Re: [PATCH v10 4/5] io_uring: add fsetxattr and setxattr support

From: Al Viro <viro@zeniv.linux.org.uk>
To: Jens Axboe <axboe@kernel.dk>
Cc: Christian Brauner <christian.brauner@ubuntu.com>,
	Stefan Roesch <shr@fb.com>,
	io-uring@vger.kernel.org, linux-fsdevel@vger.kernel.org,
	kernel-team@fb.com, torvalds@linux-foundation.org
Subject: Re: [PATCH v10 4/5] io_uring: add fsetxattr and setxattr support
Date: Thu, 30 Dec 2021 22:24:08 +0000	[thread overview]
Message-ID: <Yc4xiLus+Os4GWZf@zeniv-ca.linux.org.uk> (raw)
In-Reply-To: <5030f5fa-79c3-b3b7-857d-3ac62bf2b982@kernel.dk>

On Thu, Dec 30, 2021 at 11:09:12AM -0800, Jens Axboe wrote:

> For each command, there are two steps:
> 
> - The prep of it, this happens inline from the system call where the
>   request, or requests, are submitted. The prep phase should ensure that
>   argument structs are stable. Hence a caller can prep a request and
>   have memory on stack, as long as it submits before it becomes invalid.
>   An example of that are iovecs for readv/writev. The caller does not
>   need to have them stable for the duration of the request, just across
>   submit. That's the io_${cmd}_prep() helpers.
> 
> - The execution of it. May be separate from prep and from an async
>   worker. Where the lower layers don't support a nonblocking attempt,
>   they are always done async. The statx stuff is an example of that.
> 
> Hence prep needs to copy from userland on the prep side always for the
> statx family, as execution will happen out-of-line from the submission.
> 
> Does that explain it?

The actual call chain leading to filename_lookup() is, AFAICS, this:
	io_statx()
		do_statx()
			vfs_statx()
				user_path_at()
					user_path_at_empty()
						filename_lookup()

If you are providing such warranties for the contents of pathname
arguments, you have a bug in statx in the mainline.  If you are not,
there's no point in doing getname() in getxattr prep.

Which one it is?