[PATCH AUTOSEL 5.4 17/37] ovl: skip checking lower file's i_writecount on truncate

[Sasha Levin]

From: Chengguang Xu <cgxu519@xxxxxxxxxxxx>

[ Upstream commit b71759ef1e1730db81dab98e9dab9455e8c7f5a2 ]

It is possible that a directory tree is shared between multiple overlay
instances as a lower layer.  In this case when one instance executes a file
residing on the lower layer, the other instance denies a truncate(2) call
on this file.

This only happens for truncate(2) and not for open(2) with the O_TRUNC
flag.

Fix this interference and inconsistency by removing the preliminary
i_writecount check before copy-up.

This means that unlike on normal filesystems truncate(argv[0]) will now
succeed.  If this ever causes a regression in a real world use case this
needs to be revisited.

One way to fix this properly would be to keep a correct i_writecount in the
overlay inode, but that is difficult due to memory mapping code only
dealing with the real file/inode.

Signed-off-by: Chengguang Xu <cgxu519@xxxxxxxxxxxx>
Signed-off-by: Miklos Szeredi <mszeredi@xxxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
---
 Documentation/filesystems/overlayfs.txt | 3 +++
 fs/overlayfs/inode.c                    | 6 ------
 2 files changed, 3 insertions(+), 6 deletions(-)

diff --git a/Documentation/filesystems/overlayfs.txt b/Documentation/filesystems/overlayfs.txt
index 845d689e0fd7..69ffd8ebf3a8 100644
--- a/Documentation/filesystems/overlayfs.txt
+++ b/Documentation/filesystems/overlayfs.txt
@@ -348,6 +348,9 @@ b) If a file residing on a lower layer is opened for read-only and then
 memory mapped with MAP_SHARED, then subsequent changes to the file are not
 reflected in the memory mapping.
 
+c) If a file residing on a lower layer is being executed, then opening that
+file for write or truncating the file will not be denied with ETXTBSY.
+
 The following options allow overlayfs to act more like a standards
 compliant filesystem:
 
diff --git a/fs/overlayfs/inode.c b/fs/overlayfs/inode.c
index 56b55397a7a0..0486fc925002 100644
--- a/fs/overlayfs/inode.c
+++ b/fs/overlayfs/inode.c
@@ -29,12 +29,6 @@ int ovl_setattr(struct dentry *dentry, struct iattr *attr)
 		goto out;
 
 	if (attr->ia_valid & ATTR_SIZE) {
-		struct inode *realinode = d_inode(ovl_dentry_real(dentry));
-
-		err = -ETXTBSY;
-		if (atomic_read(&realinode->i_writecount) < 0)
-			goto out_drop_write;
-
 		/* Truncate should trigger data copy up as well */
 		full_copy_up = true;
 	}
-- 
2.30.2