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

// SPDX-License-Identifier: GPL-2.0
/*
 * Test: FUSE ACL caching bug triggered by AT_STATX_FORCE_SYNC
 *
 * A FUSE mount that does not negotiate FUSE_POSIX_ACL initialises every inode
 * with i_acl = i_default_acl = ACL_DONT_CACHE.  When a fresh stat is needed
 * (e.g. AT_STATX_FORCE_SYNC), fuse_update_get_attr() calls
 * forget_all_cached_acls() before issuing FUSE_GETATTR.  On an unfixed kernel,
 * __forget_cached_acl() replaces ACL_DONT_CACHE with ACL_NOT_CACHED,
 * inadvertently enabling the kernel ACL cache for that inode.  The next
 * getxattr populates the cache.  Because fuse_set_acl() skips
 * forget_all_cached_acls() for !fc->posix_acl mounts, any subsequent change to
 * the ACL leaves the stale kernel entry in place, and the next getxattr returns
 * wrong data without ever reaching the FUSE daemon.
 *
 * Fix (fs/posix_acl.c): __forget_cached_acl() returns early when *p is
 * ACL_DONT_CACHE, preserving the "never cache" invariant for the inode's
 * lifetime.
 *
 * Test outline:
 *  1. Mount a minimal FUSE fs (no FUSE_POSIX_ACL negotiated).
 *  2. lgetxattr -> daemon called, ACL_A returned, NOT cached (ACL_DONT_CACHE).
 *  3. statx(AT_STATX_FORCE_SYNC) -> forget_all_cached_acls() called.
 *     Buggy:  ACL_DONT_CACHE -> ACL_NOT_CACHED (cache enabled).
 *     Fixed:  ACL_DONT_CACHE preserved.
 *  4. lgetxattr -> daemon called, ACL_A returned.
 *     Buggy:  result now cached (ACL_NOT_CACHED -> cached).
 *     Fixed:  result still not cached.
 *  5. Daemon switches to ACL_B internally (different size).
 *  6. lgetxattr -> should return ACL_B (44 bytes).
 *     Buggy:  cache hit, returns stale ACL_A (28 bytes). FAIL.
 *     Fixed:  no cache, daemon called, returns ACL_B (44 bytes). PASS.
 */

#define _GNU_SOURCE
#include <errno.h>
#include <fcntl.h>
#include <linux/limits.h>
#include <pthread.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/stat.h>
#include <sys/xattr.h>
#include <unistd.h>

#define FUSE_USE_VERSION 31
#include <fuse_lowlevel.h>

#include "kselftest_harness.h"

/* ---- ACL binary encoding ------------------------------------------------ */
/*
 * POSIX ACL v2 xattr format (little-endian):
 *   header: u32 version (= 0x00000002)
 *   entry:  u16 tag | u16 perm | u32 id
 *
 * Entries must appear in tag-ascending order; named USER/GROUP entries
 * require a MASK entry.  Both ACLs pass posix_acl_from_xattr() validation.
 */

/* ACL_A: 3 entries (USER_OBJ:rwx, GROUP_OBJ:r-x, OTHER:r-x) = 28 bytes */
static const uint8_t acl_a[] = {
	0x02, 0x00, 0x00, 0x00,                         /* v2 header      */
	0x01, 0x00, 0x07, 0x00, 0xff, 0xff, 0xff, 0xff, /* USER_OBJ  rwx  */
	0x04, 0x00, 0x05, 0x00, 0xff, 0xff, 0xff, 0xff, /* GROUP_OBJ r-x  */
	0x20, 0x00, 0x05, 0x00, 0xff, 0xff, 0xff, 0xff, /* OTHER     r-x  */
};

/*
 * ACL_B: 5 entries — adds USER uid=1 and MASK = 44 bytes.
 * A named USER entry requires a MASK; all tags in ascending order.
 */
static const uint8_t acl_b[] = {
	0x02, 0x00, 0x00, 0x00,                         /* v2 header       */
	0x01, 0x00, 0x07, 0x00, 0xff, 0xff, 0xff, 0xff, /* USER_OBJ   rwx  */
	0x02, 0x00, 0x07, 0x00, 0x01, 0x00, 0x00, 0x00, /* USER uid=1 rwx  */
	0x04, 0x00, 0x05, 0x00, 0xff, 0xff, 0xff, 0xff, /* GROUP_OBJ  r-x  */
	0x10, 0x00, 0x07, 0x00, 0xff, 0xff, 0xff, 0xff, /* MASK       rwx  */
	0x20, 0x00, 0x05, 0x00, 0xff, 0xff, 0xff, 0xff, /* OTHER      r-x  */
};

/* ---- Shared state (daemon thread <-> test thread) ----------------------- */

#define FILE_INO  2
#define FILE_NAME "testfile"

struct daemon_state {
	pthread_mutex_t lock;
	const uint8_t  *acl;
	size_t          acl_size;
	int             getxattr_count;
};

