fuse: Optimize statx for permission checks by requesting only needed attributes

For permission checks via inode_permission(), we only need mode, uid, and
gid attributes. Previously, we requested all STATX_BASIC_STATS, which was
inefficient.

This commit enables the optimization by:
1. Requesting only STATX_MODE | STATX_UID | STATX_GID for permission checks
2. Relaxing the condition in fuse_do_statx() from requiring all basic stats
   to accepting any subset of basic stats
3. Adding validation that the server returns at least what was requested

The preparation commit ensures partial updates work correctly by only
updating returned attributes and managing timeouts appropriately.

Signed-off-by: Bernd Schubert <bernd@bsbernd.com>
(cherry picked from commit 09ed47b)

Author: bsbernd

fs/fuse/dir.c

@@ -1182,8 +1182,13 @@ static void fuse_statx_to_attr(struct fuse_statx *sx, struct fuse_attr *attr)
 	attr->blksize = sx->blksize;
 }
 
+/*
+ * @param sx_mask request mask send to to fuse-server
+ * @param mandatory_sx_mask subset of (or complete) sx_mask that the server
+ * has to fulfill
+*/
 static int fuse_do_statx(struct inode *inode, struct file *file,
-			 struct kstat *stat)
+			 struct kstat *stat, u32 sx_mask, u32 mandatory_sx_mask)
 {
 	int err;
 	struct fuse_attr attr;
@@ -1194,6 +1199,12 @@ static int fuse_do_statx(struct inode *inode, struct file *file,
 	u64 attr_version = fuse_get_attr_version(fm->fc);
 	FUSE_ARGS(args);
 
+	/*
+	 * mandatory_sx_mask should be a subset of sx_mask.
+	 * If it's not, we have a logic error somewhere in the call chain.
+	 */
+	WARN_ON_ONCE((mandatory_sx_mask & sx_mask) != mandatory_sx_mask);
+
 	memset(&inarg, 0, sizeof(inarg));
 	memset(&outarg, 0, sizeof(outarg));
 	/* Directories have separate file-handle space */
@@ -1203,9 +1214,12 @@ static int fuse_do_statx(struct inode *inode, struct file *file,
 		inarg.getattr_flags |= FUSE_GETATTR_FH;
 		inarg.fh = ff->fh;
 	}
-	/* For now leave sync hints as the default, request all stats. */
+	/*
+	 * For permission checks, we only need mode, uid, gid.
+	 * This is an optimization to avoid fetching all stats when not needed.
+	 */
 	inarg.sx_flags = 0;
-	inarg.sx_mask = STATX_BASIC_STATS | STATX_BTIME;
+	inarg.sx_mask = sx_mask;
 	args.opcode = FUSE_STATX;
 	args.nodeid = get_node_id(inode);
 	args.in_numargs = 1;
@@ -1219,6 +1233,17 @@ static int fuse_do_statx(struct inode *inode, struct file *file,
 		return err;
 
 	sx = &outarg.stat;
+
+	/*
+	 * Verify the server returned at least what we requested.
+	 * The server may return more attributes than requested (which is fine),
+	 * but must not return fewer.
+	 */
+	if ((sx->mask & mandatory_sx_mask) != mandatory_sx_mask) {
+		fuse_make_bad(inode);
+		return -EIO;
+	}
+
 	if (((sx->mask & STATX_SIZE) && !fuse_valid_size(sx->size)) ||
 	    ((sx->mask & STATX_TYPE) && (!fuse_valid_type(sx->mode) ||
 					 inode_wrong_type(inode, sx->mode)))) {
@@ -1227,7 +1252,7 @@ static int fuse_do_statx(struct inode *inode, struct file *file,
 	}
 
 	fuse_statx_to_attr(&outarg.stat, &attr);
-	if ((sx->mask & STATX_BASIC_STATS) == STATX_BASIC_STATS) {
+	if (sx->mask & STATX_BASIC_STATS) {
 		fuse_change_attributes(inode, &attr, &outarg.stat,
 				       ATTR_TIMEOUT(&outarg), attr_version);
 	}
@@ -1292,30 +1317,31 @@ static int fuse_update_get_attr(struct inode *inode, struct file *file,
 	bool sync;
 	u32 inval_mask = READ_ONCE(fi->inval_mask);
 	u32 cache_mask = fuse_get_cache_mask(inode);
-
+	u32 mandatory_sx_mask = request_mask & STATX_BASIC_STATS;
+	u32 sx_mask = request_mask;
 
