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Date:   Wed, 8 Nov 2023 17:23:43 +0100
From:   Christian Brauner <brauner@kernel.org>
To:     Jonathan Corbet <corbet@lwn.net>
Cc:     Miklos Szeredi <mszeredi@redhat.com>,
        linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-api@vger.kernel.org, linux-man@vger.kernel.org,
        linux-security-module@vger.kernel.org, Karel Zak <kzak@redhat.com>,
        Ian Kent <raven@themaw.net>,
        David Howells <dhowells@redhat.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Christian Brauner <christian@brauner.io>,
        Amir Goldstein <amir73il@gmail.com>,
        Matthew House <mattlloydhouse@gmail.com>,
        Florian Weimer <fweimer@redhat.com>,
        Arnd Bergmann <arnd@arndb.de>
Subject: Re: [PATCH v4 5/6] add listmount(2) syscall
Message-ID: <20231108-grinsen-entfachen-f04195b92e7d@brauner>
References: <20231025140205.3586473-1-mszeredi@redhat.com>
 <20231025140205.3586473-6-mszeredi@redhat.com>
 <87il6d1cmu.fsf@meer.lwn.net>
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On Wed, Nov 08, 2023 at 09:20:28AM -0700, Jonathan Corbet wrote:
> Christian Brauner <brauner@kernel.org> writes:
> 
> >> Why use struct __mount_arg (or struct mnt_id_req :) here rather than
> >> just passing in the mount ID directly?  You don't use the request_mask
> >
> > Please see Arnd's detailed summary here:
> > https://lore.kernel.org/lkml/44631c05-6b8a-42dc-b37e-df6776baa5d4@app.fastmail.com
> 
> Ah, makes sense, I'd missed that.
> 
> Given this, though, it seems like maybe sys_listmount() should enforce
> that req->request_mask==0 ?

Good catch, it does now:

diff --git a/fs/namespace.c b/fs/namespace.c
index ae09321b0f72..21edddd75532 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -5030,6 +5030,8 @@ SYSCALL_DEFINE4(listmount, const struct mnt_id_req __user *, req,

        if (copy_from_user(&kreq, req, sizeof(kreq)))
                return -EFAULT;
+       if (kreq.request_mask != 0)
+               return -EINVAL;
        mnt_id = kreq.mnt_id;

        down_read(&namespace_sem);