[PATCH v6 1/4] x86: Clean up extable entry format (and free up a bit)

[Andy Lutomirski <luto@amacapital.net>]

This adds two bits of fixup class information to a fixup entry,
generalizing the uaccess_err hack currently in place.

Forward-ported-from-3.9-by: Tony Luck <tony.luck@intel.com>
Signed-off-by: Andy Lutomirski <luto@amacapital.net>
---
 arch/x86/include/asm/asm.h | 70 ++++++++++++++++++++++++++++++----------------
 arch/x86/mm/extable.c      | 21 ++++++++------
 2 files changed, 59 insertions(+), 32 deletions(-)

diff --git a/arch/x86/include/asm/asm.h b/arch/x86/include/asm/asm.h
index 189679aba703..b64121ffb2da 100644
--- a/arch/x86/include/asm/asm.h
+++ b/arch/x86/include/asm/asm.h
@@ -43,19 +43,47 @@
 #define _ASM_DI		__ASM_REG(di)
 
 /* Exception table entry */
-#ifdef __ASSEMBLY__
-# define _ASM_EXTABLE(from,to)					\
-	.pushsection "__ex_table","a" ;				\
-	.balign 8 ;						\
-	.long (from) - . ;					\
-	.long (to) - . ;					\
-	.popsection
 
-# define _ASM_EXTABLE_EX(from,to)				\
-	.pushsection "__ex_table","a" ;				\
-	.balign 8 ;						\
-	.long (from) - . ;					\
-	.long (to) - . + 0x7ffffff0 ;				\
+/*
+ * An exception table entry is 64 bits.  The first 32 bits are the offset
+ * from that entry to the potentially faulting instruction.  sortextable
+ * relies on that exact encoding.  The second 32 bits encode the fault
+ * handler address.
+ *
+ * We want to stick two extra bits of handler class into the fault handler
+ * address.  All of these are generated by relocations, so we can only
+ * rely on addition.  We therefore emit:
+ *
+ * (target - here) + (class) + 0x20000000
+ *
+ * This has the property that the two high bits are the class and the
+ * rest is easy to decode.
+ */
+
+/* There are two bits of extable entry class, added to a signed offset. */
+#define _EXTABLE_CLASS_DEFAULT	0		/* standard uaccess fixup */
+#define _EXTABLE_CLASS_EX	0x80000000	/* uaccess + set uaccess_err */
+
+/*
+ * The biases are the class constants + 0x20000000, as signed integers.
+ * This can't use ordinary arithmetic -- the assembler isn't that smart.
+ */
+#define _EXTABLE_BIAS_DEFAULT	0x20000000
+#define _EXTABLE_BIAS_EX	0x20000000 - 0x80000000
+
+#define _ASM_EXTABLE(from,to)						\
+	_ASM_EXTABLE_CLASS(from, to, _EXTABLE_BIAS_DEFAULT)
+
+#define _ASM_EXTABLE_EX(from,to)					\
+	_ASM_EXTABLE_CLASS(from, to, _EXTABLE_BIAS_EX)
+
+#ifdef __ASSEMBLY__
+# define _EXPAND_EXTABLE_BIAS(x) x
+# define _ASM_EXTABLE_CLASS(from,to,bias)				\
+	.pushsection "__ex_table","a" ;					\
+	.balign 8 ;							\
+	.long (from) - . ;						\
+	.long (to) - . + _EXPAND_EXTABLE_BIAS(bias) ;			\
 	.popsection
 
 # define _ASM_NOKPROBE(entry)					\
@@ -89,18 +117,12 @@
 	.endm
 
 #else
-# define _ASM_EXTABLE(from,to)					\
-	" .pushsection \"__ex_table\",\"a\"\n"			\
-	" .balign 8\n"						\
-	" .long (" #from ") - .\n"				\
-	" .long (" #to ") - .\n"				\
-	" .popsection\n"
-
-# define _ASM_EXTABLE_EX(from,to)				\
-	" .pushsection \"__ex_table\",\"a\"\n"			\
-	" .balign 8\n"						\
-	" .long (" #from ") - .\n"				\
-	" .long (" #to ") - . + 0x7ffffff0\n"			\
+# define _EXPAND_EXTABLE_BIAS(x) #x
+# define _ASM_EXTABLE_CLASS(from,to,bias)				\
+	" .pushsection \"__ex_table\",\"a\"\n"				\
+	" .balign 8\n"							\
+	" .long (" #from ") - .\n"					\
+	" .long (" #to ") - . + " _EXPAND_EXTABLE_BIAS(bias) "\n"	\
 	" .popsection\n"
 /* For C file, we already have NOKPROBE_SYMBOL macro */
 #endif
diff --git a/arch/x86/mm/extable.c b/arch/x86/mm/extable.c
index 903ec1e9c326..95e2ede71206 100644
--- a/arch/x86/mm/extable.c
+++ b/arch/x86/mm/extable.c
@@ -8,16 +8,24 @@ ex_insn_addr(const struct exception_table_entry *x)
 {
 	return (unsigned long)&x->insn + x->insn;
 }
+static inline unsigned int
+ex_class(const struct exception_table_entry *x)
+{
+	return (unsigned int)x->fixup & 0xC0000000;
+}
+
 static inline unsigned long
 ex_fixup_addr(const struct exception_table_entry *x)
 {
-	return (unsigned long)&x->fixup + x->fixup;
+	long offset = (long)((u32)x->fixup & 0x3fffffff) - (long)0x20000000;
+	return (unsigned long)&x->fixup + offset;
 }
 
 int fixup_exception(struct pt_regs *regs)
 {
 	const struct exception_table_entry *fixup;
 	unsigned long new_ip;
+	unsigned int class;
 
 #ifdef CONFIG_PNPBIOS
 	if (unlikely(SEGMENT_IS_PNP_CODE(regs->cs))) {
@@ -35,12 +43,12 @@ int fixup_exception(struct pt_regs *regs)
 
 	fixup = search_exception_tables(regs->ip);
 	if (fixup) {
+		class = ex_class(fixup);
 		new_ip = ex_fixup_addr(fixup);
 
-		if (fixup->fixup - fixup->insn >= 0x7ffffff0 - 4) {
+		if (class == _EXTABLE_CLASS_EX) {
 			/* Special hack for uaccess_err */
 			current_thread_info()->uaccess_err = 1;
-			new_ip -= 0x7ffffff0;
 		}
 		regs->ip = new_ip;
 		return 1;
@@ -53,18 +61,15 @@ int fixup_exception(struct pt_regs *regs)
 int __init early_fixup_exception(unsigned long *ip)
 {
 	const struct exception_table_entry *fixup;
-	unsigned long new_ip;
 
 	fixup = search_exception_tables(*ip);
 	if (fixup) {
-		new_ip = ex_fixup_addr(fixup);
-
-		if (fixup->fixup - fixup->insn >= 0x7ffffff0 - 4) {
+		if (ex_class(fixup) == _EXTABLE_CLASS_EX) {
 			/* uaccess handling not supported during early boot */
 			return 0;
 		}
 
-		*ip = new_ip;
+		*ip = ex_fixup_addr(fixup);
 		return 1;
 	}
 
-- 
2.1.4