linux-kernel - Re: [PATCH V9 21/21] csky: support dword access for get_user

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Message-ID: <CAK8P3a0cFhBE1tX0xisP__MYiBfaAUQHsW6ZhkmTPocRf1Zb6w@mail.gmail.com>
Date:   Thu, 18 Oct 2018 10:34:00 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Guo Ren <ren_guo@...ky.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        David Miller <davem@...emloft.net>,
        gregkh <gregkh@...uxfoundation.org>,
        Christoph Hellwig <hch@...radead.org>,
        Marc Zyngier <marc.zyngier@....com>,
        Mark Rutland <mark.rutland@....com>,
        Peter Zijlstra <peterz@...radead.org>,
        Rob Herring <robh@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        DTML <devicetree@...r.kernel.org>,
        Rob Herring <robh+dt@...nel.org>, c-sky_gcc_upstream@...ky.com
Subject: Re: [PATCH V9 21/21] csky: support dword access for get_user_size()

On Thu, Oct 18, 2018 at 5:41 AM Guo Ren <ren_guo@...ky.com> wrote:
>
> On Wed, Oct 17, 2018 at 05:44:17PM +0200, Arnd Bergmann wrote:
> > On Tue, Oct 16, 2018 at 5:33 AM Guo Ren <ren_guo@...ky.com> wrote:
> > >
> > > Support dword access for get_user_size and redesign put_user_size with
> > > the same style of get_user_size. It's Ok to use xxx_user_asm_common for
> > > all size of variable with ldb, ldh, ldw, ld.d
> > >
> > > ld.d rx, (addr, 0) could "rx <= addr" "and r(x+1) <= addr+4" and this also
> > > follow abiv2 gcc ABI for dword access.
> >
> > Are you sure this is correct for this?
> >
> > static inline u32 get_64_to_32(__u64 __user *p)
> > {
> >       u32 ret;
> >       get_user(ret, p);
> >       return ret;
> > }
> >
> > If I read __get_user_asm_common() right, the ld.d would overwrite
> > two registers, but the caller only expects one, so it clobbers one
> > that might be in use.
> Ah... BUG! I only consider the get_user(u64, u64 *) :P
>
> Change to:
> case 8:                                                 \
> __get_user_asm_dword((x), ptr, "ld.d", retval);         \
> break;
>
> #define __get_user_asm_dword(x, ptr, err)               \
> do {                                                    \
>         u64 tmp;                                        \
>         __get_user_asm_common(tmp, ptr, "ld.d", err);   \
>         x = typeof(x) tmp;                              \
> } while(0)
>
> #define __put_user_asm_dword(x, ptr, err)               \
> do {                                                    \
>         u64 tmp = (u64) x;                              \
>         __put_user_asm_common(tmp, ptr, "st.d", err);   \
> } while(0)
>

I think this will cause warnings for code that passes a pointer.

The 64-bit __get_user() is really hard, and most 32-bit architectures don't
implement it at all. If you really want to add it, have a look at what
x86 and arm do. IIRC they both use __builtin_choose_expr(),

       Arnd