linux-kernel - Re: [PATCH] seccomp: remove unused arg from secure

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Date:   Tue, 24 Sep 2019 08:30:36 +0200
From:   Christian Brauner <christian.brauner@...ntu.com>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Borislav Petkov <bp@...en8.de>,
        LKML <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Kees Cook <keescook@...omium.org>,
        Will Drewry <wad@...omium.org>,
        Oleg Nesterov <oleg@...hat.com>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        Parisc List <linux-parisc@...r.kernel.org>,
        linux-s390 <linux-s390@...r.kernel.org>,
        linux-um@...ts.infradead.org, X86 ML <x86@...nel.org>
Subject: Re: [PATCH] seccomp: remove unused arg from secure_computing()

On Mon, Sep 23, 2019 at 11:41:59AM -0700, Andy Lutomirski wrote:
> On Mon, Sep 23, 2019 at 2:49 AM Borislav Petkov <bp@...en8.de> wrote:
> >
> > On Fri, Sep 20, 2019 at 03:19:09PM +0200, Christian Brauner wrote:
> > > While touching seccomp code I realized that the struct seccomp_data
> > > argument to secure_computing() seems to be unused by all current
> > > callers. So let's remove it unless there is some subtlety I missed.
> > > Note, I only tested this on x86.
> >
> > What was amluto thinking in
> >
> > 2f275de5d1ed ("seccomp: Add a seccomp_data parameter secure_computing()")
> 
> IIRC there was a period of time in which x86 used secure_computing()
> for normal syscalls, and it was a good deal faster to have the arch
> code supply seccomp_data.  x86 no longer works like this, and syscalls
> aren't fast anymore ayway :(

I started looking at this and actually had a slightly bigger cleanup in
mind. It seems odd that we have secure_computing() and
__secure_computing(). Especially in the mips and x86 case. From what I
can tell they could both rely on secure_computing() and don't need
__secure_computing().
If I can make those changes, we can make __secure_computing() static and
have only a single function secure_computing() that is used by all
arches which would make this code simpler.
Apparenly mips once switched from secure_computing() to
__secure_computing() because of bpf and tracepoints. The last change to
this was:

commit 3d729deaf287c43e415c5d791c9ac8414dbeff70
Author: James Hogan <jhogan@...nel.org>
Date:   Fri Aug 11 21:56:50 2017 +0100

    MIPS: seccomp: Fix indirect syscall args

which references a broken samples/bpf/tracex5 test. But in the thread to
this last change Kees and others were less than sure that this makes
sense. So I'm not sure. Maybe I should just try and send it out...

Christian