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ovl: skip checking lower file's i_writecount on truncate

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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@mykernel.net>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
This commit is contained in:
Chengguang Xu 2021-04-24 22:03:15 +08:00 committed by Miklos Szeredi
parent ffb24e3c65
commit b71759ef1e
2 changed files with 3 additions and 6 deletions
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3
Documentation/filesystems/overlayfs.rst
View File

@ -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:

6
fs/overlayfs/inode.c
View File

@ -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;
}