linux-kernel - [PATCH V3 3/4] fuse: add API to set kernel mount options

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Message-ID: <20260107153443.64794-4-john@groves.net>
Date: Wed,  7 Jan 2026 09:34:42 -0600
From: John Groves <John@...ves.net>
To: John Groves <John@...ves.net>,
	Miklos Szeredi <miklos@...redi.hu>,
	Dan Williams <dan.j.williams@...el.com>,
	Bernd Schubert <bschubert@....com>,
	Alison Schofield <alison.schofield@...el.com>
Cc: John Groves <jgroves@...ron.com>,
	Jonathan Corbet <corbet@....net>,
	Vishal Verma <vishal.l.verma@...el.com>,
	Dave Jiang <dave.jiang@...el.com>,
	Matthew Wilcox <willy@...radead.org>,
	Jan Kara <jack@...e.cz>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	David Hildenbrand <david@...nel.org>,
	Christian Brauner <brauner@...nel.org>,
	"Darrick J . Wong" <djwong@...nel.org>,
	Randy Dunlap <rdunlap@...radead.org>,
	Jeff Layton <jlayton@...nel.org>,
	Amir Goldstein <amir73il@...il.com>,
	Jonathan Cameron <Jonathan.Cameron@...wei.com>,
	Stefan Hajnoczi <shajnocz@...hat.com>,
	Joanne Koong <joannelkoong@...il.com>,
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	Bagas Sanjaya <bagasdotme@...il.com>,
	Chen Linxuan <chenlinxuan@...ontech.com>,
	James Morse <james.morse@....com>,
	Fuad Tabba <tabba@...gle.com>,
	Sean Christopherson <seanjc@...gle.com>,
	Shivank Garg <shivankg@....com>,
	Ackerley Tng <ackerleytng@...gle.com>,
	Gregory Price <gourry@...rry.net>,
	Aravind Ramesh <arramesh@...ron.com>,
	Ajay Joshi <ajayjoshi@...ron.com>,
	venkataravis@...ron.com,
	linux-doc@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	nvdimm@...ts.linux.dev,
	linux-cxl@...r.kernel.org,
	linux-fsdevel@...r.kernel.org,
	John Groves <john@...ves.net>
Subject: [PATCH V3 3/4] fuse: add API to set kernel mount options

Add fuse_add_kernel_mount_opt() to allow libfuse callers to pass
additional mount options directly to the kernel. This enables
filesystem-specific kernel mount options that aren't exposed through
the standard libfuse mount option parsing.

For example, famfs uses this to set the "shadow=" mount option
for shadow file system mounts.

API addition:
  int fuse_add_kernel_mount_opt(struct fuse_session *se, const char *mount_opt)

Signed-off-by: John Groves <john@...ves.net>
---
 include/fuse_lowlevel.h | 10 ++++++++++
 lib/fuse_i.h            |  1 +
 lib/fuse_lowlevel.c     |  5 +++++
 lib/fuse_versionscript  |  1 +
 lib/mount.c             |  8 ++++++++
 5 files changed, 25 insertions(+)

diff --git a/include/fuse_lowlevel.h b/include/fuse_lowlevel.h
index 016f831..d2bbcca 100644
--- a/include/fuse_lowlevel.h
+++ b/include/fuse_lowlevel.h
@@ -2195,6 +2195,16 @@ static inline int fuse_session_custom_io(struct fuse_session *se,
 }
 #endif
 
+/**
+ * Allow a libfuse caller to directly add kernel mount opts
+ *
+ * @param se session object
+ * @param mount_opt the option to add
+ *
+ * @return 0 on success, -1 on failure
+ */
+int fuse_add_kernel_mount_opt(struct fuse_session *se, const char *mount_opt);
+
 /**
  * Mount a FUSE file system.
  *
diff --git a/lib/fuse_i.h b/lib/fuse_i.h
index 65d2f68..41285d2 100644
--- a/lib/fuse_i.h
+++ b/lib/fuse_i.h
@@ -220,6 +220,7 @@ void destroy_mount_opts(struct mount_opts *mo);
 void fuse_mount_version(void);
 unsigned get_max_read(struct mount_opts *o);
 void fuse_kern_unmount(const char *mountpoint, int fd);
+int __fuse_add_kernel_mount_opt(struct fuse_session *se, const char *mount_opt);
 int fuse_kern_mount(const char *mountpoint, struct mount_opts *mo);
 
 int fuse_send_reply_iov_nofree(fuse_req_t req, int error, struct iovec *iov,
diff --git a/lib/fuse_lowlevel.c b/lib/fuse_lowlevel.c
index 0cde3d4..413e7c3 100644
--- a/lib/fuse_lowlevel.c
+++ b/lib/fuse_lowlevel.c
@@ -4349,6 +4349,11 @@ int fuse_session_custom_io_30(struct fuse_session *se,
 			offsetof(struct fuse_custom_io, clone_fd), fd);
 }
 
+int fuse_add_kernel_mount_opt(struct fuse_session *se, const char *mount_opt)
+{
+	return __fuse_add_kernel_mount_opt(se, mount_opt);
+}
+
 int fuse_session_mount(struct fuse_session *se, const char *_mountpoint)
 {
 	int fd;
diff --git a/lib/fuse_versionscript b/lib/fuse_versionscript
index f9562b6..536569a 100644
--- a/lib/fuse_versionscript
+++ b/lib/fuse_versionscript
@@ -220,6 +220,7 @@ FUSE_3.18 {
 
 		fuse_reply_statx;
 		fuse_fs_statx;
+		fuse_add_kernel_mount_opt;
 } FUSE_3.17;
 
 FUSE_3.19 {
diff --git a/lib/mount.c b/lib/mount.c
index 7a856c1..e6c2305 100644
--- a/lib/mount.c
+++ b/lib/mount.c
@@ -674,6 +674,14 @@ void destroy_mount_opts(struct mount_opts *mo)
 	free(mo);
 }
 
+int __fuse_add_kernel_mount_opt(struct fuse_session *se, const char *mount_opt)
+{
+	if (!se->mo)
+		return -1;
+	if (!mount_opt)
+		return -1;
+	return fuse_opt_add_opt(&se->mo->kernel_opts, mount_opt);
+}
 
 int fuse_kern_mount(const char *mountpoint, struct mount_opts *mo)
 {
-- 
2.49.0