[PATCH 11/14] 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_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 17ab74255cbf33..7a1226d6bc2c0a 100644
--- a/include/fuse_kernel.h
+++ b/include/fuse_kernel.h
@@ -1142,6 +1142,17 @@ 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_FILEIO	(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)
@@ -1150,6 +1161,8 @@ struct fuse_backing_map {
 #define FUSE_DEV_IOC_BACKING_CLOSE	_IOW(FUSE_DEV_IOC_MAGIC, 2, uint32_t)
 #define FUSE_DEV_IOC_IOMAP_DEV_ADD	_IOW(FUSE_DEV_IOC_MAGIC, 3, \
 					     struct fuse_backing_map)
+#define FUSE_DEV_IOC_IOMAP_SUPPORT	_IOR(FUSE_DEV_IOC_MAGIC, 4, \
+					     struct fuse_iomap_support)
 
 struct fuse_lseek_in {
 	uint64_t	fh;
diff --git a/include/fuse_lowlevel.h b/include/fuse_lowlevel.h
index 07748abcf079cf..a529a112998d6e 100644
--- a/include/fuse_lowlevel.h
+++ b/include/fuse_lowlevel.h
@@ -2503,6 +2503,11 @@ 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 d354b947a4fb6b..0c7d5cc99945ee 100644
--- a/lib/fuse_lowlevel.c
+++ b/lib/fuse_lowlevel.c
@@ -4490,3 +4490,31 @@ int fuse_session_exited(struct fuse_session *se)
 
 	return exited ? 1 : 0;
 }
+
+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_FILEIO)
+		ret |= FUSE_CAP_IOMAP_FILEIO;
+
+out_close:
+	close(fd);
+	return ret;
+}
diff --git a/lib/fuse_versionscript b/lib/fuse_versionscript
index 9207145624ba83..606fdc6127462e 100644
--- a/lib/fuse_versionscript
+++ b/lib/fuse_versionscript
@@ -219,6 +219,7 @@ FUSE_3.18 {
 		fuse_reply_create_iflags;
 		fuse_reply_entry_iflags;
 		fuse_add_direntry_plus_iflags;
+		fuse_discover_iomap;
 } FUSE_3.17;
 
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