 	/* FUSE only supports basic stats and possibly btime */
-	request_mask &= STATX_BASIC_STATS | STATX_BTIME;
+	sx_mask &= STATX_BASIC_STATS | STATX_BTIME;
 retry:
 	if (fc->no_statx)
-		request_mask &= STATX_BASIC_STATS;
+		sx_mask &= STATX_BASIC_STATS;
 
-	if (!request_mask)
+	if (!sx_mask)
 		sync = false;
 	else if (flags & AT_STATX_FORCE_SYNC)
 		sync = true;
 	else if (flags & AT_STATX_DONT_SYNC)
 		sync = false;
-	else if (request_mask & inval_mask & ~cache_mask)
+	else if (sx_mask & inval_mask & ~cache_mask)
 		sync = true;
 	else
 		sync = time_before64(fi->i_time, get_jiffies_64());
 
 	if (sync) {
 		forget_all_cached_acls(inode);
-		/* Try statx if BTIME is requested */
-		if (!fc->no_statx && (request_mask & ~STATX_BASIC_STATS)) {
-			err = fuse_do_statx(inode, file, stat);
+		if (!fc->no_statx) {
+			err = fuse_do_statx(inode, file, stat, sx_mask,
+					    mandatory_sx_mask);
 			if (err == -ENOSYS) {
 				fc->no_statx = 1;
 				err = 0;
@@ -1477,13 +1503,14 @@ static int fuse_access(struct inode *inode, int mask)
 	return err;
 }
 
-static int fuse_perm_getattr(struct inode *inode, int mask)
+static int fuse_perm_getattr(struct inode *inode, int mask, int perm_mask)
 {
 	if (mask & MAY_NOT_BLOCK)
 		return -ECHILD;
 
 	forget_all_cached_acls(inode);
-	return fuse_do_getattr(inode, NULL, NULL);
+	return fuse_update_get_attr(inode, NULL, NULL, perm_mask,
+				    AT_STATX_FORCE_SYNC);
 }
 
 /*
@@ -1505,6 +1532,7 @@ static int fuse_permission(struct mnt_idmap *idmap,
 	struct fuse_conn *fc = get_fuse_conn(inode);
 	bool refreshed = false;
 	int err = 0;
+	int perm_mask = STATX_MODE | STATX_UID | STATX_GID;
 
 	if (fuse_is_bad(inode))
 		return -EIO;
@@ -1518,13 +1546,12 @@ static int fuse_permission(struct mnt_idmap *idmap,
 	if (fc->default_permissions ||
 	    ((mask & MAY_EXEC) && S_ISREG(inode->i_mode))) {
 		struct fuse_inode *fi = get_fuse_inode(inode);
-		u32 perm_mask = STATX_MODE | STATX_UID | STATX_GID;
 
 		if (perm_mask & READ_ONCE(fi->inval_mask) ||
 		    time_before64(fi->i_time, get_jiffies_64())) {
 			refreshed = true;
 
-			err = fuse_perm_getattr(inode, mask);
+			err = fuse_perm_getattr(inode, mask, perm_mask);
 			if (err)
 				return err;
 		}
@@ -1537,7 +1564,7 @@ static int fuse_permission(struct mnt_idmap *idmap,
 		   attributes.  This is also needed, because the root
 		   node will at first have no permissions */
 		if (err == -EACCES && !refreshed) {
-			err = fuse_perm_getattr(inode, mask);
+			err = fuse_perm_getattr(inode, mask, perm_mask);
 			if (!err)
 				err = generic_permission(&nop_mnt_idmap,
 							 inode, mask);
@@ -1554,7 +1581,7 @@ static int fuse_permission(struct mnt_idmap *idmap,
 			if (refreshed)
 				return -EACCES;
 
-			err = fuse_perm_getattr(inode, mask);
+			err = fuse_perm_getattr(inode, mask, perm_mask);
 			if (!err && !(inode->i_mode & S_IXUGO))
 				return -EACCES;
 		}