linux-kernel - [PATCH v2] loop: fix zero sized loop for block special file

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Message-Id: <20250825093205.3684121-1-yukuai1@huaweicloud.com>
Date: Mon, 25 Aug 2025 17:32:05 +0800
From: Yu Kuai <yukuai1@...weicloud.com>
To: ming.lei@...hat.com,
	hch@...radead.org,
	axboe@...nel.dk,
	yukuai3@...wei.com,
	rajeevm@....com
Cc: linux-block@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	yukuai1@...weicloud.com,
	yi.zhang@...wei.com,
	yangerkun@...wei.com,
	johnny.chenyi@...wei.com
Subject: [PATCH v2] loop: fix zero sized loop for block special file

From: Yu Kuai <yukuai3@...wei.com>

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@...el.com>
Closes: https://lore.kernel.org/oe-lkp/202508200409.b2459c02-lkp@intel.com
Signed-off-by: Yu Kuai <yukuai3@...wei.com>
---
Changes in v2:
 - don't call vfs_getattr_nosec() for block special file path, by Ming

 drivers/block/loop.c | 26 ++++++++++++++++----------
 1 file changed, 16 insertions(+), 10 deletions(-)

diff --git a/drivers/block/loop.c b/drivers/block/loop.c
index 57263c273f0f..053a086d547e 100644
--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c
@@ -139,20 +139,26 @@ static int part_shift;
 
 static loff_t lo_calculate_size(struct loop_device *lo, struct file *file)
 {
-	struct kstat stat;
 	loff_t loopsize;
 	int ret;
 
-	/*
-	 * Get the accurate file size. This provides better results than
-	 * cached inode data, particularly for network filesystems where
-	 * metadata may be stale.
-	 */
-	ret = vfs_getattr_nosec(&file->f_path, &stat, STATX_SIZE, 0);
-	if (ret)
-		return 0;
+	if (S_ISBLK(file_inode(file)->i_mode)) {
+		loopsize = i_size_read(file->f_mapping->host);
+	} else {
+		struct kstat stat;
+
+		/*
+		 * Get the accurate file size. This provides better results than
+		 * cached inode data, particularly for network filesystems where
+		 * metadata may be stale.
+		 */
+		ret = vfs_getattr_nosec(&file->f_path, &stat, STATX_SIZE, 0);
+		if (ret)
+			return 0;
+
+		loopsize = stat.size;
+	}
 
-	loopsize = stat.size;
 	if (lo->lo_offset > 0)
 		loopsize -= lo->lo_offset;
 	/* offset is beyond i_size, weird but possible */
-- 
2.39.2