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>

4 files changed, 55 insertions, 17 deletions

diff --git a/arch/loongarch/include/asm/asm-extable.h b/arch/loongarch/include/asm/asm-extable.h
index 4f615bf56727..74f8bc75472a 100644
--- a/arch/loongarch/include/asm/asm-extable.h
+++ b/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)	\
diff --git a/arch/loongarch/include/asm/extable.h b/arch/loongarch/include/asm/extable.h
new file mode 100644
index 000000000000..b571c89705d1
--- /dev/null
+++ b/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
diff --git a/arch/loongarch/include/asm/uaccess.h b/arch/loongarch/include/asm/uaccess.h
index bf9a4e218ac0..e33282e0bdef 100644
--- a/arch/loongarch/include/asm/uaccess.h
+++ b/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;
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);
 }