linux-kernel - Re: Is ->sendmsg() allowed to change the msghdr struct it is given?

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Message-ID: <1f4271105ac5be66e5130d487464680fc65bacc8.camel@redhat.com>
Date:   Tue, 27 Jun 2023 14:54:02 +0200
From:   Paolo Abeni <pabeni@...hat.com>
To:     Jakub Kicinski <kuba@...nel.org>,
        David Howells <dhowells@...hat.com>
Cc:     Ilya Dryomov <idryomov@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>, ceph-devel@...r.kernel.org,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: Is ->sendmsg() allowed to change the msghdr struct it is given?

On Tue, 2023-06-27 at 14:51 +0200, Paolo Abeni wrote:
> On Mon, 2023-06-26 at 14:22 -0700, Jakub Kicinski wrote:
> > On Mon, 26 Jun 2023 22:14:41 +0100 David Howells wrote:
> > > Do you know if ->sendmsg() might alter the msghdr struct it is passed as an
> > > argument? Certainly it can alter msg_iter, but can it also modify,
> > > say, msg_flags?
> > 
> > I'm not aware of a precedent either way.
> > Eric or Paolo would know better than me, tho.
> 
> udp_sendmsg() can set the MSG_TRUNC bit in msg->msg_flags, so I guess
> that kind of actions are sort of allowed.

Sorry, ENOCOFFEE here. It's actually udp_recvmsg() updating msg-
>msg_flags.

>  Still, AFAICS, the kernel
> based msghdr is not copied back to the user-space, so such change
> should be almost a no-op in practice.

This part should be correct.

> @David: which would be the end goal for such action?

Sorry for the noisy reply,

Paolo