linux-ext4 - [PATCH 2/4] fuse2fs: wrap the fuse_set_feature

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Message-ID: <175798160511.389044.5108322625829000877.stgit@frogsfrogsfrogs>
Date: Mon, 15 Sep 2025 17:50:07 -0700
From: "Darrick J. Wong" <djwong@...nel.org>
To: tytso@....edu
Cc: linux-ext4@...r.kernel.org
Subject: [PATCH 2/4] fuse2fs: wrap the fuse_set_feature_flag helper for older
 libfuse

From: Darrick J. Wong <djwong@...nel.org>

Create a compatibility wrapper for fuse_set_feature_flag if the libfuse
version is older than the one where that function was introduced (3.17).

Signed-off-by: "Darrick J. Wong" <djwong@...nel.org>
---
 misc/fuse2fs.c |   32 +++++++++++++++++++++++++++++---
 1 file changed, 29 insertions(+), 3 deletions(-)


diff --git a/misc/fuse2fs.c b/misc/fuse2fs.c
index 101f0fa03c397d..aa51b8f55b0f50 100644
--- a/misc/fuse2fs.c
+++ b/misc/fuse2fs.c
@@ -1298,6 +1298,19 @@ static int fuse2fs_read_bitmaps(struct fuse2fs *ff)
 	return 0;
 }
 
+#if FUSE_VERSION < FUSE_MAKE_VERSION(3, 17)
+static inline int fuse_set_feature_flag(struct fuse_conn_info *conn,
+					 uint64_t flag)
+{
+	if (conn->capable & flag) {
+		conn->want |= flag;
+		return 1;
+	}
+
+	return 0;
+}
+#endif
+
 static void *op_init(struct fuse_conn_info *conn
 #if FUSE_VERSION >= FUSE_MAKE_VERSION(3, 0)
 			, struct fuse_config *cfg EXT2FS_ATTR((unused))
@@ -1322,14 +1335,14 @@ static void *op_init(struct fuse_conn_info *conn
 	fs = ff->fs;
 	dbg_printf(ff, "%s: dev=%s\n", __func__, fs->device_name);
 #ifdef FUSE_CAP_IOCTL_DIR
-	conn->want |= FUSE_CAP_IOCTL_DIR;
+	fuse_set_feature_flag(conn, FUSE_CAP_IOCTL_DIR);
 #endif
 #ifdef FUSE_CAP_POSIX_ACL
 	if (ff->acl)
-		conn->want |= FUSE_CAP_POSIX_ACL;
+		fuse_set_feature_flag(conn, FUSE_CAP_POSIX_ACL);
 #endif
 #ifdef FUSE_CAP_CACHE_SYMLINKS
-	conn->want |= FUSE_CAP_CACHE_SYMLINKS;
+	fuse_set_feature_flag(conn, FUSE_CAP_CACHE_SYMLINKS);
 #endif
 #if FUSE_VERSION >= FUSE_MAKE_VERSION(3, 0)
 	conn->time_gran = 1;
@@ -1349,6 +1362,19 @@ static void *op_init(struct fuse_conn_info *conn
 	if (ff->opstate == F2OP_WRITABLE)
 		fuse2fs_read_bitmaps(ff);
 
+#if FUSE_VERSION >= FUSE_MAKE_VERSION(3, 17)
+	/*
+	 * THIS MUST GO LAST!
+	 *
+	 * fuse_set_feature_flag in 3.17.0 has a strange bug: it sets feature
+	 * flags in conn->want_ext, but not conn->want.  Upon return to
+	 * libfuse, the lower level library observes that want and want_ext
+	 * have gotten out of sync, and refuses to mount.  Therefore,
+	 * synchronize the two.  This bug went away in 3.17.3, but we're stuck
+	 * with this forever because Debian trixie released with 3.17.2.
+	 */
+	conn->want = conn->want_ext & 0xFFFFFFFF;
+#endif
 	return ff;
 }