[PATCH] loop: fix zero sized loop for block special file

[Yu Kuai]

From: Yu Kuai <yukuai3@xxxxxxxxxx>

By default, /dev/sda is block specail file from devtmpfs, getattr will
return file size as zero, causing loop failed for raw block device.

We can add bdev_statx() to return device size, however this may introduce
changes that are not acknowledged by user. Fix this problem by reverting
changes for block special file, file mapping host is set to bdev inode
while opening, and use i_size_read() directly to get device size.

Fixes: 47b71abd5846 ("loop: use vfs_getattr_nosec for accurate file size")
Reported-by: kernel test robot <oliver.sang@xxxxxxxxx>
Closes: https://lore.kernel.org/oe-lkp/202508200409.b2459c02-lkp@xxxxxxxxx
Signed-off-by: Yu Kuai <yukuai3@xxxxxxxxxx>
---
 drivers/block/loop.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/drivers/block/loop.c b/drivers/block/loop.c
index 57263c273f0f..cde235bd883c 100644
--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c
@@ -153,6 +153,9 @@ static loff_t lo_calculate_size(struct loop_device *lo, struct file *file)
 		return 0;
 
 	loopsize = stat.size;
+	if (!loopsize && S_ISBLK(stat.mode))
+		loopsize = i_size_read(file->f_mapping->host);
+
 	if (lo->lo_offset > 0)
 		loopsize -= lo->lo_offset;
 	/* offset is beyond i_size, weird but possible */
-- 
2.39.2