Re: [RFC PATCH] vfs: don't check may_create_in_sticky if the file is already open/created

[Jeff Layton]

On Tue, 2022-07-26 at 16:23 -0400, Jeff Layton wrote:
> NFS server is exporting a sticky directory (mode 01777) with root
> squashing enabled. Client has protect_regular enabled and then tries to
> open a file as root in that directory. File is created (with ownership
> set to nobody:nobody) but the open syscall returns an error.
> 
> The problem is may_create_in_sticky, which rejects the open even though
> the file has already been created/opened. Only call may_create_in_sticky
> if the file hasn't already been opened or created.
> 
> Cc: Christian Brauner <brauner@xxxxxxxxxx>
> Link: https://bugzilla.redhat.com/show_bug.cgi?id=1976829
> Reported-by: Yongchen Yang <yoyang@xxxxxxxxxx>
> Signed-off-by: Jeff Layton <jlayton@xxxxxxxxxx>
> ---
>  fs/namei.c | 13 +++++++++----
>  1 file changed, 9 insertions(+), 4 deletions(-)
> 
> diff --git a/fs/namei.c b/fs/namei.c
> index 1f28d3f463c3..7480b6dc8d27 100644
> --- a/fs/namei.c
> +++ b/fs/namei.c
> @@ -3495,10 +3495,15 @@ static int do_open(struct nameidata *nd,
>  			return -EEXIST;
>  		if (d_is_dir(nd->path.dentry))
>  			return -EISDIR;
> -		error = may_create_in_sticky(mnt_userns, nd,
> -					     d_backing_inode(nd->path.dentry));
> -		if (unlikely(error))
> -			return error;
> +		if (!(file->f_mode & (FMODE_OPENED | FMODE_CREATED))) {
> +			error = may_create_in_sticky(mnt_userns, nd,
> +						d_backing_inode(nd->path.dentry));
> +			if (unlikely(error)) {
> +				printk("%s: f_mode=0x%x oflag=0x%x\n",
> +					__func__, file->f_mode, open_flag);
> +				return error;
> +			}
> +		}
>  	}
>  	if ((nd->flags & LOOKUP_DIRECTORY) && !d_can_lookup(nd->path.dentry))
>  		return -ENOTDIR;

I'm pretty sure this patch is the wrong approach, actually, since it
doesn't fix the regular (non-atomic) open codepath. Any thoughts on what
the right fix might be?
-- 
Jeff Layton <jlayton@xxxxxxxxxx>