loop: Clean up LOOP_SET_STATUS lo_flags handling

LOOP_SET_STATUS(64) will actually allow some lo_flags to be modified; in
particular, LO_FLAGS_AUTOCLEAR can be set and cleared, whereas
LO_FLAGS_PARTSCAN can be set to request a partition scan. Make this
explicit by updating the UAPI to include the flags that can be
set/cleared using this ioctl.

The implementation can then blindly take over the passed in flags,
and use the previous flags for those flags that can't be set / cleared
using LOOP_SET_STATUS.

Signed-off-by: Martijn Coenen <[e-mail]>
Reviewed-by: Christoph Hellwig <[e-mail]>
Signed-off-by: Jens Axboe <[e-mail]>

drivers/block/loop.c

···
 	lo->transfer = xfer->transfer;
 	lo->ioctl = xfer->ioctl;
 
-	if ((lo->lo_flags & LO_FLAGS_AUTOCLEAR) !=
-	     (info->lo_flags & LO_FLAGS_AUTOCLEAR))
-		lo->lo_flags ^= LO_FLAGS_AUTOCLEAR;
+	lo->lo_flags = info->lo_flags;
 
 	lo->lo_encrypt_key_size = info->lo_encrypt_key_size;
 	lo->lo_init[0] = info->lo_init[0];
···
 	int err;
 	struct block_device *bdev;
 	kuid_t uid = current_uid();
+	int prev_lo_flags;
 	bool partscan = false;
 	bool size_changed = false;
 
···
 		goto out_unfreeze;
 	}
 
+	prev_lo_flags = lo->lo_flags;
+
 	err = loop_set_status_from_info(lo, info);
 	if (err)
 		goto out_unfreeze;
+
+	/* Mask out flags that can't be set using LOOP_SET_STATUS. */
+	lo->lo_flags &= ~LOOP_SET_STATUS_SETTABLE_FLAGS;
+	/* For those flags, use the previous values instead */
+	lo->lo_flags |= prev_lo_flags & ~LOOP_SET_STATUS_SETTABLE_FLAGS;
+	/* For flags that can't be cleared, use previous values too */
+	lo->lo_flags |= prev_lo_flags & ~LOOP_SET_STATUS_CLEARABLE_FLAGS;
 
 	if (size_changed) {
 		loff_t new_size = get_size(lo->lo_offset, lo->lo_sizelimit,
···
 out_unfreeze:
 	blk_mq_unfreeze_queue(lo->lo_queue);
 
-	if (!err && (info->lo_flags & LO_FLAGS_PARTSCAN) &&
-	     !(lo->lo_flags & LO_FLAGS_PARTSCAN)) {
-		lo->lo_flags |= LO_FLAGS_PARTSCAN;
+	if (!err && (lo->lo_flags & LO_FLAGS_PARTSCAN) &&
+	     !(prev_lo_flags & LO_FLAGS_PARTSCAN)) {
 		lo->lo_disk->flags &= ~GENHD_FL_NO_PART_SCAN;
 		bdev = lo->lo_device;
 		partscan = true;

include/uapi/linux/loop.h

···
 	LO_FLAGS_DIRECT_IO	= 16,
 };
 
+/* LO_FLAGS that can be set using LOOP_SET_STATUS(64) */
+#define LOOP_SET_STATUS_SETTABLE_FLAGS (LO_FLAGS_AUTOCLEAR | LO_FLAGS_PARTSCAN)
+
+/* LO_FLAGS that can be cleared using LOOP_SET_STATUS(64) */
+#define LOOP_SET_STATUS_CLEARABLE_FLAGS (LO_FLAGS_AUTOCLEAR)
+
 #include <asm/posix_types.h>	/* for __kernel_old_dev_t */
 #include <linux/types.h>	/* for __u64 */
 
···
 	int		   lo_offset;
 	int		   lo_encrypt_type;
 	int		   lo_encrypt_key_size; 	/* ioctl w/o */
-	int		   lo_flags;			/* ioctl r/o */
+	int		   lo_flags;
 	char		   lo_name[LO_NAME_SIZE];
 	unsigned char	   lo_encrypt_key[LO_KEY_SIZE]; /* ioctl w/o */
 	unsigned long	   lo_init[2];
···
 	__u32		   lo_number;			/* ioctl r/o */
 	__u32		   lo_encrypt_type;
 	__u32		   lo_encrypt_key_size;		/* ioctl w/o */
-	__u32		   lo_flags;			/* ioctl r/o */
+	__u32		   lo_flags;
 	__u8		   lo_file_name[LO_NAME_SIZE];
 	__u8		   lo_crypt_name[LO_NAME_SIZE];
 	__u8		   lo_encrypt_key[LO_KEY_SIZE]; /* ioctl w/o */