linux-kernel - Re: [PATCH v2 3/4] x86/fpu: Fix _

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Message-ID: <20190329194208.GE2217@ZenIV.linux.org.uk>
Date:   Fri, 29 Mar 2019 19:42:08 +0000
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Jann Horn <jannh@...gle.com>
Cc:     Luc Van Oostenryck <luc.vanoostenryck@...il.com>,
        linux-sparse@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        the arch/x86 maintainers <x86@...nel.org>,
        kernel list <linux-kernel@...r.kernel.org>,
        Qiaowei Ren <qiaowei.ren@...el.com>
Subject: Re: [PATCH v2 3/4] x86/fpu: Fix __user annotations

On Fri, Mar 29, 2019 at 08:25:25PM +0100, Jann Horn wrote:
> Oooh, it's a sparse bug.

It's *not* a bug.  

> Apparently it's significant that the user pointer is stored as a
> __u64, and __u64 is defined as unsigned long long.

Yes, it is.  Casts to uintptr_t (== unsigned long on all targets)
are OK; any other arithmetical type gives a warning, and quite
deliberately so.

Don't do it.  If you want to say "I'm converting it to integer,
all traces of its origin are gone", use an idiomatic cast.