ublk: widen ublk_shmem_buf_reg.len to __u64 for 4GB buffer support

The __u32 len field cannot represent a 4GB buffer (0x100000000
overflows to 0). Change it to __u64 so buffers up to 4GB can be
registered. Add a reserved field for alignment and validate it
is zero.

The kernel enforces a default max of 4GB (UBLK_SHMEM_BUF_SIZE_MAX)
which may be increased in future.

Signed-off-by: Ming Lei <tom.leiming@gmail.com>
Link: https://patch.msgid.link/20260409133020.3780098-2-tom.leiming@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Author: ming1

drivers/block/ublk_drv.c

@@ -63,6 +63,9 @@
 #define UBLK_CMD_REG_BUF	_IOC_NR(UBLK_U_CMD_REG_BUF)
 #define UBLK_CMD_UNREG_BUF	_IOC_NR(UBLK_U_CMD_UNREG_BUF)
 
+/* Default max shmem buffer size: 4GB (may be increased in future) */
+#define UBLK_SHMEM_BUF_SIZE_MAX	(1ULL << 32)
+
 #define UBLK_IO_REGISTER_IO_BUF		_IOC_NR(UBLK_U_IO_REGISTER_IO_BUF)
 #define UBLK_IO_UNREGISTER_IO_BUF	_IOC_NR(UBLK_U_IO_UNREGISTER_IO_BUF)
 
@@ -5351,11 +5354,15 @@ static int ublk_ctrl_reg_buf(struct ublk_device *ub,
 	if (buf_reg.flags & ~UBLK_SHMEM_BUF_READ_ONLY)
 		return -EINVAL;
 
+	if (buf_reg.reserved)
+		return -EINVAL;
+
 	addr = buf_reg.addr;
 	size = buf_reg.len;
 	nr_pages = size >> PAGE_SHIFT;
 
-	if (!size || !PAGE_ALIGNED(size) || !PAGE_ALIGNED(addr))
+	if (!size || size > UBLK_SHMEM_BUF_SIZE_MAX ||
+	    !PAGE_ALIGNED(size) || !PAGE_ALIGNED(addr))
 		return -EINVAL;
 
 	disk = ublk_get_disk(ub);

include/uapi/linux/ublk_cmd.h

@@ -89,8 +89,9 @@
 /* Parameter buffer for UBLK_U_CMD_REG_BUF, pointed to by ctrl_cmd.addr */
 struct ublk_shmem_buf_reg {
 	__u64	addr;	/* userspace virtual address of shared memory */
-	__u32	len;	/* buffer size in bytes (page-aligned, max 4GB) */
+	__u64	len;	/* buffer size in bytes, page-aligned, default max 4GB */
 	__u32	flags;
+	__u32	reserved;
 };
 
 /* Pin pages without FOLL_WRITE; usable with write-sealed memfd */