[PATCH 6/6] arm64: switch to relative exception tables

[Ard Biesheuvel]

Instead of using absolute addresses for both the exception location
and the fixup, use offsets relative to the exception table entry values.
Not only does this cut the size of the exception table in half, it is
also a prerequisite for KASLR, since absolute exception table entries
are subject to dynamic relocation, which is incompatible with the sorting
of the exception table that occurs at build time.

Cc: Catalin Marinas <catalin.marinas at arm.com>
Cc: Will Deacon <will.deacon at arm.com>
Signed-off-by: Ard Biesheuvel <ard.biesheuvel at linaro.org>
---

Note that this patch supersedes the version I sent as part of the series that
implements KASLR for arm64: http://thread.gmane.org/gmane.linux.kernel/2116531

 arch/arm64/include/asm/assembler.h   |  2 +-
 arch/arm64/include/asm/futex.h       |  4 ++--
 arch/arm64/include/asm/uaccess.h     | 18 ++++++++++--------
 arch/arm64/kernel/armv8_deprecated.c |  4 ++--
 arch/arm64/mm/extable.c              |  2 +-
 scripts/sortextable.c                |  2 +-
 6 files changed, 17 insertions(+), 15 deletions(-)

diff --git a/arch/arm64/include/asm/assembler.h b/arch/arm64/include/asm/assembler.h
index 12eff928ef8b..8094d50f05bc 100644
--- a/arch/arm64/include/asm/assembler.h
+++ b/arch/arm64/include/asm/assembler.h
@@ -98,7 +98,7 @@
 9999:	x;					\
 	.section __ex_table,"a";		\
 	.align	3;				\
-	.quad	9999b,l;			\
+	.long	(9999b - .), (l - .);		\
 	.previous
 
 /*
diff --git a/arch/arm64/include/asm/futex.h b/arch/arm64/include/asm/futex.h
index 007a69fc4f40..35e73e255ad3 100644
--- a/arch/arm64/include/asm/futex.h
+++ b/arch/arm64/include/asm/futex.h
@@ -44,7 +44,7 @@
 "	.popsection\n"							\
 "	.pushsection __ex_table,\"a\"\n"				\
 "	.align	3\n"							\
-"	.quad	1b, 4b, 2b, 4b\n"					\
+"	.long	(1b - .), (4b - .), (2b - .), (4b - .)\n"		\
 "	.popsection\n"							\
 	ALTERNATIVE("nop", SET_PSTATE_PAN(1), ARM64_HAS_PAN,		\
 		    CONFIG_ARM64_PAN)					\
@@ -135,7 +135,7 @@ futex_atomic_cmpxchg_inatomic(u32 *uval, u32 __user *uaddr,
 "	.popsection\n"
 "	.pushsection __ex_table,\"a\"\n"
 "	.align	3\n"
-"	.quad	1b, 4b, 2b, 4b\n"
+"	.long	(1b - .), (4b - .), (2b - .), (4b - .)\n"
 "	.popsection\n"
 	: "+r" (ret), "=&r" (val), "+Q" (*uaddr), "=&r" (tmp)
 	: "r" (oldval), "r" (newval), "Ir" (-EFAULT)
diff --git a/arch/arm64/include/asm/uaccess.h b/arch/arm64/include/asm/uaccess.h
index b2ede967fe7d..ab627e6c06c9 100644
--- a/arch/arm64/include/asm/uaccess.h
+++ b/arch/arm64/include/asm/uaccess.h
@@ -36,11 +36,11 @@
 #define VERIFY_WRITE 1
 
 /*
- * The exception table consists of pairs of addresses: the first is the
- * address of an instruction that is allowed to fault, and the second is
- * the address at which the program should continue.  No registers are
- * modified, so it is entirely up to the continuation code to figure out
- * what to do.
+ * The exception table consists of pairs of relative offsets: the first
+ * is the relative offset to an instruction that is allowed to fault,
+ * and the second is the relative offset at which the program should
+ * continue. No registers are modified, so it is entirely up to the
+ * continuation code to figure out what to do.
  *
  * All the routines below use bits of fixup code that are out of line
  * with the main instruction path.  This means when everything is well,
@@ -50,9 +50,11 @@
 
 struct exception_table_entry
 {
-	unsigned long insn, fixup;
+	int insn, fixup;
 };
 
+#define ARCH_HAS_RELATIVE_EXTABLE
+
 extern int fixup_exception(struct pt_regs *regs);
 
 #define KERNEL_DS	(-1UL)
@@ -125,7 +127,7 @@ static inline void set_fs(mm_segment_t fs)
 	"	.previous\n"						\
 	"	.section __ex_table,\"a\"\n"				\
 	"	.align	3\n"						\
-	"	.quad	1b, 3b\n"					\
+	"	.long	(1b - .), (3b - .)\n"				\
 	"	.previous"						\
 	: "+r" (err), "=&r" (x)						\
 	: "r" (addr), "i" (-EFAULT))
@@ -192,7 +194,7 @@ do {									\
 	"	.previous\n"						\
 	"	.section __ex_table,\"a\"\n"				\
 	"	.align	3\n"						\
-	"	.quad	1b, 3b\n"					\
+	"	.long	(1b - .), (3b - .)\n"				\
 	"	.previous"						\
 	: "+r" (err)							\
 	: "r" (x), "r" (addr), "i" (-EFAULT))
diff --git a/arch/arm64/kernel/armv8_deprecated.c b/arch/arm64/kernel/armv8_deprecated.c
index 937f5e58a4d3..8f21b1363387 100644
--- a/arch/arm64/kernel/armv8_deprecated.c
+++ b/arch/arm64/kernel/armv8_deprecated.c
@@ -299,8 +299,8 @@ static void register_insn_emulation_sysctl(struct ctl_table *table)
 	"	.popsection"					\
 	"	.pushsection	 __ex_table,\"a\"\n"		\
 	"	.align		3\n"				\
-	"	.quad		0b, 4b\n"			\
-	"	.quad		1b, 4b\n"			\
+	"	.long		(0b - .), (4b - .)\n"		\
+	"	.long		(1b - .), (4b - .)\n"		\
 	"	.popsection\n"					\
 	ALTERNATIVE("nop", SET_PSTATE_PAN(1), ARM64_HAS_PAN,	\
 		CONFIG_ARM64_PAN)				\
diff --git a/arch/arm64/mm/extable.c b/arch/arm64/mm/extable.c
index 79444279ba8c..81acd4706878 100644
--- a/arch/arm64/mm/extable.c
+++ b/arch/arm64/mm/extable.c
@@ -11,7 +11,7 @@ int fixup_exception(struct pt_regs *regs)
 
 	fixup = search_exception_tables(instruction_pointer(regs));
 	if (fixup)
-		regs->pc = fixup->fixup;
+		regs->pc = (unsigned long)&fixup->fixup + fixup->fixup;
 
 	return fixup != NULL;
 }
diff --git a/scripts/sortextable.c b/scripts/sortextable.c
index c2423d913b46..af247c70fb66 100644
--- a/scripts/sortextable.c
+++ b/scripts/sortextable.c
@@ -282,12 +282,12 @@ do_file(char const *const fname)
 	case EM_386:
 	case EM_X86_64:
 	case EM_S390:
+	case EM_AARCH64:
 		custom_sort = sort_relative_table;
 		break;
 	case EM_ARCOMPACT:
 	case EM_ARCV2:
 	case EM_ARM:
-	case EM_AARCH64:
 	case EM_MICROBLAZE:
 	case EM_MIPS:
 	case EM_XTENSA:
-- 
1.9.1