riscv: switch to relative exception tables

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>
Reviewed-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>

diff --git a/arch/riscv/include/asm/Kbuild b/arch/riscv/include/asm/Kbuild
index 445ccc97305a5ed8d7f395f2df1c932b8397dad6..57b86fd9916c338cc86d0b9e916d6f7a149da23e 100644 (file)
--- 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 (file)
index 0000000..8476039
--- /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
+
+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 714cd311d9f101e76875895edcecbfb0f4b83e3b..0f2c5b9d2e8f84ed788458a3a8b3e0fdef2fd9e9 100644 (file)
--- 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 63bc691cff91b275100a8339f1cc387555e79ae0..55f80f84e23fbb077ab34442d8ed6ce76a3aa72a 100644 (file)
--- 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 ddb7d3b99e891d328de720c51c5456703d009374..d8d239c2c1bd39a6c556517b329d553d036a7e3e 100644 (file)
--- a/arch/riscv/mm/extable.c
+++ b/arch/riscv/mm/extable.c
@@ -28,6 +28,6 @@ int fixup_exception(struct pt_regs *regs)
                return rv_bpf_fixup_exception(fixup, regs);
 #endif
 
-       regs->epc = fixup->fixup;
+       regs->epc = (unsigned long)&fixup->fixup + fixup->fixup;
        return 1;
 }

diff --git a/scripts/mod/modpost.c b/scripts/mod/modpost.c
index cb8ab7d91d30722b2653537abe5a41d7cdc69030..6bfa332179140bee888179e24c9346d05736c16e 100644 (file)
--- a/scripts/mod/modpost.c
+++ b/scripts/mod/modpost.c
@@ -1830,6 +1830,14 @@ 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 void section_rela(const char *modname, struct elf_info *elf,
                         Elf_Shdr *sechdr)
 {
@@ -1866,6 +1874,13 @@ 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 (!strcmp("__ex_table", fromsec) &&
+                           ELF_R_TYPE(r.r_info) == R_RISCV_SUB32)
+                               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 ca9db62bf766eed2679dfeb381859041c167ddb3..f4a8255036b512bf3a61cdd74f8484dc34747187 100644 (file)
--- 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: