linux-ext4 - [PATCH 1/5] truncate: Zero bytes after 'oldsize' if we're expanding the file

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Message-Id: <20230202204428.3267832-2-willy@infradead.org>
Date:   Thu,  2 Feb 2023 20:44:23 +0000
From:   "Matthew Wilcox (Oracle)" <willy@...radead.org>
To:     linux-fsdevel@...r.kernel.org
Cc:     "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        linux-afs@...ts.infradead.org, linux-btrfs@...r.kernel.org,
        linux-ext4@...r.kernel.org, linux-mm@...ck.org,
        Hugh Dickins <hughd@...gle.com>, linux-kernel@...r.kernel.org,
        fstests@...r.kernel.org
Subject: [PATCH 1/5] truncate: Zero bytes after 'oldsize' if we're expanding the file

POSIX requires that "If the file size is increased, the extended area
shall appear as if it were zero-filled".  It is possible to use mmap to
write past EOF and that data will become visible instead of zeroes.
This fixes the problem for the filesystems which simply call
truncate_setsize().  More complex filesystems will need their own
patches.

Signed-off-by: Matthew Wilcox (Oracle) <willy@...radead.org>
---
 mm/truncate.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/mm/truncate.c b/mm/truncate.c
index 7b4ea4c4a46b..cebfc5415e9a 100644
--- a/mm/truncate.c
+++ b/mm/truncate.c
@@ -763,9 +763,12 @@ void truncate_setsize(struct inode *inode, loff_t newsize)
 	loff_t oldsize = inode->i_size;
 
 	i_size_write(inode, newsize);
-	if (newsize > oldsize)
+	if (newsize > oldsize) {
 		pagecache_isize_extended(inode, oldsize, newsize);
-	truncate_pagecache(inode, newsize);
+		truncate_pagecache(inode, oldsize);
+	} else {
+		truncate_pagecache(inode, newsize);
+	}
 }
 EXPORT_SYMBOL(truncate_setsize);
 
-- 
2.35.1