Re: [PATCH v2] arm64/module: use mod->klp_info section header information for livepatch modules

[Miroslav Benes]

> >Does this mean we can drop the plt pointer from this struct altogether, and
> >simply offset into the section headers when applying the relocations?
> 
> Hmm, if everyone is OK with dropping the plt pointer from struct
> mod_plt_sec, then I think we can simplify this patch even further.
> 
> With the plt shndx saved, we can additionally pass a pointer to
> sechdrs to module_emit_plt_entry(), and with that just offset into the
> section headers as you suggest. Since livepatch *already* passes in
> the correct copy of the section headers (mod->klp_info->sechdrs) to
> apply_relocate_add(), we wouldn't even need to modify the arm64
> module_finalize() to change mod->arch.core.plt to point into
> mod->klp_info->sechdrs anymore and we can drop all the changes to the
> module loader too.
> 
> Something like the following maybe?
> 
> diff --git a/arch/arm64/include/asm/module.h b/arch/arm64/include/asm/module.h
> index fef773c94e9d..ac10fa066487 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;
> };
> @@ -37,10 +37,12 @@ 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..3cd744a1cbc2 100644
> --- a/arch/arm64/kernel/module-plts.c
> +++ b/arch/arm64/kernel/module-plts.c
> @@ -16,13 +16,15 @@ 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;
> -	int i = pltsec->plt_num_entries;
> +	struct mod_plt_sec *plt_info = !in_init(mod, loc) ? &mod->arch.core :
> +							    &mod->arch.init;
> +	Elf64_Shdr *pltsec = sechdrs + plt_info->plt_shndx;
> +	struct plt_entry *plt = (struct plt_entry *)pltsec->sh_addr;
> +	int i = plt_info->plt_num_entries;
> 	u64 val = sym->st_value + rela->r_addend;
> 
> 	plt[i] = get_plt_entry(val);
> @@ -35,24 +37,26 @@ u64 module_emit_plt_entry(struct module *mod, void *loc,
> const Elf64_Rela *rela,
> 	if (i > 0 && plt_entries_equal(plt + i, plt + i - 1))
> 		return (u64)&plt[i - 1];
> 
> -	pltsec->plt_num_entries++;
> -	if (WARN_ON(pltsec->plt_num_entries > pltsec->plt_max_entries))
> +	plt_info->plt_num_entries++;
> +	if (WARN_ON(plt_info->plt_num_entries > plt_info->plt_max_entries))
> 		return 0;
> 
> 	return (u64)&plt[i];
> }
> 
> #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;
> -	int i = pltsec->plt_num_entries++;
> +	struct mod_plt_sec *plt_info = !in_init(mod, loc) ? &mod->arch.core :
> +							    &mod->arch.init;
> +	Elf64_Shdr *pltsec = sechdrs + plt_info->plt_shndx;
> +	struct plt_entry *plt = (struct plt_entry *)pltsec->sh_addr;
> +	int i = plt_info->plt_num_entries++;
> 	u32 mov0, mov1, mov2, br;
> 	int rd;
> 
> -	if (WARN_ON(pltsec->plt_num_entries > pltsec->plt_max_entries))
> +	if (WARN_ON(plt_info->plt_num_entries > plt_info->plt_max_entries))
> 		return 0;
> 
> 	/* get the destination register of the ADRP instruction */
> @@ -202,7 +206,7 @@ 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 +215,9 @@ 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 +226,7 @@ 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 +258,19 @@ 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 dd23655fda3a..8e6444db2d8e 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,
> 
> Perhaps this approach is better. Miroslav and Petr, do you think this
> would work? (Apologies for the efforts to review the last two
> versions, if we end up scrapping the old patch :-/)

No problem. I think it should work and it looks good to me (I did not 
compile it though). I'm glad we don't have to touch load_module(). The 
function is complicated enough.

Thanks,
Miroslav