Messages in this thread Patch in this message | | | Date | Thu, 21 Oct 2021 19:42:48 +0800 | Subject | Re: [PATCH v2 2/2] riscv: switch to relative exception tables | From | Kefeng Wang <> |
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On 2021/10/20 22:07, Jisheng Zhang wrote: > 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> > ---
... > 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)) directly use 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 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: >
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