[PATCH 17/27] r-o-bind-mounts-elevate-write-count-opend-files

Dave Hansen
2007-11-01T23:14:39Z

This is an old patch combined with a couple other ones I've
been working on. It should the issues that Miklos has
spotted.

--

This is the first really tricky patch in the series. It
elevates the writer count on a mount each time a non-special
file is opened for write.

We used to do this in may_open(), but Miklos pointed out
that __dentry_open() is used as well to create filps. This
will cover even those cases, while a call in may_open()
would not have.

There is also an elevated count around the vfs_create() call
in open_namei(). See the comments for more details, but we
need this to fix a 'create, remount, fail r/w open()' race.

Some filesystems forego the use of normal vfs calls to create
struct files. Make sure that these users elevate the mnt
writer count because they will get __fput(), and we need
to make sure they're balanced.

Signed-off-by: Dave Hansen <haveblue@us.ibm.com>
---

linux-2.6.git-dave/fs/file_table.c | 16 +++++++-
linux-2.6.git-dave/fs/namei.c | 74 ++++++++++++++++++++++++++++++++-----
linux-2.6.git-dave/fs/open.c | 36 +++++++++++++++++-
linux-2.6.git-dave/ipc/mqueue.c | 3 +
4 files changed, 116 insertions(+), 13 deletions(-)

diff -puN fs/file_table.c~r-o-bind-mounts-elevate-write-count-opend-files fs/file_table.c
--- linux-2.6.git/fs/file_table.c~r-o-bind-mounts-elevate-write-count-opend-files 2007-11-01 14:46:16.000000000 -0700
+++ linux-2.6.git-dave/fs/file_table.c 2007-11-01 14:46:16.000000000 -0700
@@ -193,6 +193,17 @@ int init_file(struct file *file, struct
file->f_mapping = dentry->d_inode->i_mapping;
file->f_mode = mode;
file->f_op = fop;
+
+ /*
+ * These mounts don't really matter in practice
+ * for r/o bind mounts. They aren't userspace-
+ * visible. We do this for consistency, and so
+ * that we can do debugging checks at __fput()e
+ */
+ if ((mode & FMODE_WRITE) && !special_file(dentry->d_inode->i_mode)) {
+ error = mnt_want_write(mnt);
+ WARN_ON(error);
+ }
return error;
}
EXPORT_SYMBOL(init_file);
@@ -230,8 +241,11 @@ void fastcall __fput(struct file *file)
if (unlikely(S_ISCHR(inode->i_mode) && inode->i_cdev != NULL))
cdev_put(inode->i_cdev);
fops_put(file->f_op);
- if (file->f_mode & FMODE_WRITE)
+ if (file->f_mode & FMODE_WRITE) {
put_write_access(inode);
+ if (!special_file(inode->i_mode))
+ mnt_drop_write(mnt);
+ }
put_pid(file->f_owner.pid);
file_kill(file);
file->f_path.dentry = NULL;
diff -puN fs/namei.c~r-o-bind-mounts-elevate-write-count-opend-files fs/namei.c
--- linux-2.6.git/fs/namei.c~r-o-bind-mounts-elevate-write-count-opend-files 2007-11-01 14:46:16.000000000 -0700
+++ linux-2.6.git-dave/fs/namei.c 2007-11-01 14:46:16.000000000 -0700
@@ -1620,12 +1620,12 @@ int may_open(struct nameidata *nd, int a
return -EACCES;

flag &= ~O_TRUNC;
- } else if (IS_RDONLY(inode) && (flag & FMODE_WRITE))
- return -EROFS;
+ }

error = vfs_permission(nd, acc_mode);
if (error)
return error;
+
/*
* An append-only file must be opened in append mode for writing.
*/
@@ -1721,18 +1721,31 @@ static inline int sys_open_flags_to_name
return flag;
}

+static inline int open_will_write_to_fs(int flag, struct inode *inode)
+{
+ /*
+ * We'll never write to the fs underlying
+ * a device file.
+ */
+ if (special_file(inode->i_mode))
+ return 0;
+ return (flag & O_TRUNC);
+}
+
/*
* open_pathname()
*
* namei for open - this is in fact almost the whole open-routine.
*
- * Note that the low bits of "flag" aren't the same as in the open
- * system call. See sys_open_flags_to_namei_flags().
+ * Note that the low bits of the passed in "sys_open_flag"
+ * are not the same as in the local variable "flag". See
+ * sys_open_flags_to_namei_flags() for more details.
* SMP-safe
*/
struct file *open_pathname(int dfd, const char *pathname,
int sys_open_flag, int mode)
{
+ struct file *filp;
struct nameidata nd;
int acc_mode, error;
struct path path;
@@ -1792,17 +1805,30 @@ do_last:
}

if (IS_ERR(nd.intent.open.file)) {
- mutex_unlock(&dir->d_inode->i_mutex);
error = PTR_ERR(nd.intent.open.file);
- goto exit_dput;
+ goto exit_mutex_unlock;
}

