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

Kefeng Wang
2021-10-21T11:42:55Z

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:
>