[PATCH v1 2/3] exfat: print warning upon block size calibration

[David Timber <dxdt@dev.snart.me>]

If the block size specified in the exFAT volume boot sector is different
from the actual logical block size of the device, many implementations
including FUSE-exfat, macos and previous versions of Windows are not
able to mount the volume.

A possible scenario in which this can happen is when the user dd's the
volume in a 4K-blocksize device("Advanced Format") to a 512-blocksize
device. This is a design issue inherent to the exFAT format itself
which layouts the structures of exFAT volumes aligned to the sector
size rather than large byte sizes as seen with other modern file
systems.

Print a kind warning about this potential compatibility issue.

Link: https://github.com/exfatprogs/exfatprogs/issues/349
Signed-off-by: David Timber <dxdt@dev.snart.me>
---
 fs/exfat/super.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/fs/exfat/super.c b/fs/exfat/super.c
index 15e99766cb8c..e0a80d8f5f80 100644
--- a/fs/exfat/super.c
+++ b/fs/exfat/super.c
@@ -409,6 +409,8 @@ static int exfat_calibrate_blocksize(struct super_block *sb, int logical_sect)
 	}
 
 	if (logical_sect > sb->s_blocksize) {
+		const unsigned long saved_bs = sb->s_blocksize;
+
 		brelse(sbi->boot_bh);
 		sbi->boot_bh = NULL;
 
@@ -423,6 +425,10 @@ static int exfat_calibrate_blocksize(struct super_block *sb, int logical_sect)
 				  sb->s_blocksize);
 			return -EIO;
 		}
+
+		exfat_warn(sb, "blocksize calibrated from device logical block size(%lu) to volume sector size(%d)!\n"
+			   "Other implementations may not be able to handle this volume.",
+			   saved_bs, logical_sect);
 	}
 	return 0;
 }
-- 
2.53.0.1.ga224b40d3f.dirty