/* Negative dentry, just create the file */
if (!path.dentry->d_inode) {
- error = __open_namei_create(&nd, &path, flag, mode);
+ /*
+ * This write is needed to ensure that a
+ * ro->rw transition does not occur between
+ * the time when the file is created and when
+ * a permanent write count is taken through
+ * the 'struct file' in nameidata_to_filp().
+ */
+ error = mnt_want_write(nd.mnt);
if (error)
+ goto exit_mutex_unlock;
+ error = __open_namei_create(&nd, &path, flag, mode);
+ if (error) {
+ mnt_drop_write(nd.mnt);
goto exit;
- return nameidata_to_filp(&nd, sys_open_flag);
+ }
+ filp = nameidata_to_filp(&nd, sys_open_flag);
+ mnt_drop_write(nd.mnt);
+ return filp;
}

/*
@@ -1832,11 +1858,39 @@ do_last:
if (path.dentry->d_inode && S_ISDIR(path.dentry->d_inode->i_mode))
goto exit;
ok:
+ /*
+ * Consider:
+ * 1. may_open() truncates a file
+ * 2. a rw->ro mount transition occurs
+ * 3. nameidata_to_filp() fails due to
+ * the ro mount.
+ * That would be inconsistent, and should
+ * be avoided. Taking this mnt write here
+ * ensures that (2) can not occur.
+ */
+ if (open_will_write_to_fs(flag, nd.dentry->d_inode)) {
+ error = mnt_want_write(nd.mnt);
+ if (error)
+ goto exit;
+ }
error = may_open(&nd, acc_mode, flag);
- if (error)
+ if (error) {
+ if (open_will_write_to_fs(flag, nd.dentry->d_inode))
+ mnt_drop_write(nd.mnt);
goto exit;
- return nameidata_to_filp(&nd, sys_open_flag);
+ }
+ filp = nameidata_to_filp(&nd, sys_open_flag);
+ /*
+ * It is now safe to drop the mnt write
+ * because the filp has had a write taken
+ * on its behalf.
+ */
+ if (open_will_write_to_fs(flag, nd.dentry->d_inode))
+ mnt_drop_write(nd.mnt);
+ return filp;

+exit_mutex_unlock:
+ mutex_unlock(&dir->d_inode->i_mutex);
exit_dput:
dput_path(&path, &nd);
exit:
diff -puN fs/open.c~r-o-bind-mounts-elevate-write-count-opend-files fs/open.c
--- linux-2.6.git/fs/open.c~r-o-bind-mounts-elevate-write-count-opend-files 2007-11-01 14:46:16.000000000 -0700
+++ linux-2.6.git-dave/fs/open.c 2007-11-01 14:46:16.000000000 -0700
@@ -734,6 +734,35 @@ out:
return error;
}

+/*
+ * You have to be very careful that these write
+ * counts get cleaned up in error cases and
+ * upon __fput(). This should probably never
+ * be called outside of __dentry_open().
+ */
+static inline int __get_file_write_access(struct inode *inode,
+ struct vfsmount *mnt)
+{
+ int error;
+ error = get_write_access(inode);
+ if (error)
+ return error;
+ /*
+ * Do not take mount writer counts on
+ * special files since no writes to
+ * the mount itself will occur.
+ */
+ if (!special_file(inode->i_mode)) {
+ /*
+ * Balanced in __fput()
+ */
+ error = mnt_want_write(mnt);
+ if (error)
+ put_write_access(inode);
+ }
+ return error;
+}
+
static struct file *__dentry_open(struct dentry *dentry, struct vfsmount *mnt,
int flags, struct file *f,
int (*open)(struct inode *, struct file *))
@@ -746,7 +775,7 @@ static struct file *__dentry_open(struct
FMODE_PREAD | FMODE_PWRITE;
inode = dentry->d_inode;
if (f->f_mode & FMODE_WRITE) {
- error = get_write_access(inode);
+ error = __get_file_write_access(inode, mnt);
if (error)
goto cleanup_file;
}
@@ -788,8 +817,11 @@ static struct file *__dentry_open(struct

cleanup_all:
fops_put(f->f_op);
- if (f->f_mode & FMODE_WRITE)
+ if (f->f_mode & FMODE_WRITE) {
put_write_access(inode);
+ if (!special_file(inode->i_mode))
+ mnt_drop_write(mnt);
+ }
file_kill(f);
f->f_path.dentry = NULL;
f->f_path.mnt = NULL;
diff -puN ipc/mqueue.c~r-o-bind-mounts-elevate-write-count-opend-files ipc/mqueue.c
--- linux-2.6.git/ipc/mqueue.c~r-o-bind-mounts-elevate-write-count-opend-files 2007-11-01 14:46:16.000000000 -0700
+++ linux-2.6.git-dave/ipc/mqueue.c 2007-11-01 14:46:16.000000000 -0700
@@ -682,6 +682,9 @@ asmlinkage long sys_mq_open(const char _
goto out;
filp = do_open(dentry, oflag);
} else {
+ error = mnt_want_write(mqueue_mnt);
+ if (error)
+ goto out;
filp = do_create(mqueue_mnt->mnt_root, dentry,
oflag, mode, u_attr);
}
_