/*
 * Global: callbacks are stateless fns so we use a single global.
 * Safe because only one test instance runs at a time.
 */
static struct daemon_state g_ds = {
	.lock = PTHREAD_MUTEX_INITIALIZER,
};

/* ---- FUSE lowlevel callbacks -------------------------------------------- */

static void fs_lookup(fuse_req_t req, fuse_ino_t parent, const char *name)
{
	if (parent != FUSE_ROOT_ID || strcmp(name, FILE_NAME)) {
		fuse_reply_err(req, ENOENT);
		return;
	}
	struct fuse_entry_param e = {};

	/*
	 * Long attr/entry timeouts so that normal stat() calls do not
	 * expire and trigger forget_all_cached_acls() on their own;
	 * only the explicit AT_STATX_FORCE_SYNC should trigger it.
	 */
	e.ino             = FILE_INO;
	e.generation      = 1;
	e.attr_timeout    = 10.0;
	e.entry_timeout   = 10.0;
	e.attr.st_ino     = FILE_INO;
	e.attr.st_mode    = S_IFREG | 0644;
	e.attr.st_nlink   = 1;
	fuse_reply_entry(req, &e);
}

static void fs_getattr(fuse_req_t req, fuse_ino_t ino,
		       struct fuse_file_info *fi)
{
	struct stat st = {};

	(void)fi;
	if (ino == FUSE_ROOT_ID) {
		st.st_ino   = FUSE_ROOT_ID;
		st.st_mode  = S_IFDIR | 0755;
		st.st_nlink = 2;
	} else if (ino == FILE_INO) {
		st.st_ino   = FILE_INO;
		st.st_mode  = S_IFREG | 0644;
		st.st_nlink = 1;
	} else {
		fuse_reply_err(req, ENOENT);
		return;
	}
	fuse_reply_attr(req, &st, 10);
}

static void fs_getxattr(fuse_req_t req, fuse_ino_t ino, const char *name,
			size_t size)
{
	if (ino != FILE_INO ||
	    strcmp(name, "system.posix_acl_access") != 0) {
		fuse_reply_err(req, ENODATA);
		return;
	}

	pthread_mutex_lock(&g_ds.lock);
	const uint8_t *acl      = g_ds.acl;
	size_t         acl_size = g_ds.acl_size;
	g_ds.getxattr_count++;
	pthread_mutex_unlock(&g_ds.lock);

	if (size == 0)
		fuse_reply_xattr(req, acl_size);
	else if (size < acl_size)
		fuse_reply_err(req, ERANGE);
	else
		fuse_reply_buf(req, (const char *)acl, acl_size);
}

static const struct fuse_lowlevel_ops fs_ops = {
	.lookup   = fs_lookup,
	.getattr  = fs_getattr,
	.getxattr = fs_getxattr,
};

/* ---- Daemon thread ------------------------------------------------------- */

static void *run_daemon(void *arg)
{
	fuse_session_loop((struct fuse_session *)arg);
	return NULL;
}

/* ---- kselftest harness --------------------------------------------------- */

FIXTURE(acl_cache) {
	struct fuse_session *se;
	char                 mountpoint[PATH_MAX];
	char                 file_path[PATH_MAX];
	pthread_t            thread;
};

FIXTURE_SETUP(acl_cache)
{
	char *fuse_argv[] = { "fuse_acl_cache_test", NULL };
	struct fuse_args args = FUSE_ARGS_INIT(1, fuse_argv);

	g_ds.acl            = acl_a;
	g_ds.acl_size       = sizeof(acl_a);
	g_ds.getxattr_count = 0;

	strcpy(self->mountpoint, "/tmp/acl_cache_test_XXXXXX");
	if (!mkdtemp(self->mountpoint))
		SKIP(return, "mkdtemp: %s", strerror(errno));

	snprintf(self->file_path, sizeof(self->file_path),
		 "%s/" FILE_NAME, self->mountpoint);

	self->se = fuse_session_new(&args, &fs_ops, sizeof(fs_ops), NULL);
	if (!self->se) {
		rmdir(self->mountpoint);
		SKIP(return, "fuse_session_new failed");
	}

	if (fuse_session_mount(self->se, self->mountpoint)) {
		fuse_session_destroy(self->se);
		rmdir(self->mountpoint);
		SKIP(return, "fuse_session_mount failed "
			     "(missing fusermount3 or insufficient privileges)");
	}

	if (pthread_create(&self->thread, NULL, run_daemon, self->se)) {
		fuse_session_unmount(self->se);
		fuse_session_destroy(self->se);
		rmdir(self->mountpoint);
		SKIP(return, "pthread_create: %s", strerror(errno));
	}

	fuse_opt_free_args(&args);
}

FIXTURE_TEARDOWN(acl_cache)
{
	fuse_session_exit(self->se);
	fuse_session_unmount(self->se);
	pthread_join(self->thread, NULL);
	fuse_session_destroy(self->se);
	rmdir(self->mountpoint);
}

static int do_force_statx(const char *path)
{
	struct statx stx;

	return statx(AT_FDCWD, path, AT_STATX_FORCE_SYNC, STATX_BASIC_STATS,
		     &stx);
}

