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>

1 files changed, 22 insertions, 10 deletions

diff --git a/arch/loongarch/mm/extable.c b/arch/loongarch/mm/extable.c
index bc20988f2b87..08a9a7d6357a 100644
--- a/arch/loongarch/mm/extable.c
+++ b/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>
+
+static inline unsigned long
+get_ex_fixup(const struct exception_table_entry *ex)
+{
+	return ((unsigned long)&ex->fixup + ex->fixup);
+}
 
-int fixup_exception(struct pt_regs *regs)
+static bool ex_handler_fixup(const struct exception_table_entry *ex,
+			     struct pt_regs *regs)
 {
-	const struct exception_table_entry *fixup;
+	regs->csr_era = get_ex_fixup(ex);
 
-	fixup = search_exception_tables(exception_era(regs));
-	if (fixup) {
-		regs->csr_era = fixup->fixup;
+	return true;
+}
+
+
+bool fixup_exception(struct pt_regs *regs)
+{
+	const struct exception_table_entry *ex;
 
-		return 1;
-	}
+	ex = search_exception_tables(exception_era(regs));
+	if (!ex)
+		return false;
 
-	return 0;
+	return ex_handler_fixup(ex, regs);
 }