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Date:   Sun, 17 May 2020 09:02:15 -0600
From:   Tycho Andersen <tycho@...ho.ws>
To:     Christian Brauner <christian.brauner@...ntu.com>
Cc:     Aleksa Sarai <asarai@...e.de>, Kees Cook <keescook@...omium.org>,
        linux-api@...r.kernel.org, containers@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] seccomp: Add group_leader pid to seccomp_notif

On Sun, May 17, 2020 at 08:46:03AM -0600, Tycho Andersen wrote:
> On Sun, May 17, 2020 at 04:33:11PM +0200, Christian Brauner wrote:
> > struct seccomp_notif2 {
> > 	__u32 notif_size;
> > 	__u64 id;
> > 	__u32 pid;
> > 	__u32 flags;
> > 	struct seccomp_data data;
> > 	__u32 data_size;
> > };
> 
> I guess you need to put data_size before data, otherwise old userspace
> with a smaller struct seccomp_data will look in the wrong place.
> 
> But yes, that'll work if you put two sizes in, which is probably
> reasonable since we're talking about two structs.

Well, no, it doesn't either. Suppose we add a new field first to
struct seccomp_notif2:

struct seccomp_notif2 {
    __u32 notif_size;
    __u64 id;
    __u32 pid;
    __u32 flags;
    struct seccomp_data data;
    __u32 data_size;
    __u32 new_field;
};

And next we add a new field to struct secccomp_data. When a userspace
compiled with just the new seccomp_notif2 field does:

seccomp_notif2.new_field = ...;

the compiler will put it in the wrong place for the kernel with the
new seccomp_data field too.

Sort of feels like we should do:

struct seccomp_notif2 {
    struct seccomp_notif *notif;
    struct seccomp_data *data;
};

?

Tycho

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