Re: livepatch: reuse module loader code to write relocations

Josh Poimboeuf
2016-03-21T19:24:03Z

On Mon, Mar 21, 2016 at 03:18:32PM -0400, Jessica Yu wrote:
> +++ Miroslav Benes [21/03/16 14:55 +0100]:
> >On Wed, 16 Mar 2016, Jessica Yu wrote:
> >
> >[...]
> >
> >>+struct klp_buf {
> >>+ char symname[KSYM_SYMBOL_LEN];
> >
> >I think it is better to make this KSYM_NAME_LEN. KSYM_SYMBOL_LEN looks
> >like something different and KSYM_NAME_LEN is 128 which you reference
> >below.
> >
>
> Ack, I did mean to use KSYM_NAME_LEN, thanks.
>
> >>+ char objname[MODULE_NAME_LEN];
> >>+};
> >
> >[...]
> >
> >>+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);
> >
> >It would be really nice to change actual values for their macro
> >definitions, but this would be a mess which is not worth it. Anyway
> >shouldn't those width modifiers be %63 and %127 to make a room for \0?
> >
>
> Yes, this is a concern and I'm not sure what the best way to fix it
> is. If both MODULE_NAME_LEN and KSYM_NAME_LEN were straight up
> constants, then I think Josh's stringify approach would have worked
> perfectly. However since MODULE_NAME_LEN translates to an expression
> (64 - sizeof(unsigned long)), which the preprocessor cannot evaluate,
> we will need another approach. Building the format strings at run time
> might be messier than we'd like. Alternatively we could just go the
> simple route and simply be a bit more aggressive on the upper bound
> for the format width; though the size of long varies on different
> architectures, afaik the max size it could ever be on any arch is 8
> bytes, so perhaps 64 - 8 = 56 (then - 1 to make room for \0) might be
> an appropriate field width. This would deserve a comment as well.

I think something like that would be good, along with:

BUILD_BUG_ON(MODULE_NAME_LEN < 56);

and a comment explaining why.

--
Josh