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@groves.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