linux-kernel - Re: [PATCH] f2fs: fix 32-bit linking

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Message-ID: <20190628175835.hwzfrgrtwphi6kka@shell.armlinux.org.uk>
Date:   Fri, 28 Jun 2019 18:58:35 +0100
From:   Russell King - ARM Linux admin <linux@...linux.org.uk>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Christoph Hellwig <hch@...radead.org>,
        Jaegeuk Kim <jaegeuk@...nel.org>, Chao Yu <yuchao0@...wei.com>,
        Qiuyang Sun <sunqiuyang@...wei.com>,
        Sahitya Tummala <stummala@...eaurora.org>,
        Eric Biggers <ebiggers@...gle.com>,
        Wang Shilong <wangshilong1991@...il.com>,
        "Linux F2FS DEV, Mailing List" 
        <linux-f2fs-devel@...ts.sourceforge.net>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] f2fs: fix 32-bit linking

On Fri, Jun 28, 2019 at 04:46:14PM +0200, Arnd Bergmann wrote:
> On Fri, Jun 28, 2019 at 3:17 PM Christoph Hellwig <hch@...radead.org> wrote:
> >
> > On Fri, Jun 28, 2019 at 03:09:47PM +0200, Arnd Bergmann wrote:
> > > I came across this on arm-nommu (which disables
> > > CONFIG_CPU_SPECTRE) during randconfig testing.
> > >
> > > I don't see an easy way to add this in there, short of rewriting the
> > > whole __get_user_err() function. Any suggestions?
> >
> > Can't we just fall back to using copy_from_user with a little wrapper
> > that switches based on sizeof()?
> 
> I came up with something now. It's not pretty, but seems to satisfy the
> compiler. Not a proper patch yet, but let me know if you find a bug.

Have you checked what the behaviour is when "ptr" is a pointer to a
pointer?  I think you'll end up with a compiler warning for every
case, complaining about casting an unsigned long long to a pointer.

> 
> This might contain a double uaccess_save_and_enable/uaccess_restore,
> not sure how much we care about that.
> 
>      Arnd
> 
> index 7e0d2727c6b5..c21cdecadf26 100644
> --- a/arch/arm/include/asm/uaccess.h
> +++ b/arch/arm/include/asm/uaccess.h
> @@ -307,6 +307,7 @@ static inline void set_fs(mm_segment_t fs)
>  do {                                                                   \
>         unsigned long __gu_addr = (unsigned long)(ptr);                 \
>         unsigned long __gu_val;                                         \
> +       unsigned long long __gu_val8;                                   \
>         unsigned int __ua_flags;                                        \
>         __chk_user_ptr(ptr);                                            \
>         might_fault();                                                  \
> @@ -315,10 +316,13 @@ do {
>                          \
>         case 1: __get_user_asm_byte(__gu_val, __gu_addr, err);  break;  \
>         case 2: __get_user_asm_half(__gu_val, __gu_addr, err);  break;  \
>         case 4: __get_user_asm_word(__gu_val, __gu_addr, err);  break;  \
> +       case 8: __get_user_asm_dword(__gu_val8, __gu_addr, err);break;  \
>         default: (__gu_val) = __get_user_bad();                         \
>         }                                                               \
>         uaccess_restore(__ua_flags);                                    \
> -       (x) = (__typeof__(*(ptr)))__gu_val;                             \
> +       (x) = __builtin_choose_expr(sizeof(*(ptr)) == 8,                \
> +               (__typeof__(*(ptr)))__gu_val8,                          \
> +               (__typeof__(*(ptr)))__gu_val);                          \
>  } while (0)
> 
>  #define __get_user_asm(x, addr, err, instr)                    \
> @@ -373,6 +377,8 @@ do {
>                          \
>         __get_user_asm(x, addr, err, ldr)
>  #endif
> 
> +#define __get_user_asm_dword(x, addr, err)                     \
> +       do { err = raw_copy_from_user(&x, (void __user *)addr, 8) ?
> -EFAULT : 0; } while (0)
> 
>  #define __put_user_switch(x, ptr, __err, __fn)                         \
>         do {                                                            \
> 

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
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