Re: [PATCH] arm64/module: use plt section indices for relocations

From: Ard Biesheuvel
Date: Mon Nov 05 2018 - 13:04:08 EST

On 5 November 2018 at 18:57, Jessica Yu <jeyu@xxxxxxxxxx> wrote:
> Instead of saving a pointer to the .plt and .init.plt sections to apply
> plt-based relocations, save and use their section indices instead.
>
> The mod->arch.{core,init}.plt pointers were problematic for livepatch
> because they pointed within temporary section headers (provided by the
> module loader via info->sechdrs) that would be freed after module load.
> Since livepatch modules may need to apply relocations post-module-load
> (for example, to patch a module that is loaded later), using section
> indices to offset into the section headers (instead of accessing them
> through a saved pointer) allows livepatch modules on arm64 to pass in
> their own copy of the section headers to apply_relocate_add() to apply
> delayed relocations.
>
> Signed-off-by: Jessica Yu <jeyu@xxxxxxxxxx>
> ---
>
> Note: Addressed Will's comment about the pltsec -> plt_info rename and
> removed that change to reduce unnecessary code churn. That is the only
> change from the first version. I didn't include the Ack's for this reason
> so let me know if this version is OK as well.
>
> arch/arm64/include/asm/module.h | 8 +++++---
> arch/arm64/kernel/module-plts.c | 36 ++++++++++++++++++++----------------
> arch/arm64/kernel/module.c | 9 +++++----
> 3 files changed, 30 insertions(+), 23 deletions(-)
>
> diff --git a/arch/arm64/include/asm/module.h
> b/arch/arm64/include/asm/module.h
> index 97d0ef12e2ff..f81c1847312d 100644
> --- a/arch/arm64/include/asm/module.h
> +++ b/arch/arm64/include/asm/module.h
> @@ -22,7 +22,7 @@
>
> #ifdef CONFIG_ARM64_MODULE_PLTS
> struct mod_plt_sec {
> - struct elf64_shdr *plt;
> + int plt_shndx;
> int plt_num_entries;
> int plt_max_entries;
> };
> @@ -36,10 +36,12 @@ struct mod_arch_specific {
> };
> #endif
>
> -u64 module_emit_plt_entry(struct module *mod, void *loc, const Elf64_Rela
> *rela,
> +u64 module_emit_plt_entry(struct module *mod, Elf64_Shdr *sechdrs,
> + void *loc, const Elf64_Rela *rela,
> Elf64_Sym *sym);
>
> -u64 module_emit_veneer_for_adrp(struct module *mod, void *loc, u64 val);
> +u64 module_emit_veneer_for_adrp(struct module *mod, Elf64_Shdr *sechdrs,
> + void *loc, u64 val);
>
> #ifdef CONFIG_RANDOMIZE_BASE
> extern u64 module_alloc_base;
> diff --git a/arch/arm64/kernel/module-plts.c
> b/arch/arm64/kernel/module-plts.c
> index f0690c2ca3e0..c04b0d6abde0 100644
> --- a/arch/arm64/kernel/module-plts.c
> +++ b/arch/arm64/kernel/module-plts.c
> @@ -16,12 +16,13 @@ static bool in_init(const struct module *mod, void *loc)
> return (u64)loc - (u64)mod->init_layout.base <
> mod->init_layout.size;
> }
>
> -u64 module_emit_plt_entry(struct module *mod, void *loc, const Elf64_Rela
> *rela,
> +u64 module_emit_plt_entry(struct module *mod, Elf64_Shdr *sechdrs,
> + void *loc, const Elf64_Rela *rela,
> Elf64_Sym *sym)
> {
> struct mod_plt_sec *pltsec = !in_init(mod, loc) ? &mod->arch.core :
> &mod->arch.init;
> - struct plt_entry *plt = (struct plt_entry *)pltsec->plt->sh_addr;
> + struct plt_entry *plt = (struct plt_entry *)(sechdrs +
> pltsec->plt_shndx)->sh_addr;

What's wrong with

(struct plt_entry *)sechdrs[pltsec->plt_shndx].sh_addr

?


