3 files changed, 60 insertions, 43 deletions

diff --git a/arch/x86/include/asm/extable_fixup_types.h b/arch/x86/include/asm/extable_fixup_types.h
index 503622627400..991e31cfde94 100644
--- a/arch/x86/include/asm/extable_fixup_types.h
+++ b/arch/x86/include/asm/extable_fixup_types.h
@@ -64,4 +64,6 @@
 #define	EX_TYPE_UCOPY_LEN4		(EX_TYPE_UCOPY_LEN | EX_DATA_IMM(4))
 #define	EX_TYPE_UCOPY_LEN8		(EX_TYPE_UCOPY_LEN | EX_DATA_IMM(8))
 
+#define EX_TYPE_ZEROPAD			20 /* longword load with zeropad on fault */
+
 #endif
diff --git a/arch/x86/include/asm/word-at-a-time.h b/arch/x86/include/asm/word-at-a-time.h
index 8338b0432b50..46b4f1f7f354 100644
--- a/arch/x86/include/asm/word-at-a-time.h
+++ b/arch/x86/include/asm/word-at-a-time.h
@@ -77,58 +77,18 @@ static inline unsigned long find_zero(unsigned long mask)
  * and the next page not being mapped, take the exception and
  * return zeroes in the non-existing part.
  */
-#ifdef CONFIG_CC_HAS_ASM_GOTO_OUTPUT
-
 static inline unsigned long load_unaligned_zeropad(const void *addr)
 {
-	unsigned long offset, data;
 	unsigned long ret;
 
-	asm_volatile_goto(
+	asm volatile(
 		"1:	mov %[mem], %[ret]\n"
-
-		_ASM_EXTABLE(1b, %l[do_exception])
-
-		: [ret] "=r" (ret)
-		: [mem] "m" (*(unsigned long *)addr)
-		: : do_exception);
-
-	return ret;
-
-do_exception:
-	offset = (unsigned long)addr & (sizeof(long) - 1);
-	addr = (void *)((unsigned long)addr & ~(sizeof(long) - 1));
-	data = *(unsigned long *)addr;
-	ret = data >> offset * 8;
-
-	return ret;
-}
-
-#else /* !CONFIG_CC_HAS_ASM_GOTO_OUTPUT */
-
-static inline unsigned long load_unaligned_zeropad(const void *addr)
-{
-	unsigned long offset, data;
-	unsigned long ret, err = 0;
-
-	asm(	"1:	mov %[mem], %[ret]\n"
 		"2:\n"
-
-		_ASM_EXTABLE_FAULT(1b, 2b)
-
-		: [ret] "=&r" (ret), "+a" (err)
+		_ASM_EXTABLE_TYPE(1b, 2b, EX_TYPE_ZEROPAD)
+		: [ret] "=r" (ret)
 		: [mem] "m" (*(unsigned long *)addr));
 
-	if (unlikely(err)) {
-		offset = (unsigned long)addr & (sizeof(long) - 1);
-		addr = (void *)((unsigned long)addr & ~(sizeof(long) - 1));
-		data = *(unsigned long *)addr;
-		ret = data >> offset * 8;
-	}
-
 	return ret;
 }
 
-#endif /* CONFIG_CC_HAS_ASM_GOTO_OUTPUT */
-
 #endif /* _ASM_WORD_AT_A_TIME_H */
diff --git a/arch/x86/mm/extable.c b/arch/x86/mm/extable.c
index 331310c29349..60814e110a54 100644
--- a/arch/x86/mm/extable.c
+++ b/arch/x86/mm/extable.c
@@ -41,6 +41,59 @@ static bool ex_handler_default(const struct exception_table_entry *e,
 	return true;
 }
 
+/*
+ * This is the *very* rare case where we do a "load_unaligned_zeropad()"
+ * and it's a page crosser into a non-existent page.
+ *
+ * This happens when we optimistically load a pathname a word-at-a-time
+ * and the name is less than the full word and the  next page is not
+ * mapped. Typically that only happens for CONFIG_DEBUG_PAGEALLOC.
+ *
+ * NOTE! The faulting address is always a 'mov mem,reg' type instruction
+ * of size 'long', and the exception fixup must always point to right
+ * after the instruction.
+ */
+static bool ex_handler_zeropad(const struct exception_table_entry *e,
+			       struct pt_regs *regs,
+			       unsigned long fault_addr)
+{
+	struct insn insn;
+	const unsigned long mask = sizeof(long) - 1;
+	unsigned long offset, addr, next_ip, len;
+	unsigned long *reg;
+
+	next_ip = ex_fixup_addr(e);
+	len = next_ip - regs->ip;
+	if (len > MAX_INSN_SIZE)
+		return false;
+
+	if (insn_decode(&insn, (void *) regs->ip, len, INSN_MODE_KERN))
+		return false;
+	if (insn.length != len)
+		return false;
+
+	if (insn.opcode.bytes[0] != 0x8b)
+		return false;
+	if (insn.opnd_bytes != sizeof(long))
+		return false;
+
+	addr = (unsigned long) insn_get_addr_ref(&insn, regs);
+	if (addr == ~0ul)
+		return false;
+
+	offset = addr & mask;
+	addr = addr & ~mask;
+	if (fault_addr != addr + sizeof(long))
+		return false;
+
+	reg = insn_get_modrm_reg_ptr(&insn, regs);
+	if (!reg)
+		return false;
+
+	*reg = *(unsigned long *)addr >> (offset * 8);
+	return ex_handler_default(e, regs);
+}
+
 static bool ex_handler_fault(const struct exception_table_entry *fixup,
 			     struct pt_regs *regs, int trapnr)
 {
@@ -217,6 +270,8 @@ int fixup_exception(struct pt_regs *regs, int trapnr, unsigned long error_code,
 		return ex_handler_sgx(e, regs, trapnr);
 	case EX_TYPE_UCOPY_LEN:
 		return ex_handler_ucopy_len(e, regs, trapnr, reg, imm);
+	case EX_TYPE_ZEROPAD:
+		return ex_handler_zeropad(e, regs, fault_addr);
 	}
 	BUG();
 }