Commit 4a3fd211 authored by Dave Hansen's avatar Dave Hansen Committed by Al Viro

[PATCH] r/o bind mounts: elevate write count for open()s

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.
Acked-by: default avatarAl Viro <viro@ZenIV.linux.org.uk>
Signed-off-by: default avatarChristoph Hellwig <hch@lst.de>
Signed-off-by: default avatarDave Hansen <haveblue@us.ibm.com>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarAl Viro <viro@zeniv.linux.org.uk>
parent 42a74f20
......@@ -199,6 +199,17 @@ int init_file(struct file *file, struct vfsmount *mnt, struct dentry *dentry,
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()
*/
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);
......@@ -221,10 +232,13 @@ EXPORT_SYMBOL(fput);
*/
void drop_file_write_access(struct file *file)
{
struct vfsmount *mnt = file->f_path.mnt;
struct dentry *dentry = file->f_path.dentry;
struct inode *inode = dentry->d_inode;
put_write_access(inode);
if (!special_file(inode->i_mode))
mnt_drop_write(mnt);
}
EXPORT_SYMBOL_GPL(drop_file_write_access);
......
......@@ -1623,8 +1623,7 @@ int may_open(struct nameidata *nd, int acc_mode, int flag)
return -EACCES;
flag &= ~O_TRUNC;
} else if (IS_RDONLY(inode) && (acc_mode & MAY_WRITE))
return -EROFS;
}
error = vfs_permission(nd, acc_mode);
if (error)
......@@ -1724,18 +1723,32 @@ static inline int open_to_namei_flags(int flag)
return flag;
}
static 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);
}
/*
* Note that the low bits of "flag" aren't the same as in the open
* system call. See open_to_namei_flags().
* Note that the low bits of the passed in "open_flag"
* are not the same as in the local variable "flag". See
* open_to_namei_flags() for more details.
*/
struct file *do_filp_open(int dfd, const char *pathname,
int open_flag, int mode)
{
struct file *filp;
struct nameidata nd;
int acc_mode, error;
struct path path;
struct dentry *dir;
int count = 0;
int will_write;
int flag = open_to_namei_flags(open_flag);
acc_mode = ACC_MODE(flag);
......@@ -1791,17 +1804,30 @@ 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.path.mnt);
if (error)
goto exit_mutex_unlock;
error = __open_namei_create(&nd, &path, flag, mode);
if (error) {
mnt_drop_write(nd.path.mnt);
goto exit;
return nameidata_to_filp(&nd, open_flag);
}
filp = nameidata_to_filp(&nd, open_flag);
mnt_drop_write(nd.path.mnt);
return filp;
}
/*
......@@ -1831,11 +1857,40 @@ 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.
*/
will_write = open_will_write_to_fs(flag, nd.path.dentry->d_inode);
if (will_write) {
error = mnt_want_write(nd.path.mnt);
if (error)
goto exit;
}
error = may_open(&nd, acc_mode, flag);
if (error)
if (error) {
if (will_write)
mnt_drop_write(nd.path.mnt);
goto exit;
return nameidata_to_filp(&nd, open_flag);
}
filp = nameidata_to_filp(&nd, open_flag);
/*
* It is now safe to drop the mnt write
* because the filp has had a write taken
* on its behalf.
*/
if (will_write)
mnt_drop_write(nd.path.mnt);
return filp;
exit_mutex_unlock:
mutex_unlock(&dir->d_inode->i_mutex);
exit_dput:
path_put_conditional(&path, &nd);
exit:
......
......@@ -730,6 +730,35 @@ 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 *))
......@@ -742,7 +771,7 @@ static struct file *__dentry_open(struct dentry *dentry, struct vfsmount *mnt,
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;
}
......@@ -784,8 +813,11 @@ static struct file *__dentry_open(struct dentry *dentry, struct vfsmount *mnt,
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;
......
......@@ -598,6 +598,7 @@ static struct file *do_create(struct dentry *dir, struct dentry *dentry,
int oflag, mode_t mode, struct mq_attr __user *u_attr)
{
struct mq_attr attr;
struct file *result;
int ret;
if (u_attr) {
......@@ -612,13 +613,24 @@ static struct file *do_create(struct dentry *dir, struct dentry *dentry,
}
mode &= ~current->fs->umask;
ret = mnt_want_write(mqueue_mnt);
if (ret)
goto out;
ret = vfs_create(dir->d_inode, dentry, mode, NULL);
dentry->d_fsdata = NULL;
if (ret)
goto out;
goto out_drop_write;
return dentry_open(dentry, mqueue_mnt, oflag);
result = dentry_open(dentry, mqueue_mnt, oflag);
/*
* dentry_open() took a persistent mnt_want_write(),
* so we can now drop this one.
*/
mnt_drop_write(mqueue_mnt);
return result;
out_drop_write:
mnt_drop_write(mqueue_mnt);
out:
dput(dentry);
mntput(mqueue_mnt);
......
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