Re: [PATCH v6] livepatch: Clear relocation targets on a module removal

Song Liu
2022-11-29T01:57:23Z

On Fri, Nov 18, 2022 at 8:24 AM Petr Mladek <pmladek@suse.com> wrote:
>
[...]

> >
> > +#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("Clearing ADD relocate section %u to %u\n", relsec,
> > + 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;
> > + /*
> > + * reverse the operations in apply_relocate_add() for case
> > + * R_PPC_REL24.
> > + */
> > + if (sym->st_shndx != SHN_UNDEF &&
> > + sym->st_shndx != SHN_LIVEPATCH)
> > + continue;
> > +
> > + instruction = (u32 *)location;
> > + if (is_mprofile_ftrace_call(symname))
> > + continue;
> > +
> > + if (!instr_is_relative_link_branch(ppc_inst(*instruction)))
> > + continue;
> > +
> > + instruction += 1;
> > + patch_instruction(instruction, ppc_inst(PPC_RAW_NOP()));
> > + }
> > +
> > +}
>
> This looks like a lot of duplicated code. Isn't it?

TBH, I think the duplicated code is not really bad.

apply_relocate_add() is a much more complicated function, I would
rather not mess it up to make this function a little simpler.

[...]

>
> This duplicates a lot of code. Please, rename apply_relocate_add() the
> same way as __apply_clear_relocate_add() and add the "apply" parameter.
> Then add the wrappers for this:
>
> int write_relocate_add(Elf64_Shdr *sechdrs,
> const char *strtab,
> unsigned int symindex,
> unsigned int relsec,
> struct module *me,
> bool apply)
> {
> int ret;
> bool early = me->state == MODULE_STATE_UNFORMED;
> void *(*write)(void *, const void *, size_t) = memcpy;
>
> if (!early) {
> write = text_poke;
> mutex_lock(&text_mutex);
> }

How about we move the "early" logic into __write_relocate_add()?

>
> ret = __write_relocate_add(sechdrs, strtab, symindex, relsec, me,
> write, apply);
>
> if (!early) {
> text_poke_sync();
> mutex_unlock(&text_mutex);
> }
>
> return ret;
> }
>
> int apply_relocate_add(Elf64_Shdr *sechdrs,
> const char *strtab,
> unsigned int symindex,
> unsigned int relsec,
> struct module *me)
> {
> return write_relocate_add(sechdrs, strtab, symindex, relsec, me, true);

Then we just call __write_relocate_add() from here...

> }
>
> #ifdef CONFIG_LIVEPATCH
> void apply_relocate_add(Elf64_Shdr *sechdrs,
> const char *strtab,
> unsigned int symindex,
> unsigned int relsec,
> struct module *me)
> {
> write_relocate_add(sechdrs, strtab, symindex, relsec, me, false);

and here.

> }
> #endif
>
>
> > +#endif
> > +
> > #endif
> >
> > int module_finalize(const Elf_Ehdr *hdr,
> > --- a/kernel/livepatch/core.c
> > +++ b/kernel/livepatch/core.c
> > @@ -316,6 +316,45 @@ int klp_apply_section_relocs(struct module *pmod, Elf_Shdr *sechdrs,
> > return apply_relocate_add(sechdrs, strtab, symndx, secndx, pmod);
> > }
> >
> > +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;
> > + }

Actually, I think we don't need the cnt check here. Once it is removed,
there isn't much duplicated logic.

> > +
> > + if (strcmp(objname, sec_objname))
> > + continue;
> > +
> > + clear_relocate_add(pmod->klp_info->sechdrs,
> > + pmod->core_kallsyms.strtab,
> > + pmod->klp_info->symndx, i, pmod);
> > + }
> > +}
>
> Huh, this duplicates a lot of tricky code.
>
> It is even worse because this squashed code from two functions
> klp_apply_section_relocs() and klp_apply_object_relocs()
> into a single function. As a result, the code duplication is not
> even obvious.
>
> Also the suffix "_reloacations() does not match the suffix of
> the related funciton:
>
> + klp_apply_object_relocs() (existing)
> + klp_clear_object_relocations() (new)
>
> This all would complicate maintenance of the code.
>
> Please, implement a common:
>
> int klp_write_section_relocs(struct module *pmod, Elf_Shdr *sechdrs,
> const char *shstrtab, const char *strtab,
> unsigned int symndx, unsigned int secndx,
> const char *objname, bool apply);
>
> and
>
> int klp_write_object_relocs(struct klp_patch *patch,
> struct klp_object *obj,
> bool apply);
>
> and add the respective wrappers:
>
> int klp_apply_section_relocs(); /* also needed in module/main.c */
> int klp_apply_object_relocs();
> void klp_clear_object_relocs();

With the above simplification (removing cnt check), do we still need
all these wrappers? Personally, I think they will make the code more
difficult to follow..

Thanks,
Song