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.

Jisheng Zhang committed 4 years ago

Commit bb1f85d1 parent f8f2ad07 files · +46 −7Browse files

Showing the changes to scripts (2 of 7 files). Show all

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2 files

scripts/mod/modpost.c +15 −0
scripts/sorttable.c +1 −1

scripts/mod/modpost.c+15−0

@@ -1830,6 +1830,14 @@ static int addend_mips_rel(struct elf_info elf, Elf_Shdr sechdr, Elf_Rela *r) · 1829 unmodified lines
1830	1830	return 0;
1831	1831	}
1832	1832
	1833	+#ifndef EM_RISCV
	1834	+#define EM_RISCV 243
	1835	+#endif
	1836	+
	1837	+#ifndef R_RISCV_SUB32
	1838	+#define R_RISCV_SUB32 39
	1839	+#endif
	1840	+
1833	1841	static void section_rela(const char modname, struct elf_info elf,
1834	1842	Elf_Shdr *sechdr)
1835	1843	{
@@ -1866,6 +1874,13 @@ static void section_rela(const char modname, struct elf_info elf, · 30 unmodified lines
1866	1874	r_sym = ELF_R_SYM(r.r_info);
1867	1875	#endif
1868	1876	r.r_addend = TO_NATIVE(rela->r_addend);
	1877	+ switch (elf->hdr->e_machine) {
	1878	+ case EM_RISCV:
	1879	+ if (!strcmp("__ex_table", fromsec) &&
	1880	+ ELF_R_TYPE(r.r_info) == R_RISCV_SUB32)
	1881	+ continue;
	1882	+ break;
	1883	+ }
1869	1884	sym = elf->symtab_start + r_sym;
1870	1885	/* Skip special sections */
1871	1886	if (is_shndx_special(sym->st_shndx))

scripts/sorttable.c+1−1

@@ -346,6 +346,7 @@ static int do_file(char const const fname, void addr) · 345 unmodified lines
346	346	case EM_PARISC:
347	347	case EM_PPC:
348	348	case EM_PPC64:
	349	+ case EM_RISCV:
349	350	custom_sort = sort_relative_table;
350	351	break;
351	352	case EM_ARCOMPACT:
@@ -353,7 +354,6 @@ static int do_file(char const const fname, void addr) · 1 unmodified line
353	354	case EM_ARM:
354	355	case EM_MICROBLAZE:
355	356	case EM_MIPS:
356		− case EM_RISCV:
357	357	case EM_XTENSA:
358	358	break;
359	359	default:

riscv: switch to relative exception tables

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