[PATCH v2 2/2] selftests/fuse: test FUSE_EXCL_CREATE

[Jimmy Zuber <jamz@amazon.com>]

Check that mkdir(), mknod(), symlink(), link() and renameat2() with
RENAME_NOREPLACE of new names send no FUSE_LOOKUP when the server sets
FUSE_EXCL_CREATE, that the created names are cached, and that creating
an existing name fails with EEXIST and leaves the name uncached.
The FUSE_EXCL_CREATE cases skip on kernels that do not offer the flag.

Assisted-by: Claude:claude-opus-5-5 [Claude-Code]
Signed-off-by: Jimmy Zuber <jamz@amazon.com>
---
 .../selftests/filesystems/fuse/.gitignore     |   1 +
 .../selftests/filesystems/fuse/Makefile       |   3 +-
 .../filesystems/fuse/excl_create_test.c       | 386 ++++++++++++++++++
 3 files changed, 389 insertions(+), 1 deletion(-)
 create mode 100644 tools/testing/selftests/filesystems/fuse/excl_create_test.c

diff --git a/tools/testing/selftests/filesystems/fuse/.gitignore b/tools/testing/selftests/filesystems/fuse/.gitignore
index 7c787313e69b6..719298cb98c5f 100644
--- a/tools/testing/selftests/filesystems/fuse/.gitignore
+++ b/tools/testing/selftests/filesystems/fuse/.gitignore
@@ -1,4 +1,5 @@
 # SPDX-License-Identifier: GPL-2.0-only
+excl_create_test
 fuse_mnt
 fusectl_test
 test_syncfs
diff --git a/tools/testing/selftests/filesystems/fuse/Makefile b/tools/testing/selftests/filesystems/fuse/Makefile
index e3ae3acff1139..ef1a241b79f8f 100644
--- a/tools/testing/selftests/filesystems/fuse/Makefile
+++ b/tools/testing/selftests/filesystems/fuse/Makefile
@@ -3,7 +3,7 @@
 CFLAGS += -Wall -O2 -g $(KHDR_INCLUDES)
 
 TEST_GEN_PROGS := fusectl_test test_syncfs
-TEST_GEN_PROGS += write_extend_eof_test
+TEST_GEN_PROGS += write_extend_eof_test excl_create_test
 TEST_GEN_PROGS += fuse_acl_cache_test
 TEST_GEN_PROGS += fuse_symlink_cache_test
 TEST_GEN_PROGS += fuse_readdir_cache_test
@@ -12,6 +12,7 @@ TEST_GEN_FILES := fuse_mnt
 include ../../lib.mk
 
 $(OUTPUT)/write_extend_eof_test: LDLIBS += -lpthread
+$(OUTPUT)/excl_create_test: LDLIBS += -lpthread
 
