linux-ext4 - [PATCH 2/2] fuse: set iomap backing device block size

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Message-ID: <176169812555.1427080.18405773837900501876.stgit@frogsfrogsfrogs>
Date: Tue, 28 Oct 2025 17:59:18 -0700
From: "Darrick J. Wong" <djwong@...nel.org>
To: djwong@...nel.org, miklos@...redi.hu
Cc: joannelkoong@...il.com, bernd@...ernd.com, neal@...pa.dev,
 linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: [PATCH 2/2] fuse: set iomap backing device block size

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

Add a new ioctl so that an unprivileged fuse server can set the block
size of a bdev that's opened for iomap usage.

Signed-off-by: "Darrick J. Wong" <djwong@...nel.org>
---
 fs/fuse/fuse_i.h          |    3 +++
 include/uapi/linux/fuse.h |    7 +++++++
 fs/fuse/dev.c             |    2 ++
 fs/fuse/file_iomap.c      |   24 ++++++++++++++++++++++++
 4 files changed, 36 insertions(+)


diff --git a/fs/fuse/fuse_i.h b/fs/fuse/fuse_i.h
index f45e59d16d0ebc..4b8c54cced7e07 100644
--- a/fs/fuse/fuse_i.h
+++ b/fs/fuse/fuse_i.h
@@ -1857,6 +1857,8 @@ void fuse_iomap_copied_file_range(struct inode *inode, loff_t offset,
 int fuse_dev_ioctl_add_iomap(struct file *file);
 int fuse_dev_ioctl_iomap_support(struct file *file,
 				 struct fuse_iomap_support __user *argp);
+int fuse_dev_ioctl_iomap_set_blocksize(struct file *file,
+				struct fuse_iomap_backing_info __user *argp);
 int fuse_iomap_dev_inval(struct fuse_conn *fc,
 			 const struct fuse_iomap_dev_inval_out *arg);
 
@@ -1908,6 +1910,7 @@ int fuse_iomap_inval(struct fuse_conn *fc,
 # define fuse_iomap_copied_file_range(...)	((void)0)
 # define fuse_dev_ioctl_add_iomap(...)		(-EOPNOTSUPP)
 # define fuse_dev_ioctl_iomap_support(...)	(-EOPNOTSUPP)
+# define fuse_dev_ioctl_iomap_set_blocksize(...) (-EOPNOTSUPP)
 # define fuse_iomap_dev_inval(...)		(-ENOSYS)
 # define fuse_iomap_fadvise			NULL
 # define fuse_inode_caches_iomaps(...)		(false)
diff --git a/include/uapi/linux/fuse.h b/include/uapi/linux/fuse.h
index daf72e46120c24..38e44909370e12 100644
--- a/include/uapi/linux/fuse.h
+++ b/include/uapi/linux/fuse.h
@@ -1197,6 +1197,11 @@ struct fuse_iomap_support {
 	uint64_t	padding;
 };
 
+struct fuse_iomap_backing_info {
+	uint32_t	backing_id;
+	uint32_t	blocksize;
+};
+
 /* Device ioctls: */
 #define FUSE_DEV_IOC_MAGIC		229
 #define FUSE_DEV_IOC_CLONE		_IOR(FUSE_DEV_IOC_MAGIC, 0, uint32_t)
@@ -1207,6 +1212,8 @@ struct fuse_iomap_support {
 #define FUSE_DEV_IOC_ADD_IOMAP		_IO(FUSE_DEV_IOC_MAGIC, 99)
 #define FUSE_DEV_IOC_IOMAP_SUPPORT	_IOR(FUSE_DEV_IOC_MAGIC, 99, \
 					     struct fuse_iomap_support)
+#define FUSE_DEV_IOC_IOMAP_SET_BLOCKSIZE _IOW(FUSE_DEV_IOC_MAGIC, 99, \
+					      struct fuse_iomap_backing_info)
 
 struct fuse_lseek_in {
 	uint64_t	fh;
diff --git a/fs/fuse/dev.c b/fs/fuse/dev.c
index 4dfad6c33fac8f..a457d31d8e252c 100644
--- a/fs/fuse/dev.c
+++ b/fs/fuse/dev.c
@@ -2791,6 +2791,8 @@ static long fuse_dev_ioctl(struct file *file, unsigned int cmd,
 		return fuse_dev_ioctl_iomap_support(file, argp);
 	case FUSE_DEV_IOC_ADD_IOMAP:
 		return fuse_dev_ioctl_add_iomap(file);
+	case FUSE_DEV_IOC_IOMAP_SET_BLOCKSIZE:
+		return fuse_dev_ioctl_iomap_set_blocksize(file, argp);
 
 	default:
 		return -ENOTTY;
diff --git a/fs/fuse/file_iomap.c b/fs/fuse/file_iomap.c
index 08e7e4f924a65a..3e6bdb53b1bfc9 100644
--- a/fs/fuse/file_iomap.c
+++ b/fs/fuse/file_iomap.c
@@ -2800,3 +2800,27 @@ int fuse_iomap_inval(struct fuse_conn *fc,
 	up_read(&fc->killsb);
 	return ret ? ret : ret2;
 }
+
+int fuse_dev_ioctl_iomap_set_blocksize(struct file *file,
+				struct fuse_iomap_backing_info __user *argp)
+{
+	struct fuse_iomap_backing_info fbi;
+	struct fuse_dev *fud = fuse_get_dev(file);
+	struct fuse_backing *fb;
+	int ret;
+
+	if (IS_ERR(fud))
+		return PTR_ERR(fud);
+
+	if (copy_from_user(&fbi, argp, sizeof(fbi)))
+		return -EFAULT;
+
+	fb = fuse_backing_lookup(fud->fc, &fuse_iomap_backing_ops,
+				 fbi.backing_id);
+	if (!fb)
+		return -ENOENT;
+
+	ret = set_blocksize(fb->file, fbi.blocksize);
+	fuse_backing_put(fb);
+	return ret;
+}