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    Subject[RFC][PATCH v2 4/5] x86/uaccess: Implement unsafe_try_cmpxchg_user()
    Do try_cmpxchg() loops on userspace addresses.

    Cc: Sean Christopherson <seanjc@google.com>
    Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
    ---
    arch/x86/include/asm/uaccess.h | 67 +++++++++++++++++++++++++++++++++++++++++
    1 file changed, 67 insertions(+)

    --- a/arch/x86/include/asm/uaccess.h
    +++ b/arch/x86/include/asm/uaccess.h
    @@ -342,6 +342,24 @@ do { \
    : [umem] "m" (__m(addr)) \
    : : label)

    +#define __try_cmpxchg_user_asm(itype, ltype, _ptr, _pold, _new, label) ({ \
    + bool success; \
    + __typeof__(_ptr) _old = (__typeof__(_ptr))(_pold); \
    + __typeof__(*(_ptr)) __old = *_old; \
    + __typeof__(*(_ptr)) __new = (_new); \
    + asm_volatile_goto("\n" \
    + "1: " LOCK_PREFIX "cmpxchg"itype" %[new], %[ptr]\n"\
    + _ASM_EXTABLE_UA(1b, %l[label]) \
    + : CC_OUT(z) (success), \
    + [ptr] "+m" (*_ptr), \
    + [old] "+a" (__old) \
    + : [new] ltype (__new) \
    + : "memory", "cc" \
    + : label); \
    + if (unlikely(!success)) \
    + *_old = __old; \
    + likely(success); })
    +
    #else // !CONFIG_CC_HAS_ASM_GOTO_OUTPUT

    #ifdef CONFIG_X86_32
    @@ -407,6 +425,30 @@ do { \
    : [umem] "m" (__m(addr)), \
    "0" (err))

    +#define __try_cmpxchg_user_asm(itype, ltype, _ptr, _pold, _new, label) ({ \
    + int __err = 0; \
    + bool success; \
    + __typeof__(_ptr) _old = (__typeof__(_ptr))(_pold); \
    + __typeof__(*(_ptr)) __old = *_old; \
    + __typeof__(*(_ptr)) __new = (_new); \
    + asm volatile("\n" \
    + "1: " LOCK_PREFIX "cmpxchg"itype" %[new], %[ptr]\n"\
    + CC_SET(z) \
    + "2:\n" \
    + _ASM_EXTABLE_TYPE_REG(1b, 2b, EX_TYPE_EFAULT_REG, \
    + %[errout]) \
    + : CC_OUT(z) (success), \
    + [errout] "+r" (__err), \
    + [ptr] "+m" (*_ptr), \
    + [old] "+a" (__old) \
    + : [new] ltype (__new) \
    + : "memory", "cc"); \
    + if (unlikely(__err)) \
    + goto label; \
    + if (unlikely(!success)) \
    + *_old = __old; \
    + likely(success); })
    +
    #endif // CONFIG_CC_HAS_ASM_GOTO_OUTPUT

    /* FIXME: this hack is definitely wrong -AK */
    @@ -501,6 +543,31 @@ do { \
    } while (0)
    #endif // CONFIG_CC_HAS_ASM_GOTO_OUTPUT

    +extern void __try_cmpxchg_user_wrong_size(void);
    +
    +#define unsafe_try_cmpxchg_user(_ptr, _oldp, _nval, _label) ({ \
    + __typeof__(*(_ptr)) __ret; \
    + switch (sizeof(__ret)) { \
    + case 1: __ret = __try_cmpxchg_user_asm("b", "q", \
    + (_ptr), (_oldp), \
    + (_nval), _label); \
    + break; \
    + case 2: __ret = __try_cmpxchg_user_asm("w", "r", \
    + (_ptr), (_oldp), \
    + (_nval), _label); \
    + break; \
    + case 4: __ret = __try_cmpxchg_user_asm("l", "r", \
    + (_ptr), (_oldp), \
    + (_nval), _label); \
    + break; \
    + case 8: __ret = __try_cmpxchg_user_asm("q", "r", \
    + (_ptr), (_oldp), \
    + (_nval), _label); \
    + break; \
    + default: __try_cmpxchg_user_wrong_size(); \
    + } \
    + __ret; })
    +
    /*
    * We want the unsafe accessors to always be inlined and use
    * the error labels - thus the macro games.

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