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From: David Howells <>
Subject: [PATCH] overlayfs: Warn on copy up if a process has a R/O fd open to the lower file
Date: Tue, 07 Jul 2015 15:04:44 +0100	[thread overview]
Message-ID: <> (raw)

Print a warning when overlayfs copies up a file if the process that triggered
the copy up has a R/O fd open to the lower file being copied up.

This can help catch applications that do things like the following:

	fd1 = open("foo", O_RDONLY);
	fd2 = open("foo", O_RDWR);

where they expect fd1 and fd2 to refer to the same file - which will no longer
be the case post-copy up.

With this patch, the following commands:

	bash 5</mnt/a/foo128

assuming /mnt/a/foo128 to be an un-copied up file on an overlay will produce
the following warning in the kernel log:

	overlayfs: Copying up foo129, but open R/O on fd 5 which will cease to
	be coherent [pid=3818 bash]

This is enabled by setting:


to 1.

The warnings are ratelimited and are also limited to one warning per file -
assuming the copy up completes in each case.

Signed-off-by: David Howells <>
diff --git a/fs/overlayfs/copy_up.c b/fs/overlayfs/copy_up.c
index 84d693d37428..e1aef2ba2f08 100644
--- a/fs/overlayfs/copy_up.c
+++ b/fs/overlayfs/copy_up.c
@@ -7,6 +7,7 @@
  * the Free Software Foundation.
+#include <linux/module.h>
 #include <linux/fs.h>
 #include <linux/slab.h>
 #include <linux/file.h>
@@ -16,10 +17,41 @@
 #include <linux/uaccess.h>
 #include <linux/sched.h>
 #include <linux/namei.h>
+#include <linux/fdtable.h>
+#include <linux/ratelimit.h>
 #include "overlayfs.h"
 #define OVL_COPY_UP_CHUNK_SIZE (1 << 20)
+static bool __read_mostly ovl_check_copy_up;
+module_param_named(check_copy_up, ovl_check_copy_up, bool,
+		   S_IWUSR | S_IRUGO);
+		 "Warn on copy-up when causing process also has a R/O fd open");
+static int ovl_check_fd(const void *data, struct file *f, unsigned fd)
+	const struct dentry *dentry = data;
+	if (f->f_inode == d_inode(dentry))
+		pr_warn_ratelimited("overlayfs: Warning: Copying up %pD, but open R/O on fd %u which will cease to be coherent [pid=%d %s]\n",
+				    f, fd, current->pid, current->comm);
+	return 0;
+ * Check the fds open by this process and warn if something like the following
+ * scenario is about to occur:
+ *
+ *	fd1 = open("foo", O_RDONLY);
+ *	fd2 = open("foo", O_RDWR);
+ */
+static void ovl_do_check_copy_up(struct dentry *dentry)
+	if (ovl_check_copy_up)
+		iterate_fd(current->files, 0, ovl_check_fd, dentry);
 int ovl_copy_xattr(struct dentry *old, struct dentry *new)
 	ssize_t list_size, size;
@@ -302,6 +334,8 @@ int ovl_copy_up_one(struct dentry *parent, struct dentry *dentry,
 	if (WARN_ON(!workdir))
 		return -EROFS;
+	ovl_do_check_copy_up(lowerpath->dentry);
 	ovl_path_upper(parent, &parentpath);
 	upperdir = parentpath.dentry;

             reply	other threads:[~2015-07-07 14:04 UTC|newest]

Thread overview: 8+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2015-07-07 14:04 David Howells [this message]
2016-03-07 10:06 ` Miklos Szeredi
  -- strict thread matches above, loose matches on Subject: below --
2015-05-28 18:46 David Howells
2015-06-10 13:23 ` David Howells
2015-06-10 15:33 ` Colin Walters
2015-06-10 16:12 ` David Howells
2015-06-22 15:10 ` David Howells

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