe: [PATCH v4 5/6] selftests: add memfd/sealing page-pinning tests
On Sun, 20 Jul 2014, David Herrmann wrote:

> Setting SEAL_WRITE is not possible if there're pending GUP users. This
> commit adds selftests for memfd+sealing that use FUSE to create pending
> page-references. FUSE is very helpful here in that it allows us to delay
> direct-IO operations for an arbitrary amount of time. This way, we can
> force the kernel to pin pages and then run our normal selftests.
>
> Signed-off-by: David Herrmann <dh.herrmann@gmail.com>

Acked-by: Hugh Dickins <hughd@google.com>

My "Permission denied" problem was actually not with /dev/fuse,
but with the executability (or not) of ./run_fuse_test.sh.
I see now that your git patch has create mode 100755, but that
got missed when I applied it to my tree with "patch -p1".
I would not be surprised if it goes missing on its way through
the quilt-style mmotm, or I may be under-rating akpm. Personally,
I'd change the Makefile one way or another, not to rely on 755.

> ---
> tools/testing/selftests/memfd/.gitignore | 2 +
> tools/testing/selftests/memfd/Makefile | 14 +-
> tools/testing/selftests/memfd/fuse_mnt.c | 110 +++++++++
> tools/testing/selftests/memfd/fuse_test.c | 311 +++++++++++++++++++++++++
> tools/testing/selftests/memfd/run_fuse_test.sh | 14 ++
> 5 files changed, 450 insertions(+), 1 deletion(-)
> create mode 100755 tools/testing/selftests/memfd/fuse_mnt.c
> create mode 100644 tools/testing/selftests/memfd/fuse_test.c
> create mode 100755 tools/testing/selftests/memfd/run_fuse_test.sh
>
> diff --git a/tools/testing/selftests/memfd/.gitignore b/tools/testing/selftests/memfd/.gitignore
> index bcc8ee2..afe87c4 100644
> --- a/tools/testing/selftests/memfd/.gitignore
> +++ b/tools/testing/selftests/memfd/.gitignore
> @@ -1,2 +1,4 @@
> +fuse_mnt
> +fuse_test
> memfd_test
> memfd-test-file
> diff --git a/tools/testing/selftests/memfd/Makefile b/tools/testing/selftests/memfd/Makefile
> index 36653b9..6816c49 100644
> --- a/tools/testing/selftests/memfd/Makefile
> +++ b/tools/testing/selftests/memfd/Makefile
> @@ -7,6 +7,7 @@ ifeq ($(ARCH),x86_64)
> ARCH := X86
> endif
>
> +CFLAGS += -D_FILE_OFFSET_BITS=64
> CFLAGS += -I../../../../arch/x86/include/generated/uapi/
> CFLAGS += -I../../../../arch/x86/include/uapi/
> CFLAGS += -I../../../../include/uapi/
> @@ -25,5 +26,16 @@ ifeq ($(ARCH),X86)
> endif
> @./memfd_test || echo "memfd_test: [FAIL]"
>
> +build_fuse:
> +ifeq ($(ARCH),X86)
> + gcc $(CFLAGS) fuse_mnt.c `pkg-config fuse --cflags --libs` -o fuse_mnt
> + gcc $(CFLAGS) fuse_test.c -o fuse_test
> +else
> + echo "Not an x86 target, can't build memfd selftest"
> +endif
> +
> +run_fuse: build_fuse
> + @./run_fuse_test.sh || echo "fuse_test: [FAIL]"
> +
> clean:
> - $(RM) memfd_test
> + $(RM) memfd_test fuse_test
> diff --git a/tools/testing/selftests/memfd/fuse_mnt.c b/tools/testing/selftests/memfd/fuse_mnt.c
> new file mode 100755
> index 0000000..feacf12
> --- /dev/null
> +++ b/tools/testing/selftests/memfd/fuse_mnt.c
> @@ -0,0 +1,110 @@
> +/*
> + * memfd test file-system
> + * This file uses FUSE to create a dummy file-system with only one file /memfd.
> + * This file is read-only and takes 1s per read.
> + *
> + * This file-system is used by the memfd test-cases to force the kernel to pin
> + * pages during reads(). Due to the 1s delay of this file-system, this is a
> + * nice way to test race-conditions against get_user_pages() in the kernel.