 VAR_CFLAGS := $(shell pkg-config fuse3 --cflags 2>/dev/null)
 ifeq ($(VAR_CFLAGS),)
diff --git a/tools/testing/selftests/filesystems/fuse/excl_create_test.c b/tools/testing/selftests/filesystems/fuse/excl_create_test.c
new file mode 100644
index 0000000000000..25c8764556630
--- /dev/null
+++ b/tools/testing/selftests/filesystems/fuse/excl_create_test.c
@@ -0,0 +1,386 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Test mkdir(), mknod(), symlink(), link() and renameat2() with
+ * RENAME_NOREPLACE, with and without FUSE_EXCL_CREATE.
+ *
+ * Requires root to mount fuse.
+ */
+#define _GNU_SOURCE
+#include <errno.h>
+#include <fcntl.h>
+#include <pthread.h>
+#include <sched.h>
+#include <stdint.h>
+#include <stdio.h>
+#include <string.h>
+#include <unistd.h>
+#include <sys/mman.h>
+#include <sys/mount.h>
+#include <sys/stat.h>
+#include <sys/uio.h>
+#include <linux/fuse.h>
+
+#include "../../kselftest_harness.h"
+
+#define FUSE_ROOT_ID	1
+#define MAX_WRITE	(128 * 1024)
+#define MAX_ENTRIES	16
+
+struct entry {
+	char name[64];
+	uint64_t nodeid;
+	uint32_t mode;
+};
+
+/* Server state, shared with the responder thread. */
+struct server {
+	int fd;
+	int excl;
+	/* The kernel offers FUSE_EXCL_CREATE in FUSE_INIT */
+	int offered;
+	struct entry ent[MAX_ENTRIES];
+	int nent;
+	int nr_lookup, nr_mkobj, nr_rename;
+};
+
+static void reply(int fd, uint64_t unique, int error, void *data, size_t len)
+{
+	struct fuse_out_header oh = {
+		.len = sizeof(oh) + (data ? len : 0),
+		.error = error,
+		.unique = unique,
+	};
+	struct iovec iov[2] = { { &oh, sizeof(oh) }, { data, len } };
+
+	/* Errors here are teardown races (device closed on unmount); ignore. */
+	if (writev(fd, iov, data ? 2 : 1) < 0)
+		return;
+}
+
+static struct entry *find(struct server *s, const char *name)
+{
+	int i;
+
+	for (i = 0; i < s->nent; i++)
+		if (!strcmp(s->ent[i].name, name))
+			return &s->ent[i];
+	return NULL;
+}
+
+static struct entry *add(struct server *s, const char *name, uint32_t mode)
+{
+	struct entry *e = &s->ent[s->nent];
+
+	snprintf(e->name, sizeof(e->name), "%.63s", name);
+	e->nodeid = FUSE_ROOT_ID + 1 + s->nent++;
+	e->mode = mode;
+	return e;
+}
+
+/* A NULL entry is a negative entry, cached like a found one. */
+static void fill_entry(struct fuse_entry_out *out, struct entry *e)
+{
+	memset(out, 0, sizeof(*out));
+	out->entry_valid = 3600;
+	if (!e)
+		return;
+	out->nodeid = e->nodeid;
+	out->attr_valid = 3600;
+	out->attr.ino = e->nodeid;
+	out->attr.mode = e->mode;
+	out->attr.nlink = S_ISDIR(e->mode) ? 2 : 1;
+	out->attr.blksize = 4096;
+}
+
+static void *server_thread(void *arg)
+{
+	struct server *s = arg;
+	static char buf[MAX_WRITE + 4096];
+
+	for (;;) {
+		ssize_t n = read(s->fd, buf, sizeof(buf));
+		struct fuse_in_header *ih = (void *)buf;
+		char *in = (char *)(ih + 1);
+
+		if (n < 0) {
+			if (errno == EINTR || errno == EAGAIN)
+				continue;
+			return NULL;	/* device closed on unmount */
+		}
+		if (n < (ssize_t)sizeof(*ih))
+			continue;
+
+		switch (ih->opcode) {
+		case FUSE_INIT: {
+			struct fuse_init_in *init = (void *)in;
+			struct fuse_init_out out = {0};
+
+			s->offered = (init->flags & FUSE_INIT_EXT) &&
+				     (init->flags2 & (FUSE_EXCL_CREATE >> 32));
+			out.major = FUSE_KERNEL_VERSION;
+			out.minor = FUSE_KERNEL_MINOR_VERSION;
+			out.max_readahead = init->max_readahead;
+			out.max_write = MAX_WRITE;
+			out.max_background = 16;
