[PATCH 1/2] exfat: fix used_clusters accounting during cluster allocation

[Chi Zhiling]

From: Chi Zhiling <chizhiling@xxxxxxxxxx>

In the current exfat_alloc_cluster(), p_chain->size is incremented as
each cluster is allocated so that the partially allocated chain can
be freed by __exfat_free_cluster() on the error path.

However, sbi->used_clusters is still updated only after all clusters
have been allocated, while __exfat_free_cluster() unconditionally
subtracts the number of freed clusters from sbi->used_clusters.
Therefore, when allocation fails partway through, the partially
allocated clusters are subtracted from sbi->used_clusters even
though they were never added to it. This corrupts the free-space
accounting and can cause sbi->used_clusters to underflow.

Increment sbi->used_clusters together with p_chain->size as each
cluster is allocated. This keeps the accounting consistent with
__exfat_free_cluster() on both the success and error paths.

This issue is caught by generic/476 with 256k cluster size:

    Ran: generic/476
    Failures: generic/476
    Failed 1 of 1 tests

    *** fsck.exfat output ***
    ERROR: <path>: more clusters are allocated. truncate to N bytes
    ...
    /dev/vdc: corrupted. directories 331, files 526
    /dev/vdc: files corrupted 9, files fixed 0
    *** end fsck.exfat output ***

Fixes: d5c514b6a0c0 ("exfat: fix the newly allocated clusters are not freed in error handling")
Signed-off-by: Chi Zhiling <chizhiling@xxxxxxxxxx>
---
 fs/exfat/fatent.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/exfat/fatent.c b/fs/exfat/fatent.c
index a8b11e2ce43f..a6728c361289 100644
--- a/fs/exfat/fatent.c
+++ b/fs/exfat/fatent.c
@@ -509,13 +509,13 @@ int exfat_alloc_cluster(struct inode *inode, unsigned int num_alloc,
 			}
 		}
 		p_chain->size++;
+		sbi->used_clusters++;
 
 		last_clu = new_clu;
 
 		if (p_chain->size == num_alloc) {
 done:
 			sbi->clu_srch_ptr = hint_clu;
-			sbi->used_clusters += p_chain->size;
 			mutex_unlock(&sbi->bitmap_lock);
 			return 0;
 		}
-- 
2.53.0