[PATCH 8/8] libfuse: allow discovery of the kernel's iomap capabilities

["Darrick J. Wong" <djwong@xxxxxxxxxx>]

From: Darrick J. Wong <djwong@xxxxxxxxxx>

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

Signed-off-by: "Darrick J. Wong" <djwong@xxxxxxxxxx>
---
 include/fuse_kernel.h   |   13 +++++++++++++
 include/fuse_lowlevel.h |    5 +++++
 lib/fuse_lowlevel.c     |   28 ++++++++++++++++++++++++++++
 lib/fuse_versionscript  |    1 +
 4 files changed, 47 insertions(+)


diff --git a/include/fuse_kernel.h b/include/fuse_kernel.h
index 93ecb98a0bc20f..71077eb9f49fef 100644
--- a/include/fuse_kernel.h
+++ b/include/fuse_kernel.h
@@ -1129,12 +1129,25 @@ struct fuse_backing_map {
 	uint64_t	padding;
 };
 
+/* basic reporting functionality */
+#define FUSE_IOMAP_SUPPORT_BASICS	(1ULL << 0)
+/* fuse driver can do direct io */
+#define FUSE_IOMAP_SUPPORT_DIRECTIO	(1ULL << 1)
+/* fuse driver can do buffered io */
+#define FUSE_IOMAP_SUPPORT_PAGECACHE	(1ULL << 2)
+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 eb457007a72cbc..a74d287f9012e9 100644
--- a/include/fuse_lowlevel.h
+++ b/include/fuse_lowlevel.h
@@ -2410,6 +2410,11 @@ 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_discover_iomap(void);
+
 #ifdef __cplusplus
 }
 #endif
diff --git a/lib/fuse_lowlevel.c b/lib/fuse_lowlevel.c
index fd12daf509cebf..9779e6ea7cc8ac 100644
--- a/lib/fuse_lowlevel.c
+++ b/lib/fuse_lowlevel.c
@@ -4341,3 +4341,31 @@ int fuse_session_exited(struct fuse_session *se)
 {
 	return se->exited;
 }
+
+uint64_t fuse_discover_iomap(void)
+{
+	struct fuse_iomap_support ios;
+	uint64_t ret = 0;
+	int fd;
+
+	fd = open("/dev/fuse", O_RDONLY | O_CLOEXEC);
+	if (fd < 0)
+		return 0;
+
+	ret = ioctl(fd, FUSE_DEV_IOC_IOMAP_SUPPORT, &ios);
+	if (ret) {
+		ret = 0;
+		goto out_close;
+	}
+
+	if (ios.flags & FUSE_IOMAP_SUPPORT_BASICS)
+		ret |= FUSE_CAP_IOMAP;
+	if (ios.flags & FUSE_IOMAP_SUPPORT_DIRECTIO)
+		ret |= FUSE_CAP_IOMAP_DIRECTIO;
+	if (ios.flags & FUSE_IOMAP_SUPPORT_PAGECACHE)
+		ret |= FUSE_CAP_IOMAP_PAGECACHE;
+
+out_close:
+	close(fd);
+	return ret;
+}
diff --git a/lib/fuse_versionscript b/lib/fuse_versionscript
index 5c04e204adba33..210527ce9dd283 100644
--- a/lib/fuse_versionscript
+++ b/lib/fuse_versionscript
@@ -206,6 +206,7 @@ FUSE_3.18 {
 	global:
 		fuse_req_is_uring;
 		fuse_lowlevel_notify_iomap_add_device;
+		fuse_discover_iomap;
 } FUSE_3.17;
 
 # Local Variables: