linux-kernel - Re: [PATCH v2 3/3] pidfs: implement file handle support

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Message-ID: <1128f3cd-38de-43a0-981e-ec1485ec9e3b@e43.eu>
Date: Thu, 14 Nov 2024 14:13:06 +0100
From: Erin Shepherd <erin.shepherd@....eu>
To: Christian Brauner <brauner@...nel.org>
Cc: Alexander Viro <viro@...iv.linux.org.uk>, Jan Kara <jack@...e.cz>,
 Chuck Lever <chuck.lever@...cle.com>, linux-fsdevel@...r.kernel.org,
 linux-kernel@...r.kernel.org, Jeff Layton <jlayton@...nel.org>,
 Amir Goldstein <amir73il@...il.com>, linux-nfs@...r.kernel.org
Subject: Re: [PATCH v2 3/3] pidfs: implement file handle support

On 14/11/2024 13:52, Christian Brauner wrote:

> I think you need at least something like the following completely
> untested draft on top:
>
> - the pidfs_finish_open_by_handle_at() is somewhat of a clutch to handle
>   thread vs thread-group pidfds but it works.
>
> - In contrast to pidfd_open() that uses dentry_open() to create a pidfd
>   open_by_handle_at() uses file_open_root(). That's overall fine but
>   makes pidfds subject to security hooks which they aren't via
>   pidfd_open(). It also necessitats pidfs_finish_open_by_handle_at().
>   There's probably other solutions I'm not currently seeing.

These two concerns combined with the special flag make me wonder if pidfs
is so much of a special snowflake we should just special case it up front
and skip all of the shared handle decode logic?

> - The exportfs_decode_fh_raw() call that's used to decode the pidfd is
>   passed vfs_dentry_acceptable() as acceptability callback. For pidfds
>   we don't need any of that functionality and we don't need any of the
>   disconnected dentry handling logic. So the easiest way to fix that is
>   to rely on EXPORT_OP_UNRESTRICTED_OPEN to skip everything. That in
>   turns means the only acceptability we have is the nop->fh_to_dentry()
>   callback for pidfs.

With the current logic we go exportfs_decode_fh_raw(...) ->
find_acceptable_alias(result) -> vfs_dentry_acceptable(context, result).


vfs_dentry_acceptable immediately returns 1 if ctx->flags is 0, which will
always be the case if EXPORT_OP_UNRESTRICTED_OPEN was set, so we immediately
fall back out of the call tree and return result.

So I'm not 100% sure we actually need this special case but I'm not opposed.

> - This all really needs rigorous selftests before we can even think of
>   merging any of this.