ovl: get_write_access() in truncate

When truncating a file we should check write access on the underlying
inode.  And we should do so on the lower file as well (before copy-up) for
consistency.

Original patch and test case by Aihua Zhang.

 - - >o >o - - test.c - - >o >o - -
#include <stdio.h>
#include <errno.h>
#include <unistd.h>

int main(int argc, char *argv[])
{
	int ret;

	ret = truncate(argv[0], 4096);
	if (ret != -1) {
		fprintf(stderr, "truncate(argv[0]) should have failed\n");
		return 1;
	}
	if (errno != ETXTBSY) {
		perror("truncate(argv[0])");
		return 1;
	}

	return 0;
}
 - - >o >o - - >o >o - - >o >o - -

Reported-by: Aihua Zhang <zhangaihua1@huawei.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Cc: <stable@vger.kernel.org>

1 files changed, 21 insertions, 0 deletions

diff --git a/fs/overlayfs/inode.c b/fs/overlayfs/inode.c
index 8514d692042b..c831c2e5f803 100644
--- a/fs/overlayfs/inode.c
+++ b/fs/overlayfs/inode.c
@@ -59,16 +59,37 @@ int ovl_setattr(struct dentry *dentry, struct iattr *attr)
         if (err)
                 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;
+        }
+
         err = ovl_copy_up(dentry);
         if (!err) {
+                struct inode *winode = NULL;
+
                 upperdentry = ovl_dentry_upper(dentry);
 
+                if (attr->ia_valid & ATTR_SIZE) {
+                        winode = d_inode(upperdentry);
+                        err = get_write_access(winode);
+                        if (err)
+                                goto out_drop_write;
+                }
+
                 inode_lock(upperdentry->d_inode);
                 err = notify_change(upperdentry, attr, NULL);
                 if (!err)
                         ovl_copyattr(upperdentry->d_inode, dentry->d_inode);
                 inode_unlock(upperdentry->d_inode);
+
+                if (winode)
+                        put_write_access(winode);
         }
+out_drop_write:
         ovl_drop_write(dentry);
 out:
         return err;