+			out.congestion_threshold = 12;
+			out.flags = FUSE_INIT_EXT;
+			if (s->excl)
+				out.flags2 = FUSE_EXCL_CREATE >> 32;
+			reply(s->fd, ih->unique, 0, &out, sizeof(out));
+			break;
+		}
+		case FUSE_GETATTR: {
+			struct fuse_attr_out out = {0};
+			struct fuse_entry_out eo;
+			int i;
+
+			out.attr_valid = 3600;
+			if (ih->nodeid == FUSE_ROOT_ID) {
+				out.attr.ino = FUSE_ROOT_ID;
+				out.attr.mode = S_IFDIR | 0777;
+				out.attr.nlink = 2;
+			}
+			for (i = 0; i < s->nent; i++) {
+				if (s->ent[i].nodeid == ih->nodeid) {
+					fill_entry(&eo, &s->ent[i]);
+					out.attr = eo.attr;
+				}
+			}
+			reply(s->fd, ih->unique, 0, &out, sizeof(out));
+			break;
+		}
+		case FUSE_LOOKUP: {
+			struct fuse_entry_out out;
+
+			s->nr_lookup++;
+			fill_entry(&out, find(s, in));
+			reply(s->fd, ih->unique, 0, &out, sizeof(out));
+			break;
+		}
+		case FUSE_MKNOD:
+		case FUSE_MKDIR:
+		case FUSE_SYMLINK:
+		case FUSE_LINK: {
+			struct fuse_entry_out out;
+			struct entry *e;
+			const char *name;
+			uint32_t mode;
+
+			s->nr_mkobj++;
+			if (ih->opcode == FUSE_MKNOD) {
+				struct fuse_mknod_in *mi = (void *)in;
+
+				name = (char *)(mi + 1);
+				mode = mi->mode;
+			} else if (ih->opcode == FUSE_MKDIR) {
+				struct fuse_mkdir_in *mi = (void *)in;
+
+				name = (char *)(mi + 1);
+				mode = S_IFDIR | mi->mode;
+			} else if (ih->opcode == FUSE_SYMLINK) {
+				name = in;
+				mode = S_IFLNK | 0777;
+			} else {
+				name = (char *)((struct fuse_link_in *)in + 1);
+				mode = S_IFREG | 0644;
+			}
+			if (find(s, name)) {
+				reply(s->fd, ih->unique, -EEXIST, NULL, 0);
+				break;
+			}
+			e = add(s, name, mode);
+			/* A link names the inode of its source */
+			if (ih->opcode == FUSE_LINK)
+				e->nodeid = ((struct fuse_link_in *)in)->oldnodeid;
+			fill_entry(&out, e);
+			reply(s->fd, ih->unique, 0, &out, sizeof(out));
+			break;
+		}
+		case FUSE_RENAME2: {
+			struct fuse_rename2_in *ri = (void *)in;
+			const char *oldname = (char *)(ri + 1);
+			const char *newname = oldname + strlen(oldname) + 1;
+			struct entry *e = find(s, oldname);
+
+			s->nr_rename++;
+			if (!e || ri->flags != RENAME_NOREPLACE) {
+				reply(s->fd, ih->unique, -EINVAL, NULL, 0);
+				break;
+			}
+			if (find(s, newname)) {
+				reply(s->fd, ih->unique, -EEXIST, NULL, 0);
+				break;
+			}
+			snprintf(e->name, sizeof(e->name), "%.63s", newname);
+			reply(s->fd, ih->unique, 0, NULL, 0);
+			break;
+		}
+		case FUSE_FORGET:
+		case FUSE_BATCH_FORGET:
+			break;
+		default:
+			reply(s->fd, ih->unique, -EOPNOTSUPP, NULL, 0);
+			break;
+		}
+	}
+}
+
+FIXTURE(fuse)
+{
+	struct server *srv;
+	char dir[64];
+	int mounted;
+};
+
+FIXTURE_VARIANT(fuse)
+{
+	int excl;
+};
+
+FIXTURE_VARIANT_ADD(fuse, excl)
+{
+	.excl = 1,
+};
+
+FIXTURE_VARIANT_ADD(fuse, plain)
+{
+	.excl = 0,
+};
+
+static void cleanup(FIXTURE_DATA(fuse) *self)
+{
+	if (self->mounted)
+		umount2(self->dir, MNT_DETACH);
+	self->mounted = 0;
+	/* The detached server thread may still use self->srv */
+	if (self->srv && self->srv != MAP_FAILED && self->srv->fd > 0)
+		close(self->srv->fd);
+	if (self->dir[0])
+		rmdir(self->dir);
+	self->dir[0] = '\0';
+}
+
+FIXTURE_SETUP(fuse)
+{
+	char opts[128];
+	struct stat st;
+	pthread_t t;
+	int ret;
+
+	if (geteuid() != 0)
+		SKIP(return, "need root to mount fuse");
+
+	/* Mounts made by the test go away with its mount namespace */
