[PATCH 14/21] libfuse: allow discovery of the kernel's iomap capabilities

["Darrick J. Wong" <djwong@kernel.org>]

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

Create a library function so that we can discover the kernel's iomap
capabilities ahead of time.

Signed-off-by: "Darrick J. Wong" <djwong@kernel.org>
---
 include/fuse_common.h   |    7 +++++++
 include/fuse_kernel.h   |    7 +++++++
 include/fuse_lowlevel.h |    5 +++++
 lib/fuse_lowlevel.c     |   15 +++++++++++++++
 lib/fuse_versionscript  |    1 +
 5 files changed, 35 insertions(+)


diff --git a/include/fuse_common.h b/include/fuse_common.h
index 6e8b2958373258..f9cc3702411680 100644
--- a/include/fuse_common.h
+++ b/include/fuse_common.h
@@ -542,6 +542,13 @@ struct fuse_loop_config_v1 {
 
 #define FUSE_IOCTL_MAX_IOV	256
 
+/**
+ * iomap discovery flags
+ *
+ * FUSE_IOMAP_SUPPORT_FILEIO: basic file I/O functionality through iomap
+ */
+#define FUSE_IOMAP_SUPPORT_FILEIO	(1ULL << 0)
+
 /**
  * Connection information, passed to the ->init() method
  *
diff --git a/include/fuse_kernel.h b/include/fuse_kernel.h
index eafad773a1fd5f..dbd2ce1fbbe6ed 100644
--- a/include/fuse_kernel.h
+++ b/include/fuse_kernel.h
@@ -1134,12 +1134,19 @@ struct fuse_backing_map {
 	uint64_t	padding;
 };
 
+struct fuse_iomap_support {
+	uint64_t	flags;
+	uint64_t	padding;
+};
+
 /* Device ioctls: */
 #define FUSE_DEV_IOC_MAGIC		229
 #define FUSE_DEV_IOC_CLONE		_IOR(FUSE_DEV_IOC_MAGIC, 0, uint32_t)
 #define FUSE_DEV_IOC_BACKING_OPEN	_IOW(FUSE_DEV_IOC_MAGIC, 1, \
 					     struct fuse_backing_map)
 #define FUSE_DEV_IOC_BACKING_CLOSE	_IOW(FUSE_DEV_IOC_MAGIC, 2, uint32_t)
+#define FUSE_DEV_IOC_IOMAP_SUPPORT	_IOR(FUSE_DEV_IOC_MAGIC, 3, \
+					     struct fuse_iomap_support)
 
 struct fuse_lseek_in {
 	uint64_t	fh;
diff --git a/include/fuse_lowlevel.h b/include/fuse_lowlevel.h
index e0642032127686..2931a57ec4079b 100644
--- a/include/fuse_lowlevel.h
+++ b/include/fuse_lowlevel.h
@@ -2556,6 +2556,11 @@ int fuse_session_receive_buf(struct fuse_session *se, struct fuse_buf *buf);
  */
 bool fuse_req_is_uring(fuse_req_t req);
 
+/**
+ * Discover the kernel's iomap capabilities.  Returns FUSE_CAP_IOMAP_* flags.
+ */
+uint64_t fuse_lowlevel_discover_iomap(void);
+
 #ifdef __cplusplus
 }
 #endif
diff --git a/lib/fuse_lowlevel.c b/lib/fuse_lowlevel.c
index 6a96c0f62d5884..ab10204c8042d9 100644
--- a/lib/fuse_lowlevel.c
+++ b/lib/fuse_lowlevel.c
@@ -4603,3 +4603,18 @@ int fuse_session_exited(struct fuse_session *se)
 
 	return exited ? 1 : 0;
 }
+
+uint64_t fuse_lowlevel_discover_iomap(void)
+{
+	struct fuse_iomap_support ios = { };
+	int fd;
+
+	fd = open("/dev/fuse", O_RDONLY | O_CLOEXEC);
+	if (fd < 0)
+		return 0;
+
+	ioctl(fd, FUSE_DEV_IOC_IOMAP_SUPPORT, &ios);
+	close(fd);
+
+	return ios.flags;
+}
diff --git a/lib/fuse_versionscript b/lib/fuse_versionscript
index aa16efdd8a9879..5275a17ba1ed51 100644
--- a/lib/fuse_versionscript
+++ b/lib/fuse_versionscript
@@ -231,6 +231,7 @@ FUSE_3.99 {
 		fuse_add_direntry_plus_iflags;
 		fuse_fs_can_enable_iomap;
 		fuse_fs_can_enable_iomapx;
+		fuse_lowlevel_discover_iomap;
 } FUSE_3.18;
 
 # Local Variables: