On Mon, Apr 23, 2018 at 11:19:41PM -0700, Omar Sandoval wrote:
> From: Omar Sandoval <osandov@xxxxxx>
>
> One of the most common uses of temporary files is the classic atomic
> replacement pattern, i.e.,
>
> - write temporary file
> - fsync temporary file
> - rename temporary file over real file
> - fsync parent directory
>
> Now, we have O_TMPFILE, which gives us a much better way to create
> temporary files, but it's not possible to use it for this pattern.
>
> This patch introduces an AT_REPLACE flag which allows linkat() to
> replace the target file. Now, the temporary file in the pattern above
> can be a proper O_TMPFILE. Even without O_TMPFILE, this is a new
> primitive which might be useful in other contexts.
>
> The implementation on the VFS side mimics sys_renameat2().
>
> Cc: Xi Wang <xi@xxxxxxxxxxxxxxxxx>
> Signed-off-by: Omar Sandoval <osandov@xxxxxx>
> ---
> fs/ecryptfs/inode.c | 2 +-
> fs/namei.c | 181 +++++++++++++++++++++++++++++--------
> fs/nfsd/vfs.c | 2 +-
> fs/overlayfs/overlayfs.h | 2 +-
> include/linux/fs.h | 3 +-
> include/uapi/linux/fcntl.h | 1 +
> 6 files changed, 150 insertions(+), 41 deletions(-)
>
> diff --git a/fs/namei.c b/fs/namei.c
> index 186bd2464fd5..2cc2b1deaa12 100644
> --- a/fs/namei.c
> +++ b/fs/namei.c
> @@ -4149,6 +4149,7 @@ SYSCALL_DEFINE2(symlink, const char __user *, oldname, const char __user *, newn
> * @dir: new parent
> * @new_dentry: where to create the new link
> * @delegated_inode: returns inode needing a delegation break
> + * @flags: link flags
> *
> * The caller must hold dir->i_mutex
> *
> @@ -4162,16 +4163,26 @@ SYSCALL_DEFINE2(symlink, const char __user *, oldname, const char __user *, newn
> * be appropriate for callers that expect the underlying filesystem not
> * to be NFS exported.
> */
> -int vfs_link(struct dentry *old_dentry, struct inode *dir, struct dentry *new_dentry, struct inode **delegated_inode)
> +int vfs_link(struct dentry *old_dentry, struct inode *dir,
> + struct dentry *new_dentry, struct inode **delegated_inode,
> + unsigned int flags)
> {
> struct inode *inode = old_dentry->d_inode;
> + struct inode *target = new_dentry->d_inode;
> unsigned max_links = dir->i_sb->s_max_links;
> int error;
>
> if (!inode)
> return -ENOENT;
>
> - error = may_create(dir, new_dentry);
> + if (target) {
> + if (flags & AT_REPLACE)
This needs an
if (inode == target)
return 0;
I'll fix this in v4.
> + error = may_delete(dir, new_dentry, d_is_dir(old_dentry));
> + else
> + error = -EEXIST;
> + } else {
> + error = may_create(dir, new_dentry);
> + }
> if (error)
> return error;
>
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