LoongArch: Switch to relative exception tables

Similar to other architectures such as arm64, x86, riscv and so on, use
offsets relative to the exception table entry values rather than their
absolute addresses for both the exception location and the fixup.

However, LoongArch label difference because it will actually produce two
relocations, a pair of R_LARCH_ADD32 and R_LARCH_SUB32. Take simple code
below for example:

$ cat test_ex_table.S
.section .text
1:
        nop
.section __ex_table,"a"
        .balign 4
        .long (1b - .)
.previous

$ loongarch64-unknown-linux-gnu-gcc -c test_ex_table.S
$ loongarch64-unknown-linux-gnu-readelf -Wr test_ex_table.o

Relocation section '.rela__ex_table' at offset 0x100 contains 2 entries:
    Offset            Info             Type         Symbol's Value   Symbol's Name + Addend
0000000000000000 0000000600000032 R_LARCH_ADD32    0000000000000000  .L1^B1 + 0
0000000000000000 0000000500000037 R_LARCH_SUB32    0000000000000000  L0^A + 0

The modpost will complain the R_LARCH_SUB32 relocation, so we need to
patch modpost.c to skip this relocation for .rela__ex_table section.

Signed-off-by: Youling Tang <tangyouling@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>

arch/loongarch/include/asm/asm-extable.h

@@ -6,9 +6,9 @@
 
 #define __ASM_EXTABLE_RAW(insn, fixup)			\
 	.pushsection	__ex_table, "a";		\
-	.balign		8;				\
-	.quad		(insn);				\
-	.quad		(fixup);			\
+	.balign		4;				\
+	.long		((insn) - .);			\
+	.long		((fixup) - .);			\
 	.popsection;
 
 	.macro		_asm_extable, insn, fixup
@@ -22,9 +22,9 @@
 
 #define __ASM_EXTABLE_RAW(insn, fixup)			\
 	".pushsection	__ex_table, \"a\"\n"		\
-	".balign	8\n"				\
-	".quad		((" insn "))\n"			\
-	".quad		((" fixup "))\n"		\
+	".balign	4\n"				\
+	".long		((" insn ") - .)\n"		\
+	".long		((" fixup ") - .)\n"		\
 	".popsection\n"
 
 #define _ASM_EXTABLE(insn, fixup)	\

arch/loongarch/include/asm/extable.h

@@ -0,0 +1,26 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _ASM_LOONGARCH_EXTABLE_H
+#define _ASM_LOONGARCH_EXTABLE_H
+
+/*
+ * 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,
+ * we don't even have to jump over them.  Further, they do not intrude
+ * on our cache or tlb entries.
+ */
+
+struct exception_table_entry {
+	int insn, fixup;
+};
+
+#define ARCH_HAS_RELATIVE_EXTABLE
+
+bool fixup_exception(struct pt_regs *regs);
+
+#endif

arch/loongarch/include/asm/uaccess.h

@@ -15,8 +15,8 @@
 #include <linux/string.h>
 #include <linux/extable.h>
 #include <asm/pgtable.h>
+#include <asm/extable.h>
 #include <asm/asm-extable.h>
-#include <asm-generic/extable.h>
 #include <asm-generic/access_ok.h>
 
 extern u64 __ua_limit;

arch/loongarch/mm/extable.c

@@ -3,20 +3,32 @@
  * Copyright (C) 2020-2022 Loongson Technology Corporation Limited
  */
 #include <linux/extable.h>
-#include <linux/spinlock.h>
-#include <asm/branch.h>
 #include <linux/uaccess.h>
+#include <asm/asm-extable.h>
+#include <asm/branch.h>
 
-int fixup_exception(struct pt_regs *regs)
+static inline unsigned long
+get_ex_fixup(const struct exception_table_entry *ex)
 {
-	const struct exception_table_entry *fixup;
-
-	fixup = search_exception_tables(exception_era(regs));
-	if (fixup) {
-		regs->csr_era = fixup->fixup;
-
-		return 1;
-	}
-
-	return 0;
+	return ((unsigned long)&ex->fixup + ex->fixup);
+}
+
+static bool ex_handler_fixup(const struct exception_table_entry *ex,
+			     struct pt_regs *regs)
+{
+	regs->csr_era = get_ex_fixup(ex);
+
+	return true;
+}
+
+
+bool fixup_exception(struct pt_regs *regs)
+{
+	const struct exception_table_entry *ex;
+
+	ex = search_exception_tables(exception_era(regs));
+	if (!ex)
+		return false;
+
+	return ex_handler_fixup(ex, regs);
 }

scripts/mod/modpost.c

@@ -1523,6 +1523,14 @@ static int addend_mips_rel(struct elf_info *elf, Elf_Shdr *sechdr, Elf_Rela *r)
 #define R_RISCV_SUB32		39
 #endif
 
+#ifndef EM_LOONGARCH
+#define EM_LOONGARCH		258
+#endif
+
+#ifndef R_LARCH_SUB32
+#define R_LARCH_SUB32		55
+#endif
+
 static void section_rela(const char *modname, struct elf_info *elf,
 			 Elf_Shdr *sechdr)
 {
@@ -1564,6 +1572,11 @@ static void section_rela(const char *modname, struct elf_info *elf,
 			    ELF_R_TYPE(r.r_info) == R_RISCV_SUB32)
 				continue;
 			break;
+		case EM_LOONGARCH:
+			if (!strcmp("__ex_table", fromsec) &&
+			    ELF_R_TYPE(r.r_info) == R_LARCH_SUB32)
+				continue;
+			break;
 		}
 		sym = elf->symtab_start + r_sym;
 		/* Skip special sections */

scripts/sorttable.c

@@ -312,12 +312,12 @@ static int do_file(char const *const fname, void *addr)
 	case EM_PARISC:
 	case EM_PPC:
 	case EM_PPC64:
+	case EM_LOONGARCH:
 		custom_sort = sort_relative_table;
 		break;
 	case EM_ARCOMPACT:
 	case EM_ARCV2:
 	case EM_ARM:
-	case EM_LOONGARCH:
 	case EM_MICROBLAZE:
 	case EM_MIPS:
 	case EM_XTENSA: