selftests/fuse: add the missing symlink caching test

Commit 3e14f079d7 ("selftests/fuse: add fuse symlink caching test")
registered fuse_symlink_cache_test in the fuse selftest Makefile and
.gitignore, but the test source was not part of the applied commit, so
building the fuse selftests fails:

  make[1]: *** No rule to make target 'fuse_symlink_cache_test.c', needed by '.../fuse_symlink_cache_test'.

Add fuse_symlink_cache_test.c as posted in Luis Henriques's v5
submission. The test serves a file and a symlink to it from a libfuse
low-level filesystem, resolves the symlink 100 times, and counts the
readlink requests that reach the server: one when FUSE_CAP_CACHE_SYMLINKS
is set during FUSE_INIT, and all 100 when it is not.

Fixes: 3e14f079d7 ("selftests/fuse: add fuse symlink caching test")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/r/202610170822.oEJ1Autq-lkp@intel.com/
Link: https://lore.kernel.org/r/20261001164916.15311-8-luis@igalia.com
Originally-by: Luis Henriques <luis@igalia.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Reviewed-by: Luis Henriques <luis@igalia.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

tools/testing/selftests/filesystems/fuse/fuse_symlink_cache_test.c

@@ -0,0 +1,162 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Simple filesystem to test FUSE symlink cache
+ *
+ * This is a simple FUSE filesystem that contains two objects: a file named
+ * 'file' and a symlink to that file named 'link'.  If symlink caching is
+ * disabled (i.e. FUSE_CAP_CACHE_SYMLINKS is reset during FUSE_INIT), whenever
+ * the ->readlink() is executed to resolve 'link' a counter will be incremented.
+ *
+ * If symlink caching is enabled (i.e. FUSE_CAP_CACHE_SYMLINKS is set during
+ * FUSE_INIT), resolving a symlink will only call into user-space the first
+ * time.
+ */
+
+#define FUSE_USE_VERSION 31
+
+#include <stdio.h>
+#include <fuse_lowlevel.h>
+
+#include "kselftest_harness.h"
+
+#include "fuse_common.h"
+
+#define FILENAME "file"
+#define FILE_INO 42
+
+#define LINKNAME "link"
+#define LINK_INO 43
+
+#define TIMEOUT	86400.0f
+
+struct test_state {
+	pthread_mutex_t lock;
+	int readlink_counter;
+	bool cache;
+} test_state = {
+	.lock = PTHREAD_MUTEX_INITIALIZER,
+};
+
+static void fs_init(void *userdata, struct fuse_conn_info *conn)
+{
+	pthread_mutex_lock(&test_state.lock);
+	if (test_state.cache)
+		fuse_set_feature_flag(conn, FUSE_CAP_CACHE_SYMLINKS);
+	else
+		fuse_unset_feature_flag(conn, FUSE_CAP_CACHE_SYMLINKS);
+	pthread_mutex_unlock(&test_state.lock);
+}
+
+static void fs_lookup(fuse_req_t req, fuse_ino_t parent, const char *name)
+{
+	struct fuse_entry_param e = {};
+
+	if (parent != FUSE_ROOT_ID ||
+	    (strcmp(name, FILENAME) != 0 && strcmp(name, LINKNAME) != 0)) {
+		fuse_reply_err(req, ENOENT);
+		return;
+	}
+
+	if (!strcmp(name, FILENAME)) {
+		e.ino = FILE_INO;
+		e.attr.st_mode = S_IFREG | 0444;
+		e.attr.st_nlink = 2;
+	} else if (!strcmp(name, LINKNAME)) {
+		e.ino = LINK_INO;
+		e.attr.st_mode = S_IFLNK | 0444;
+		e.attr.st_nlink = 1;
+		e.attr.st_size = strlen(FILENAME);
+	}
+	e.attr_timeout = TIMEOUT;
+	e.entry_timeout = TIMEOUT;
+
+	fuse_reply_entry(req, &e);
+}
+
+static void fs_readlink(fuse_req_t req, fuse_ino_t ino)
+{
+	char buf[PATH_MAX];
+	size_t sz = strlen(FILENAME);
+
+	if (ino != LINK_INO) {
+		fuse_reply_err(req, ENOENT);
+		return;
+	}
+
+	memcpy(buf, FILENAME, sz);
+	buf[sz] = '\0';
+	pthread_mutex_lock(&test_state.lock);
+	test_state.readlink_counter++;
+	pthread_mutex_unlock(&test_state.lock);
+
+	fuse_reply_readlink(req, buf);
+}
+
+static const struct fuse_lowlevel_ops fs_ops = {
+	.init           = fs_init,
+	.lookup		= fs_lookup,
+	.readlink	= fs_readlink,
+};
+
+FIXTURE(symlink_cache)
+{
+	struct fuse_common_ctx ctx;
+};
+FIXTURE_VARIANT(symlink_cache)
+{
+	const bool cache;
+};
+FIXTURE_VARIANT_ADD(symlink_cache, symlinks_nocache)
+{
+	/* Variant with symlink cache disabled */
+	.cache = false,
+};
+FIXTURE_VARIANT_ADD(symlink_cache, symlinks_cache)
+{
+	/* Variant with symlink cache enabled */
+	.cache = true,
+};
+
+FIXTURE_SETUP(symlink_cache)
+{
+	char err[MAX_ERR_MSG];
+
+	pthread_mutex_lock(&test_state.lock);
+	test_state.readlink_counter = 0;
+	test_state.cache = variant->cache;
+	pthread_mutex_unlock(&test_state.lock);
+
+	if (fs_setup(&fs_ops, &self->ctx, err))
+		SKIP(return, err);
+}
+
+FIXTURE_TEARDOWN(symlink_cache)
+{
+	fs_teardown(&self->ctx);
+}
+
+TEST_F(symlink_cache, test_symlink_cache)
+{
+	char pathname[PATH_MAX];
+	char buf[PATH_MAX];
+	ssize_t sz;
+	int counter;
+	int i;
+
+	sprintf(pathname, "%s/%s", self->ctx.mountpoint, LINKNAME);
+	for (i = 0; i < 100; i++) {
+		sz = readlink(pathname, buf, PATH_MAX);
+		ASSERT_NE(sz, -1);
+	}
+	pthread_mutex_lock(&test_state.lock);
+	counter = test_state.readlink_counter;
+	pthread_mutex_unlock(&test_state.lock);
+
+	if (variant->cache) {
+		ASSERT_EQ(counter, 1);
+	} else {
+		ASSERT_EQ(counter, 100);
+	}
+}
+
+TEST_HARNESS_MAIN