linux-ext4 - [PATCH 13/22] libfuse: allow discovery of the kernel's iomap capabilities

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Message-ID: <176169813768.1427432.13427682700847139457.stgit@frogsfrogsfrogs>
Date: Tue, 28 Oct 2025 18:02:41 -0700
From: "Darrick J. Wong" <djwong@...nel.org>
To: djwong@...nel.org, bschubert@....com
Cc: linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org,
 bernd@...ernd.com, miklos@...redi.hu, joannelkoong@...il.com, neal@...pa.dev
Subject: [PATCH 13/22] libfuse: allow discovery of the kernel's iomap
 capabilities

From: Darrick J. Wong <djwong@...nel.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@...nel.org>
---
 include/fuse_common.h   |    7 +++++++
 include/fuse_kernel.h   |    7 +++++++
 include/fuse_lowlevel.h |   10 ++++++++++
 lib/fuse_lowlevel.c     |   19 +++++++++++++++++++
 lib/fuse_versionscript  |    1 +
 5 files changed, 44 insertions(+)


diff --git a/include/fuse_common.h b/include/fuse_common.h
index 191d9749960992..86ae8894d81dbb 100644
--- a/include/fuse_common.h
+++ b/include/fuse_common.h
@@ -534,6 +534,13 @@ 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 38aa03dce17e53..3dc00cd4cb113f 100644
--- a/include/fuse_kernel.h
+++ b/include/fuse_kernel.h
@@ -1144,12 +1144,19 @@ 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, 99, \
+					     struct fuse_iomap_support)
 
 struct fuse_lseek_in {
 	uint64_t	fh;
diff --git a/include/fuse_lowlevel.h b/include/fuse_lowlevel.h
index e2d14f2e2bd911..5ce7b4aaa2ae94 100644
--- a/include/fuse_lowlevel.h
+++ b/include/fuse_lowlevel.h
@@ -2576,6 +2576,16 @@ bool fuse_req_is_uring(fuse_req_t req);
 int fuse_req_get_payload(fuse_req_t req, char **payload, size_t *payload_sz,
 			 void **mr);
 
+
+/**
+ * Discover the kernel's iomap capabilities.  Returns FUSE_CAP_IOMAP_* flags.
+ *
+ * @param fd open file descriptor to a fuse device, or -1 if you're running
+ *           in the same process that will call mount().
+ * @return FUSE_IOMAP_SUPPORT_* flags
+ */
+uint64_t fuse_lowlevel_discover_iomap(int fd);
+
 #ifdef __cplusplus
 }
 #endif
diff --git a/lib/fuse_lowlevel.c b/lib/fuse_lowlevel.c
index e0d18844098971..4e7bf40833b578 100644
--- a/lib/fuse_lowlevel.c
+++ b/lib/fuse_lowlevel.c
@@ -4709,3 +4709,22 @@ int fuse_session_exited(struct fuse_session *se)
 
 	return exited ? 1 : 0;
 }
+
+uint64_t fuse_lowlevel_discover_iomap(int fd)
+{
+	struct fuse_iomap_support ios = { };
+
+	if (fd >= 0) {
+		ioctl(fd, FUSE_DEV_IOC_IOMAP_SUPPORT, &ios);
+		return ios.flags;
+	}
+
+	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 25a3e04c6c5ec7..704e8c2908ec4b 100644
--- a/lib/fuse_versionscript
+++ b/lib/fuse_versionscript
@@ -232,6 +232,7 @@ 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: