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

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Date:   Fri, 28 Jun 2019 16:46:14 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     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>,
        Russell King - ARM Linux <linux@...linux.org.uk>
Subject: Re: [PATCH] f2fs: fix 32-bit linking

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.

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 {                                                            \