Re: [PATCH v5 4/6] livepatch: reuse module loader code to write relocations

Petr Mladek
2016-03-21T16:31:57Z

On Wed 2016-03-16 15:47:06, Jessica Yu wrote:
> Reuse module loader code to write relocations, thereby eliminating the need
> for architecture specific relocation code in livepatch. Specifically, reuse
> the apply_relocate_add() function in the module loader to write relocations
> instead of duplicating functionality in livepatch's arch-dependent
> klp_write_module_reloc() function.
>
> In order to accomplish this, livepatch modules manage their own relocation
> sections (marked with the SHF_RELA_LIVEPATCH section flag) and
> livepatch-specific symbols (marked with SHN_LIVEPATCH symbol section
> index). To apply livepatch relocation sections, livepatch symbols
> referenced by relocs are resolved and then apply_relocate_add() is called
> to apply those relocations.
>
> In addition, remove x86 livepatch relocation code and the s390
> klp_write_module_reloc() function stub. They are no longer needed since
> relocation work has been offloaded to module loader.

Most of the problems were covered by Mirek and Josh. I agree with
them. Please read two more comments below.

> diff --git a/kernel/livepatch/core.c b/kernel/livepatch/core.c
> index 780f00c..2aa20fa 100644
> --- a/kernel/livepatch/core.c
> +++ b/kernel/livepatch/core.c
> +static int klp_resolve_symbols(Elf_Shdr *relasec, struct module *pmod)
> +{
> + int i, cnt, vmlinux, ret;
> + struct klp_buf bufs = {0};
> + Elf_Rela *relas;
> + Elf_Sym *sym;
> + char *symname;
> + unsigned long sympos;
> +
> + relas = (Elf_Rela *) relasec->sh_addr;
> + /* For each rela in this klp relocation section */
> + for (i = 0; i < relasec->sh_size / sizeof(Elf_Rela); i++) {
> + sym = pmod->core_kallsyms.symtab + ELF_R_SYM(relas[i].r_info);
> + if (sym->st_shndx != SHN_LIVEPATCH)
> + return -EINVAL;
> +
> + klp_clear_buf(&bufs);
> +
> + /* Format: .klp.sym.objname.symbol_name,sympos */
> + symname = pmod->core_kallsyms.strtab + sym->st_name;
> + cnt = sscanf(symname, ".klp.sym.%64[^.].%128[^,],%lu",
> + bufs.objname, bufs.symname, &sympos);

Note that MODULE_NAME_LEN even is not 64. It is defined by:

#define MAX_PARAM_PREFIX_LEN (64 - sizeof(unsigned long))

I strongly suggest to use the proposal from Josh.

> + if (cnt != 3)
> + return -EINVAL;
> +
> + /* klp_find_object_symbol() treats a NULL objname as vmlinux */
> + vmlinux = !strcmp(bufs.objname, "vmlinux");
> + ret = klp_find_object_symbol(vmlinux ? NULL : bufs.objname,
> + bufs.symname, sympos,
> + (unsigned long *) &sym->st_value);
> + if (ret)
> + return ret;
> }
> - preempt_enable();
>
> - /*
> - * Check if it's in another .o within the patch module. This also
> - * checks that the external symbol is unique.
> - */
> - return klp_find_object_symbol(pmod->name, name, 0, addr);
> + return 0;
> }

[...]
> @@ -842,6 +867,9 @@ int klp_register_patch(struct klp_patch *patch)
> {
> int ret;
>
> + if (!is_livepatch_module(patch->mod))
> + return -EINVAL;
> +

This breaks bisectability if livepatch-sample is used. Please, merge
the 5th patch here or move it before this one.

Best Regards,
Petr