[PATCH AUTOSEL 5.14 47/99] ovl: skip checking lower file's i_writecount on truncate

From: Sasha Levin
Date: Thu Sep 09 2021 - 20:42:58 EST

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

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.rst | 3 +++
fs/overlayfs/inode.c | 6 ------
2 files changed, 3 insertions(+), 6 deletions(-)

diff --git a/Documentation/filesystems/overlayfs.rst b/Documentation/filesystems/overlayfs.rst
index 455ca86eb4fc..7da6c30ed596 100644
--- a/Documentation/filesystems/overlayfs.rst
+++ b/Documentation/filesystems/overlayfs.rst
@@ -427,6 +427,9 @@ 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 b288843e6b42..6566294c5fd6 100644
--- a/fs/overlayfs/inode.c
+++ b/fs/overlayfs/inode.c
@@ -33,12 +33,6 @@ int ovl_setattr(struct user_namespace *mnt_userns, struct dentry *dentry,
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;