[PATCH v2 2/2] riscv: switch to relative exception tables

Jisheng Zhang
2021-10-20T14:13:56Z

From: Jisheng Zhang <jszhang@kernel.org>

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

However, RISCV label difference will actually produce two relocations,
a pair of R_RISCV_ADD32 and R_RISCV_SUB32. Take below simple code for
example:

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

$ riscv64-linux-gnu-gcc -c test.S
$ riscv64-linux-gnu-readelf -r test.o
Relocation section '.rela__ex_table' at offset 0x100 contains 2 entries:
Offset Info Type Sym. Value Sym. Name + Addend
000000000000 000600000023 R_RISCV_ADD32 0000000000000000 .L1^B1 + 0
000000000000 000500000027 R_RISCV_SUB32 0000000000000000 .L0 + 0

The modpost will complain the R_RISCV_SUB32 relocation, so we need to
patch modpost.c to skip this relocation for .rela__ex_table section.

After this patch, the __ex_table section size of defconfig vmlinux is
reduced from 7072 Bytes to 3536 Bytes.

Signed-off-by: Jisheng Zhang <jszhang@kernel.org>
---
arch/riscv/include/asm/Kbuild | 1 -
arch/riscv/include/asm/extable.h | 25 +++++++++++++++++++++++++
arch/riscv/include/asm/uaccess.h | 4 ++--
arch/riscv/lib/uaccess.S | 4 ++--
arch/riscv/mm/extable.c | 2 +-
scripts/mod/modpost.c | 27 +++++++++++++++++++++++++++
scripts/sorttable.c | 2 +-
7 files changed, 58 insertions(+), 7 deletions(-)
create mode 100644 arch/riscv/include/asm/extable.h

diff --git a/arch/riscv/include/asm/Kbuild b/arch/riscv/include/asm/Kbuild
index 445ccc97305a..57b86fd9916c 100644
--- a/arch/riscv/include/asm/Kbuild
+++ b/arch/riscv/include/asm/Kbuild
@@ -1,6 +1,5 @@
# SPDX-License-Identifier: GPL-2.0
generic-y += early_ioremap.h
-generic-y += extable.h
generic-y += flat.h
generic-y += kvm_para.h
generic-y += user.h
diff --git a/arch/riscv/include/asm/extable.h b/arch/riscv/include/asm/extable.h
new file mode 100644
index 000000000000..bc439b0fdb29
--- /dev/null
+++ b/arch/riscv/include/asm/extable.h
@@ -0,0 +1,25 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _ASM_RISCV_EXTABLE_H
+#define _ASM_RISCV_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
+
+extern int fixup_exception(struct pt_regs *regs);
+#endif
diff --git a/arch/riscv/include/asm/uaccess.h b/arch/riscv/include/asm/uaccess.h
index 35802e72ace8..540903c7d3e8 100644
--- a/arch/riscv/include/asm/uaccess.h
+++ b/arch/riscv/include/asm/uaccess.h
@@ -12,8 +12,8 @@

#define _ASM_EXTABLE(from, to) \
" .pushsection __ex_table, \"a\"\n" \
- " .balign " RISCV_SZPTR " \n" \
- " " RISCV_PTR "(" #from "), (" #to ")\n" \
+ " .balign 4\n" \
+ " .long (" #from " - .), (" #to " - .)\n" \
" .popsection\n"

/*
diff --git a/arch/riscv/lib/uaccess.S b/arch/riscv/lib/uaccess.S
index 63bc691cff91..55f80f84e23f 100644
--- a/arch/riscv/lib/uaccess.S
+++ b/arch/riscv/lib/uaccess.S
@@ -7,8 +7,8 @@
100:
\op \reg, \addr
.section __ex_table,"a"
- .balign RISCV_SZPTR
- RISCV_PTR 100b, \lbl
+ .balign 4
+ .long (100b - .), (\lbl - .)
.previous
.endm

diff --git a/arch/riscv/mm/extable.c b/arch/riscv/mm/extable.c
index 2fc729422151..6aa8ffac4be7 100644
--- a/arch/riscv/mm/extable.c
+++ b/arch/riscv/mm/extable.c
@@ -17,7 +17,7 @@ int fixup_exception(struct pt_regs *regs)

fixup = search_exception_tables(regs->epc);
if (fixup) {
- regs->epc = fixup->fixup;
+ regs->epc = (unsigned long)&fixup->fixup + fixup->fixup;
return 1;
}
return 0;
diff --git a/scripts/mod/modpost.c b/scripts/mod/modpost.c
index cb8ab7d91d30..0aa14b5bd124 100644
--- a/scripts/mod/modpost.c
+++ b/scripts/mod/modpost.c
@@ -1830,6 +1830,27 @@ static int addend_mips_rel(struct elf_info *elf, Elf_Shdr *sechdr, Elf_Rela *r)
return 0;
}

+#ifndef EM_RISCV
+#define EM_RISCV 243
+#endif
+
+#ifndef R_RISCV_SUB32
+#define R_RISCV_SUB32 39
+#endif
+
+static int addend_riscv_rela(struct elf_info *elf, Elf_Shdr *sechdr, Elf_Rela *r)
+{
+ unsigned int r_typ = ELF_R_TYPE(r->r_info);
+ const char *fromsec;
+
+ fromsec = sech_name(elf, sechdr);
+ fromsec += strlen(".rela");
+
+ if (!strcmp("__ex_table", fromsec) && r_typ == R_RISCV_SUB32)
+ return 1; /* skip this */
+ return 0;
+}
+
static void section_rela(const char *modname, struct elf_info *elf,
Elf_Shdr *sechdr)
{
@@ -1866,6 +1887,12 @@ static void section_rela(const char *modname, struct elf_info *elf,
r_sym = ELF_R_SYM(r.r_info);
#endif
r.r_addend = TO_NATIVE(rela->r_addend);
+ switch (elf->hdr->e_machine) {
+ case EM_RISCV:
+ if (addend_riscv_rela(elf, sechdr, &r))
+ continue;
+ break;
+ }
sym = elf->symtab_start + r_sym;
/* Skip special sections */
if (is_shndx_special(sym->st_shndx))
diff --git a/scripts/sorttable.c b/scripts/sorttable.c
index 6ee4fa882919..39e86e4acea3 100644
--- a/scripts/sorttable.c
+++ b/scripts/sorttable.c
@@ -346,6 +346,7 @@ static int do_file(char const *const fname, void *addr)
case EM_PARISC:
case EM_PPC:
case EM_PPC64:
+ case EM_RISCV:
custom_sort = sort_relative_table;
break;
case EM_ARCOMPACT:
@@ -353,7 +354,6 @@ static int do_file(char const *const fname, void *addr)
case EM_ARM:
case EM_MICROBLAZE:
case EM_MIPS:
- case EM_RISCV:
case EM_XTENSA:
break;
default:
--
2.33.0