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Date:	Wed, 23 Jul 2014 21:28:52 -0700 (PDT)
From:	Hugh Dickins <hughd@...gle.com>
To:	David Herrmann <dh.herrmann@...il.com>
cc:	linux-kernel@...r.kernel.org,
	Michael Kerrisk <mtk.manpages@...il.com>,
	Ryan Lortie <desrt@...rt.ca>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
	linux-fsdevel@...r.kernel.org, linux-api@...r.kernel.org,
	Greg Kroah-Hartman <greg@...ah.com>, john.stultz@...aro.org,
	Lennart Poettering <lennart@...ttering.net>,
	Daniel Mack <zonque@...il.com>, Kay Sievers <kay@...y.org>,
	Hugh Dickins <hughd@...gle.com>,
	Andy Lutomirski <luto@...capital.net>,
	Alexander Viro <viro@...iv.linux.org.uk>
Subject: Re: [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@...il.com>

Acked-by: Hugh Dickins <hughd@...gle.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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