TEST_F(acl_cache, stale_after_force_sync)
{
	char    buf[512];
	ssize_t sz;
	int     count;

	/*
	 * Step 1: two getxattr calls before any statx(FORCE_SYNC).
	 * i_acl == ACL_DONT_CACHE.  __get_acl's cmpxchg(p, ACL_NOT_CACHED,
	 * sentinel) finds *p != ACL_NOT_CACHED on every call, so the sentinel
	 * is never placed and the result is never cached.  Both calls must
	 * reach the daemon, proving ACL_DONT_CACHE suppresses caching.
	 */
	sz = lgetxattr(self->file_path, "system.posix_acl_access",
		       buf, sizeof(buf));
	ASSERT_EQ(sz, (ssize_t)sizeof(acl_a));

	sz = lgetxattr(self->file_path, "system.posix_acl_access",
		       buf, sizeof(buf));
	ASSERT_EQ(sz, (ssize_t)sizeof(acl_a));

	pthread_mutex_lock(&g_ds.lock);
	count = g_ds.getxattr_count;
	pthread_mutex_unlock(&g_ds.lock);

	ASSERT_EQ(count, 2);
	TH_LOG("step 1 OK: both pre-trigger getxattrs reached daemon (count=%d), "
	       "ACL_DONT_CACHE is working", count);

	/*
	 * Step 2: statx(AT_STATX_FORCE_SYNC).
	 * fuse_update_get_attr() calls forget_all_cached_acls() before sending
	 * FUSE_GETATTR.
	 *   Buggy kernel:  ACL_DONT_CACHE -> ACL_NOT_CACHED  (cache enabled)
	 *   Fixed kernel:  ACL_DONT_CACHE preserved           (no effect)
	 */
	ASSERT_EQ(do_force_statx(self->file_path), 0);
	TH_LOG("step 2 OK: statx(AT_STATX_FORCE_SYNC) succeeded");

	/*
	 * Step 3: getxattr — cache population attempt after the trigger.
	 *   Buggy:  *p == ACL_NOT_CACHED -> sentinel placed -> fuse_get_inode_acl
	 *           called -> ACL_A parsed and stored in the kernel cache.
	 *   Fixed:  *p == ACL_DONT_CACHE -> sentinel placement skipped ->
	 *           fuse_get_inode_acl called but result not cached.
	 * Either way the correct ACL_A is returned here.
	 */
	sz = lgetxattr(self->file_path, "system.posix_acl_access",
		       buf, sizeof(buf));
	ASSERT_EQ(sz, (ssize_t)sizeof(acl_a));

	pthread_mutex_lock(&g_ds.lock);
	count = g_ds.getxattr_count;
	pthread_mutex_unlock(&g_ds.lock);

	ASSERT_EQ(count, 3);
	TH_LOG("step 3 OK: post-trigger getxattr reached daemon (count=%d), "
	       "returned correct ACL_A (%zd bytes)", count, sz);

	/*
	 * Step 4: switch daemon to ACL_B (different size: 44 vs 28 bytes).
	 * Simulates an ACL change that fuse_set_acl() would NOT invalidate for
	 * !fc->posix_acl mounts (it skips forget_all_cached_acls in that case).
	 * On a fixed kernel the ACL was never cached, so this is moot.
	 */
	pthread_mutex_lock(&g_ds.lock);
	g_ds.acl      = acl_b;
	g_ds.acl_size = sizeof(acl_b);
	pthread_mutex_unlock(&g_ds.lock);
	TH_LOG("step 4: daemon switched to ACL_B (%zu bytes)", sizeof(acl_b));

	/*
	 * Step 5: getxattr — the decisive check.
	 *   Buggy kernel:  cache hit -> stale ACL_A (28 bytes), count stays 3.
	 *   Fixed kernel:  no cache -> daemon called -> ACL_B (44 bytes), count 4.
	 */
	sz = lgetxattr(self->file_path, "system.posix_acl_access",
		       buf, sizeof(buf));

	pthread_mutex_lock(&g_ds.lock);
	count = g_ds.getxattr_count;
	pthread_mutex_unlock(&g_ds.lock);

	if (sz == (ssize_t)sizeof(acl_a))
		TH_LOG("step 5 BUG: stale ACL_A (%zd bytes) from kernel cache "
		       "(count=%d); ACL_DONT_CACHE corrupted by "
		       "forget_all_cached_acls()", sz, count);
	else
		TH_LOG("step 5 OK: daemon reached (count=%d), "
		       "fresh ACL_B (%zd bytes)", count, sz);

	EXPECT_EQ(sz, (ssize_t)sizeof(acl_b));
	EXPECT_EQ(count, 4);
}

TEST_HARNESS_MAIN