+	ASSERT_EQ(0, unshare(CLONE_NEWNS));
+	ASSERT_EQ(0, mount(NULL, "/", NULL, MS_REC | MS_PRIVATE, NULL));
+
+	self->srv = mmap(NULL, sizeof(*self->srv), PROT_READ | PROT_WRITE,
+			 MAP_SHARED | MAP_ANONYMOUS, -1, 0);
+	ASSERT_NE(MAP_FAILED, self->srv);
+	self->srv->excl = variant->excl;
+	add(self->srv, "src", S_IFREG | 0644);
+	add(self->srv, "taken", S_IFREG | 0644);
+
+	self->srv->fd = open("/dev/fuse", O_RDWR);
+	if (self->srv->fd < 0 && (errno == ENOENT || errno == ENODEV))
+		SKIP(return, "no /dev/fuse");
+	ASSERT_GE(self->srv->fd, 0);
+
+	strcpy(self->dir, "/tmp/fuse_excl_XXXXXX");
+	ASSERT_NE(NULL, mkdtemp(self->dir));
+
+	snprintf(opts, sizeof(opts),
+		 "fd=%d,rootmode=40000,user_id=0,group_id=0", self->srv->fd);
+	ret = mount("fuse", self->dir, "fuse", 0, opts);
+	if (ret)
+		cleanup(self);
+	ASSERT_EQ(0, ret);
+	self->mounted = 1;
+
+	ret = pthread_create(&t, NULL, server_thread, self->srv);
+	if (!ret) {
+		pthread_detach(t);
+		/* Wait for FUSE_INIT: the root GETATTR is queued behind it */
+		ret = stat(self->dir, &st);
+	}
+	/* The harness skips teardown when setup fails or skips */
+	if (ret)
+		cleanup(self);
+	ASSERT_EQ(0, ret);
+	if (variant->excl && !self->srv->offered) {
+		cleanup(self);
+		SKIP(return, "kernel without FUSE_EXCL_CREATE");
+	}
+}
+
+FIXTURE_TEARDOWN(fuse)
+{
+	cleanup(self);
+}
+
+TEST_F(fuse, create_new)
+{
+	static const char *const names[] = { "d", "f", "l", "h", "r" };
+	char path[128], src[128];
+	struct stat st;
+	unsigned int i;
+
+	snprintf(path, sizeof(path), "%s/d", self->dir);
+	ASSERT_EQ(0, mkdir(path, 0755));
+	snprintf(path, sizeof(path), "%s/f", self->dir);
+	ASSERT_EQ(0, mkfifo(path, 0644));
+	snprintf(path, sizeof(path), "%s/l", self->dir);
+	ASSERT_EQ(0, symlink("src", path));
+	snprintf(src, sizeof(src), "%s/src", self->dir);
+	snprintf(path, sizeof(path), "%s/h", self->dir);
+	ASSERT_EQ(0, link(src, path));
+	snprintf(path, sizeof(path), "%s/r", self->dir);
+	ASSERT_EQ(0, renameat2(AT_FDCWD, src, AT_FDCWD, path,
+			       RENAME_NOREPLACE));
+
+	/* The created names are cached */
+	for (i = 0; i < ARRAY_SIZE(names); i++) {
+		snprintf(path, sizeof(path), "%s/%s", self->dir, names[i]);
+		ASSERT_EQ(0, lstat(path, &st));
+	}
+
+	EXPECT_EQ(4, self->srv->nr_mkobj);
+	EXPECT_EQ(1, self->srv->nr_rename);
+	/* Only "src" is looked up with FUSE_EXCL_CREATE */
+	EXPECT_EQ(variant->excl ? 1 : 6, self->srv->nr_lookup);
+}
+
+TEST_F(fuse, create_existing)
+{
+	char path[128], src[128];
+	struct stat st;
+
+	snprintf(path, sizeof(path), "%s/taken", self->dir);
+	ASSERT_EQ(-1, mkdir(path, 0755));
+	ASSERT_EQ(EEXIST, errno);
+	ASSERT_EQ(-1, symlink("src", path));
+	ASSERT_EQ(EEXIST, errno);
+	ASSERT_EQ(-1, mkfifo(path, 0644));
+	ASSERT_EQ(EEXIST, errno);
+	snprintf(src, sizeof(src), "%s/src", self->dir);
+	ASSERT_EQ(-1, link(src, path));
+	ASSERT_EQ(EEXIST, errno);
+	ASSERT_EQ(-1, renameat2(AT_FDCWD, src, AT_FDCWD, path,
+				RENAME_NOREPLACE));
+	ASSERT_EQ(EEXIST, errno);
+	ASSERT_EQ(0, stat(path, &st));
+
+	if (variant->excl) {
+		/* "src" is looked up, and "taken" once it stays uncached */
+		EXPECT_EQ(4, self->srv->nr_mkobj);
+		EXPECT_EQ(1, self->srv->nr_rename);
+		EXPECT_EQ(2, self->srv->nr_lookup);
+	} else {
+		/* The cached name is looked up again for each request */
+		EXPECT_EQ(0, self->srv->nr_mkobj);
+		EXPECT_EQ(0, self->srv->nr_rename);
+		EXPECT_EQ(6, self->srv->nr_lookup);
+	}
+}
+
+TEST_HARNESS_MAIN
-- 
2.50.1