> int i = pltsec->plt_num_entries;
> u64 val = sym->st_value + rela->r_addend;
>
> @@ -43,11 +44,12 @@ u64 module_emit_plt_entry(struct module *mod, void *loc,
> const Elf64_Rela *rela,
> }
>
> #ifdef CONFIG_ARM64_ERRATUM_843419
> -u64 module_emit_veneer_for_adrp(struct module *mod, void *loc, u64 val)
> +u64 module_emit_veneer_for_adrp(struct module *mod, Elf64_Shdr *sechdrs,
> + void *loc, u64 val)
> {
> struct mod_plt_sec *pltsec = !in_init(mod, loc) ? &mod->arch.core :
> &mod->arch.init;
> - struct plt_entry *plt = (struct plt_entry *)pltsec->plt->sh_addr;
> + struct plt_entry *plt = (struct plt_entry *)(sechdrs +
> pltsec->plt_shndx)->sh_addr;
> int i = pltsec->plt_num_entries++;
> u32 mov0, mov1, mov2, br;
> int rd;
> @@ -202,7 +204,7 @@ int module_frob_arch_sections(Elf_Ehdr *ehdr, Elf_Shdr
> *sechdrs,
> unsigned long core_plts = 0;
> unsigned long init_plts = 0;
> Elf64_Sym *syms = NULL;
> - Elf_Shdr *tramp = NULL;
> + Elf_Shdr *pltsec, *tramp = NULL;
> int i;
>
> /*
> @@ -211,9 +213,9 @@ int module_frob_arch_sections(Elf_Ehdr *ehdr, Elf_Shdr
> *sechdrs,
> */
> for (i = 0; i < ehdr->e_shnum; i++) {
> if (!strcmp(secstrings + sechdrs[i].sh_name, ".plt"))
> - mod->arch.core.plt = sechdrs + i;
> + mod->arch.core.plt_shndx = i;
> else if (!strcmp(secstrings + sechdrs[i].sh_name,
> ".init.plt"))
> - mod->arch.init.plt = sechdrs + i;
> + mod->arch.init.plt_shndx = i;
> else if (IS_ENABLED(CONFIG_DYNAMIC_FTRACE) &&
> !strcmp(secstrings + sechdrs[i].sh_name,
> ".text.ftrace_trampoline"))
> @@ -222,7 +224,7 @@ int module_frob_arch_sections(Elf_Ehdr *ehdr, Elf_Shdr
> *sechdrs,
> syms = (Elf64_Sym *)sechdrs[i].sh_addr;
> }
>
> - if (!mod->arch.core.plt || !mod->arch.init.plt) {
> + if (!mod->arch.core.plt_shndx || !mod->arch.init.plt_shndx) {
> pr_err("%s: module PLT section(s) missing\n", mod->name);
> return -ENOEXEC;
> }
> @@ -254,17 +256,19 @@ int module_frob_arch_sections(Elf_Ehdr *ehdr, Elf_Shdr
> *sechdrs,
> sechdrs[i].sh_info, dstsec);
> }
>
> - mod->arch.core.plt->sh_type = SHT_NOBITS;
> - mod->arch.core.plt->sh_flags = SHF_EXECINSTR | SHF_ALLOC;
> - mod->arch.core.plt->sh_addralign = L1_CACHE_BYTES;
> - mod->arch.core.plt->sh_size = (core_plts + 1) * sizeof(struct
> plt_entry);
> + pltsec = sechdrs + mod->arch.core.plt_shndx;
> + pltsec->sh_type = SHT_NOBITS;
> + pltsec->sh_flags = SHF_EXECINSTR | SHF_ALLOC;
> + pltsec->sh_addralign = L1_CACHE_BYTES;
> + pltsec->sh_size = (core_plts + 1) * sizeof(struct plt_entry);
> mod->arch.core.plt_num_entries = 0;
> mod->arch.core.plt_max_entries = core_plts;
>
> - mod->arch.init.plt->sh_type = SHT_NOBITS;
> - mod->arch.init.plt->sh_flags = SHF_EXECINSTR | SHF_ALLOC;
> - mod->arch.init.plt->sh_addralign = L1_CACHE_BYTES;
> - mod->arch.init.plt->sh_size = (init_plts + 1) * sizeof(struct
> plt_entry);
> + pltsec = sechdrs + mod->arch.init.plt_shndx;
> + pltsec->sh_type = SHT_NOBITS;
> + pltsec->sh_flags = SHF_EXECINSTR | SHF_ALLOC;
> + pltsec->sh_addralign = L1_CACHE_BYTES;
> + pltsec->sh_size = (init_plts + 1) * sizeof(struct plt_entry);
> mod->arch.init.plt_num_entries = 0;
> mod->arch.init.plt_max_entries = init_plts;
>
> diff --git a/arch/arm64/kernel/module.c b/arch/arm64/kernel/module.c
> index f0f27aeefb73..c2abe59c6f8f 100644
> --- a/arch/arm64/kernel/module.c
> +++ b/arch/arm64/kernel/module.c
> @@ -198,7 +198,8 @@ static int reloc_insn_imm(enum aarch64_reloc_op op,
> __le32 *place, u64 val,
> return 0;
> }
>
> -static int reloc_insn_adrp(struct module *mod, __le32 *place, u64 val)
> +static int reloc_insn_adrp(struct module *mod, Elf64_Shdr *sechdrs,
> + __le32 *place, u64 val)
> {
> u32 insn;
>
> @@ -215,7 +216,7 @@ static int reloc_insn_adrp(struct module *mod, __le32
> *place, u64 val)
> insn &= ~BIT(31);
> } else {
> /* out of range for ADR -> emit a veneer */
> - val = module_emit_veneer_for_adrp(mod, place, val & ~0xfff);
> + val = module_emit_veneer_for_adrp(mod, sechdrs, place, val &
> ~0xfff);
> if (!val)
> return -ENOEXEC;
> insn = aarch64_insn_gen_branch_imm((u64)place, val,
> @@ -368,7 +369,7 @@ int apply_relocate_add(Elf64_Shdr *sechdrs,
> case R_AARCH64_ADR_PREL_PG_HI21_NC:
> overflow_check = false;
> case R_AARCH64_ADR_PREL_PG_HI21:
> - ovf = reloc_insn_adrp(me, loc, val);
> + ovf = reloc_insn_adrp(me, sechdrs, loc, val);
> if (ovf && ovf != -ERANGE)
> return ovf;
> break;
> @@ -413,7 +414,7 @@ int apply_relocate_add(Elf64_Shdr *sechdrs,
>
> if (IS_ENABLED(CONFIG_ARM64_MODULE_PLTS) &&
> ovf == -ERANGE) {
> - val = module_emit_plt_entry(me, loc,
> &rel[i], sym);
> + val = module_emit_plt_entry(me, sechdrs,
> loc, &rel[i], sym);
> if (!val)
> return -ENOEXEC;
> ovf = reloc_insn_imm(RELOC_OP_PREL, loc,
> val, 2,
> --
> 2.14.5
>