linux-kernel - Re: [PATCH v6 11/11] arm64: annotate user pointers casts detected by sparse

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Message-ID: <20180906230858.psedqdai3dw2cvvl@ltop.local>
Date:   Fri, 7 Sep 2018 01:08:59 +0200
From:   Luc Van Oostenryck <luc.vanoostenryck@...il.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Andrey Konovalov <andreyknvl@...gle.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Mark Rutland <mark.rutland@....com>,
        Robin Murphy <robin.murphy@....com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Kees Cook <keescook@...omium.org>,
        Kate Stewart <kstewart@...uxfoundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Ingo Molnar <mingo@...nel.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Shuah Khan <shuah@...nel.org>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Kostya Serebryany <kcc@...gle.com>, eugenis@...gle.com,
        Lee.Smith@....com,
        Ramana Radhakrishnan <Ramana.Radhakrishnan@....com>,
        Jacob.Bramley@....com, Ruben.Ayrapetyan@....com,
        cpandya@...eaurora.org
Subject: Re: [PATCH v6 11/11] arm64: annotate user pointers casts detected by
 sparse

On Thu, Sep 06, 2018 at 02:16:19PM -0700, Linus Torvalds wrote:
> On Thu, Sep 6, 2018 at 2:13 PM Linus Torvalds
> <torvalds@...ux-foundation.org> wrote:
> >
> > So for example:
> >
> > >  static inline compat_uptr_t ptr_to_compat(void __user *uptr)
> > >  {
> > > -       return (u32)(unsigned long)uptr;
> > > +       return (u32)(__force unsigned long)uptr;
> > >  }
> >
> > this actually looks correct.
> 
> Side note: I do think that while the above is correct, the rest of the
> patch shows that we might be better off simply not havign the warning
> for address space changes at all for the "cast a pointer to an integer
> type" case.
> 
> When you cast to a non-pointer type, the address space issue simply
> doesn't exist at all, so the warning makes less sense.
> 
> It's really just he "pointer to one address space" being cast to
> "pointer to another address space" that should really warn, and that
> might need that "__force" thing.
> 
> Hmm? So maybe a sparse change is better for most of that patch.

Unless I'm misunderstanding something, I don't think there is
anything to change for this specific point. Sparse don't warn
(by default) on "cast from pointer with address space to integer",
as it always been the case, I think. I think it's the good choice.

It's just that recently, I've added a new flag -Wcast-from-as [1],
defaulting to 'no', specifically to *detect* these cast because of
these tagged pointers.

Note: I tend to think more and more that __force is simply too
      strong and weaker form, like __force_as and __force_bitwise
      would be more appropriate.


-- Luc Van Oostenryck

[1] d96da358c ("stricter warning for explicit cast to ulong")