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SubjectRe: [RFC PATCH v2 1/3] livepatch: Clear relocation targets on a module removal
On Thu 2019-09-05 14:45:12, Miroslav Benes wrote:
> We thus decided to reverse the relocation patching (clear all relocation
> targets on x86_64, or return back nops on powerpc). The solution is not
> universal and is too much arch-specific, but it may prove to be simpler
> in the end.
>
> diff --git a/arch/powerpc/kernel/module_64.c b/arch/powerpc/kernel/module_64.c
> index a93b10c48000..e461d456e447 100644
> --- a/arch/powerpc/kernel/module_64.c
> +++ b/arch/powerpc/kernel/module_64.c
> @@ -741,6 +741,51 @@ int apply_relocate_add(Elf64_Shdr *sechdrs,
> return 0;
> }
>
> +#ifdef CONFIG_LIVEPATCH
> +void clear_relocate_add(Elf64_Shdr *sechdrs,
> + const char *strtab,
> + unsigned int symindex,
> + unsigned int relsec,
> + struct module *me)
> +{
> + unsigned int i;
> + Elf64_Rela *rela = (void *)sechdrs[relsec].sh_addr;
> + Elf64_Sym *sym;
> + unsigned long *location;
> + const char *symname;
> + u32 *instruction;
> +
> + pr_debug("Applying ADD relocate section %u to %u\n", relsec,

s/Applying/Clearing/

> + sechdrs[relsec].sh_info);
> +
> + for (i = 0; i < sechdrs[relsec].sh_size / sizeof(*rela); i++) {
> + location = (void *)sechdrs[sechdrs[relsec].sh_info].sh_addr
> + + rela[i].r_offset;
> + sym = (Elf64_Sym *)sechdrs[symindex].sh_addr
> + + ELF64_R_SYM(rela[i].r_info);
> + symname = me->core_kallsyms.strtab
> + + sym->st_name;
> +
> + if (ELF64_R_TYPE(rela[i].r_info) != R_PPC_REL24)
> + continue;

I expected that the code below would reverse the operations
in apply_relocate_add() for case R_PPC_REL24. But it is not
obvious for me.

It might be because I am not familiar with the code. Or would
it deserve some comments?

> +
> + if (sym->st_shndx != SHN_UNDEF &&
> + sym->st_shndx != SHN_LIVEPATCH)
> + continue;
> +
> + instruction = (u32 *)location;
> + if (is_mprofile_mcount_callsite(symname, instruction))
> + continue;
> +
> + if (!instr_is_relative_link_branch(*instruction))
> + continue;
> +
> + instruction += 1;
> + *instruction = PPC_INST_NOP;
> + }
> +}
> +#endif
> +
> #ifdef CONFIG_DYNAMIC_FTRACE
>
> diff --git a/kernel/livepatch/core.c b/kernel/livepatch/core.c
> index ab4a4606d19b..f0b380d2a17a 100644
> --- a/kernel/livepatch/core.c
> +++ b/kernel/livepatch/core.c
> @@ -295,6 +295,45 @@ static int klp_write_object_relocations(struct module *pmod,
> return ret;
> }
>
> +static void klp_clear_object_relocations(struct module *pmod,
> + struct klp_object *obj)
> +{
> + int i, cnt;
> + const char *objname, *secname;
> + char sec_objname[MODULE_NAME_LEN];
> + Elf_Shdr *sec;
> +
> + objname = klp_is_module(obj) ? obj->name : "vmlinux";
> +
> + /* For each klp relocation section */
> + for (i = 1; i < pmod->klp_info->hdr.e_shnum; i++) {
> + sec = pmod->klp_info->sechdrs + i;
> + secname = pmod->klp_info->secstrings + sec->sh_name;
> + if (!(sec->sh_flags & SHF_RELA_LIVEPATCH))
> + continue;
> +
> + /*
> + * Format: .klp.rela.sec_objname.section_name
> + * See comment in klp_resolve_symbols() for an explanation
> + * of the selected field width value.
> + */
> + secname = pmod->klp_info->secstrings + sec->sh_name;
> + cnt = sscanf(secname, ".klp.rela.%55[^.]", sec_objname);
> + if (cnt != 1) {
> + pr_err("section %s has an incorrectly formatted name\n",
> + secname);
> + continue;
> + }
> +
> + if (strcmp(objname, sec_objname))
> + continue;
> +

It would make the review easier when the order of 1st and 2nd
patch was swaped. I mean that I would not need to check twice
that the two functions actually share the same code.

> + clear_relocate_add(pmod->klp_info->sechdrs,
> + pmod->core_kallsyms.strtab,
> + pmod->klp_info->symndx, i, pmod);
> + }
> +}
> +
> /*
> * Sysfs Interface
> *

I was not able to check correctness of the ppc and s390 parts.
Otherwise, it looks good to me.

Best Regards,
Petr

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