linux-kernel - Re: [PATCH 05/10] lkdtm: disable set_fs-based tests for !CONFIG_SET

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Date:   Wed, 2 Sep 2020 10:09:11 +0200
From:   Christoph Hellwig <hch@....de>
To:     Kees Cook <keescook@...omium.org>
Cc:     Christoph Hellwig <hch@....de>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Michael Ellerman <mpe@...erman.id.au>,
        the arch/x86 maintainers <x86@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>
Subject: Re: [PATCH 05/10] lkdtm: disable set_fs-based tests for
 !CONFIG_SET_FS

On Tue, Sep 01, 2020 at 11:57:37AM -0700, Kees Cook wrote:
> On Sat, Aug 29, 2020 at 11:24:06AM +0200, Christoph Hellwig wrote:
> > On Thu, Aug 27, 2020 at 11:06:28AM -0700, Linus Torvalds wrote:
> > > On Thu, Aug 27, 2020 at 8:00 AM Christoph Hellwig <hch@....de> wrote:
> > > >
> > > > Once we can't manipulate the address limit, we also can't test what
> > > > happens when the manipulation is abused.
> > > 
> > > Just remove these tests entirely.
> > > 
> > > Once set_fs() doesn't exist on x86, the tests no longer make any sense
> > > what-so-ever, because test coverage will be basically zero.
> > > 
> > > So don't make the code uglier just to maintain a fiction that
> > > something is tested when it isn't really.
> > 
> > Sure fine with me unless Kees screams.
> 
> To clarify: if any of x86, arm64, arm, powerpc, riscv, and s390 are
> using set_fs(), I want to keep this test. "ugly" is fine in lkdtm. :)

And Linus wants them gone entirely, so I'll need a stage fight between
the two of you.  At least for this merge window I'm only planning on
x86 and power, plus maybe riscv if I get the work done in time.  Although
helper from the maintainers would be welcome.  s390 has a driver that
still uses set_fs that will need some surgery, although it shouldn't
be too bad, but arm will be a piece of work.  Unless I get help it will
take a while.