Re: [PATCH bpf-next v2 1/3] libfs: support RENAME_EXCHANGE in simple_rename()

[Daniel Borkmann <daniel@iogearbox.net>]

[ Adding Miklos & Greg to Cc for review given e0e0be8a8355 ("libfs: support RENAME_NOREPLACE in
   simple_rename()"). If you have a chance, would be great if you could take a look, thanks! ]

On 10/21/21 5:15 PM, Lorenz Bauer wrote:
> Allow atomic exchange via RENAME_EXCHANGE when using simple_rename.
> This affects binderfs, ramfs, hubetlbfs and bpffs. There isn't much
> to do except update the various *time fields.
> 
> Signed-off-by: Lorenz Bauer <lmb@cloudflare.com>
> ---
>   fs/libfs.c | 6 +++++-
>   1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/libfs.c b/fs/libfs.c
> index 51b4de3b3447..93c03d593749 100644
> --- a/fs/libfs.c
> +++ b/fs/libfs.c
> @@ -455,9 +455,12 @@ int simple_rename(struct user_namespace *mnt_userns, struct inode *old_dir,
>   	struct inode *inode = d_inode(old_dentry);
>   	int they_are_dirs = d_is_dir(old_dentry);
>   
> -	if (flags & ~RENAME_NOREPLACE)
> +	if (flags & ~(RENAME_NOREPLACE | RENAME_EXCHANGE))
>   		return -EINVAL;
>   
> +	if (flags & RENAME_EXCHANGE)
> +		goto done;
> +
>   	if (!simple_empty(new_dentry))
>   		return -ENOTEMPTY;
>   
> @@ -472,6 +475,7 @@ int simple_rename(struct user_namespace *mnt_userns, struct inode *old_dir,
>   		inc_nlink(new_dir);
>   	}
>   
> +done:
>   	old_dir->i_ctime = old_dir->i_mtime = new_dir->i_ctime =
>   		new_dir->i_mtime = inode->i_ctime = current_time(old_dir);
>   
>