> + *
> + * We use direct_io==1 to force the kernel to use direct-IO for this
> + * file-system.
> + */
> +
> +#define FUSE_USE_VERSION 26
> +
> +#include <fuse.h>
> +#include <stdio.h>
> +#include <string.h>
> +#include <errno.h>
> +#include <fcntl.h>
> +#include <unistd.h>
> +
> +static const char memfd_content[] = "memfd-example-content";
> +static const char memfd_path[] = "/memfd";
> +
> +static int memfd_getattr(const char *path, struct stat *st)
> +{
> + memset(st, 0, sizeof(*st));
> +
> + if (!strcmp(path, "/")) {
> + st->st_mode = S_IFDIR | 0755;
> + st->st_nlink = 2;
> + } else if (!strcmp(path, memfd_path)) {
> + st->st_mode = S_IFREG | 0444;
> + st->st_nlink = 1;
> + st->st_size = strlen(memfd_content);
> + } else {
> + return -ENOENT;
> + }
> +
> + return 0;
> +}
> +
> +static int memfd_readdir(const char *path,
> + void *buf,
> + fuse_fill_dir_t filler,
> + off_t offset,
> + struct fuse_file_info *fi)
> +{
> + if (strcmp(path, "/"))
> + return -ENOENT;
> +
> + filler(buf, ".", NULL, 0);
> + filler(buf, "..", NULL, 0);
> + filler(buf, memfd_path + 1, NULL, 0);
> +
> + return 0;
> +}
> +
> +static int memfd_open(const char *path, struct fuse_file_info *fi)
> +{
> + if (strcmp(path, memfd_path))
> + return -ENOENT;
> +
> + if ((fi->flags & 3) != O_RDONLY)
> + return -EACCES;
> +
> + /* force direct-IO */
> + fi->direct_io = 1;
> +
> + return 0;
> +}
> +
> +static int memfd_read(const char *path,
> + char *buf,
> + size_t size,
> + off_t offset,
> + struct fuse_file_info *fi)
> +{
> + size_t len;
> +
> + if (strcmp(path, memfd_path) != 0)
> + return -ENOENT;
> +
> + sleep(1);
> +
> + len = strlen(memfd_content);
> + if (offset < len) {
> + if (offset + size > len)
> + size = len - offset;
> +
> + memcpy(buf, memfd_content + offset, size);
> + } else {
> + size = 0;
> + }
> +
> + return size;
> +}
> +
> +static struct fuse_operations memfd_ops = {
> + .getattr = memfd_getattr,
> + .readdir = memfd_readdir,
> + .open = memfd_open,
> + .read = memfd_read,
> +};
> +
> +int main(int argc, char *argv[])
> +{
> + return fuse_main(argc, argv, &memfd_ops, NULL);
> +}
> diff --git a/tools/testing/selftests/memfd/fuse_test.c b/tools/testing/selftests/memfd/fuse_test.c
> new file mode 100644
> index 0000000..67908b1
> --- /dev/null
> +++ b/tools/testing/selftests/memfd/fuse_test.c
> @@ -0,0 +1,311 @@
> +/*
> + * 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>
> +
> +#define MFD_DEF_SIZE 8192
> +#define STACK_SIZE 65535
> +
> +static int sys_memfd_create(const char *name,
> + unsigned int flags)
> +{
> + return syscall(__NR_memfd_create, name, flags);
> +}
> +
> +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)
> +{
> + static const char zero[MFD_DEF_SIZE];
> + 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();
> + }
> +
> + /* 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");
> +
> + return 0;
> +}
> diff --git a/tools/testing/selftests/memfd/run_fuse_test.sh b/tools/testing/selftests/memfd/run_fuse_test.sh
> new file mode 100755
> index 0000000..69b930e
> --- /dev/null
> +++ b/tools/testing/selftests/memfd/run_fuse_test.sh
> @@ -0,0 +1,14 @@
> +#!/bin/sh
> +
> +if test -d "./mnt" ; then
> + fusermount -u ./mnt
> + rmdir ./mnt
> +fi
> +
> +set -e
> +
> +mkdir mnt
> +./fuse_mnt ./mnt
> +./fuse_test ./mnt/memfd
> +fusermount -u ./mnt
> +rmdir ./mnt
> --
> 2.0.2
>
>


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