[RFC PATCH v2 2/5] x86/uaccess: Implement unsafe_try_cmpxchg_user()

[Peter Oskolkov <posk@google.com>]

From: Peter Zijlstra <peterz@infradead.org>

Do try_cmpxchg() loops on userspace addresses.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
 arch/x86/include/asm/uaccess.h | 57 ++++++++++++++++++++++++++++++++++
 1 file changed, 57 insertions(+)

diff --git a/arch/x86/include/asm/uaccess.h b/arch/x86/include/asm/uaccess.h
index ac96f9b2d64b..8277ec05be02 100644
--- 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, _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] "r" (__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, _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] "r" (__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 +543,21 @@ 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 4:	__ret = __try_cmpxchg_user_asm("l", (_ptr), (_oldp),	\
+					       (_nval), _label);	\
+		break;							\
+	case 8:	__ret = __try_cmpxchg_user_asm("q", (_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.
-- 
2.34.1.703.g22d0c6ccf7-goog