Message-ID: <20151210220703.GB6553@packer-debian-8-amd64.digitalocean.com>
Date:	Thu, 10 Dec 2015 17:07:03 -0500
From:	Jessica Yu <jeyu@...hat.com>
To:	Josh Poimboeuf <jpoimboe@...hat.com>
Cc:	Rusty Russell <rusty@...tcorp.com.au>,
	Seth Jennings <sjenning@...hat.com>,
	Jiri Kosina <jikos@...nel.org>,
	Vojtech Pavlik <vojtech@...e.com>,
	Jonathan Corbet <corbet@....net>,
	Miroslav Benes <mbenes@...e.cz>, linux-api@...r.kernel.org,
	live-patching@...r.kernel.org, x86@...nel.org,
	linux-kernel@...r.kernel.org, linux-s390@...r.kernel.org,
	linux-doc@...r.kernel.org
Subject: Re: livepatch: reuse module loader code to write relocations

+++ Josh Poimboeuf [10/12/15 15:41 -0600]:
>On Thu, Dec 10, 2015 at 04:33:29PM -0500, Jessica Yu wrote:
>> +++ Josh Poimboeuf [10/12/15 08:28 -0600]:
>> >On Wed, Dec 09, 2015 at 02:10:14PM -0500, Jessica Yu wrote:
>> >>+++ Josh Poimboeuf [08/12/15 12:38 -0600]:
>> >>>>+	/* For each __klp_rela section for this object */
>> >>>>+	klp_for_each_reloc_sec(obj, reloc_sec) {
>> >>>>+		relindex = reloc_sec->index;
>> >>>>+		num_relas = pmod->sechdrs[relindex].sh_size / sizeof(Elf_Rela);
>> >>>>+		rela = (Elf_Rela *) pmod->sechdrs[relindex].sh_addr;
>> >>>>+
>> >>>>+		/* For each rela in this __klp_rela section */
>> >>>>+		for (i = 0; i < num_relas; i++, rela++) {
>> >>>>+			sym = symtab + ELF_R_SYM(rela->r_info);
>> >>>>+			symname = pmod->core_strtab + sym->st_name;
>> >>>>+
>> >>>>+			if (sym->st_shndx == SHN_LIVEPATCH) {
>> >>>>+				if (sym->st_info == 'K')
>> >>>>+					ret = klp_find_external_symbol(pmod, symname, &addr);
>> >>>>+				else
>> >>>>+					ret = klp_find_object_symbol(obj->name, symname, &addr);
>> >>>>+				if (ret)
>> >>>>+					return ret;
>> >>>>+				sym->st_value = addr;
>> >>>
>> >>>So I think you're also working on removing the 'external' stuff.  Any
>> >>>idea how this code will handle that?  Specifically I'm wondering how the
>> >>>objname and sympos of the rela's symbol will be specified.  Maybe it can
>> >>>be encoded somehow in one of the symbol fields (st_value)?
>> >>
>> >>Thanks for bringing this up. I think we can just encode the symbol's
>> >>position in kallsyms in the symbol's st_other field. It isn't used
>> >>anywhere and has size char, which is plenty of bits to represent the
>> >>small ints.
>> >
>> >st_other does seem to at least have some trivial usage in the kernel,
>> >see print_absolute_symbols() and sym_visibility() in
>> >arch/x86/tools/relocs.c.  Two of the bits are used to specify the
>> >"visibility" of a symbol.  Also readelf shows a "Vis" column in the
>> >symbol table.
>>
>> Yeah, for x86 it looks like st_other is used only for SHN_ABS symbols
>> in print_absolute_symbols(). Technically SHN_LIVEPATCH symbols
>> shouldn't be affected in this case...but despite its sparse usage in the
>> kernel it does look like using st_other to encode sympos is out of the
>> question as its meaning is architecture specific..
>>
>> >>For objname, the simplest solution might be to append ".klp.objname"
>> >>to the symbol name, and extract out this suffix when resolving the
>> >>symbol. Another way might be to have st_value contain the index into
>> >>the strtab (or .kpatch.strings) that contains the objname. Then we'd
>> >>access the objname just like how we access the symbol's name (strtab +
>> >>sym->st_name). After we find the objname we can then overwrite
>> >>st_value with the real address. I think this second method is cleaner.
>> >>Thoughts?
>> >
>> >Yeah, I guess there are a lot of possibilities for ways to encode it.
>> >
>> >Personally I think it would be nice if the encoding were something that
>> >could easily be seen when dumping the symbol table with readelf.  So,
>> >for example, the objname could be encoded in the symbol name (as you
>> >suggested), and the sympos could be in st_value.
>>
>> Sure, that should be doable. So the new process might look like this:
>>
>> For every livepatch symbol referenced by a rela..
>>
>> 1) Save the sympos encoded in st_value
>>
>> 2) Save the sym_objname that is encoded in the symbol's name with the
>> 'klp' suffix (Just to clarify: the sym_objname specifies the object
>> in which the symbol lives, and recall that we need this to remove the
>> need for the "external" flag)
>>
>> 3) Resolve the symbol by using its name (without the klp suffix),
>> sympos, and sym_objname
>>
>> 4) Set st_value to the found address
>
>Sounds right to me.
>
>> >If we do that, it'd probably be good to keep the naming consistent with
>> >the '__klp_rela_objname.symname' thing.  So something like
>> >'_klp_sym_objname.symname'.
>>
>> How about 'symname.klp.objname', and renaming the klp reloc sections
>> to '.klp.objname.rela.section_name'? Special symbol suffixes and
>> section names seem to always use '.', so maybe this would look better?
>> :-) But we can keep the underscores if people like that more. Both
>> naming methods would work, it is only a matter of preference.
>
>It's your patches, so I'd say you get to pick ;-)  My only request would
>be some consistency between the symbol names and the rela section names.
>
>> >But... would there be any side effects associated with renaming it?  For
>> >example, would it cause problems with the s390 PLT?
>
>Just to verify, did you see this question? :-)

Ah, sorry I missed this! So I looked through the module loader code
for x86, s390, arm (and just grepped in a few other archs) and the
symbol's name (st_name) is only ever used in printing error or debug
messages, or not used at all. As long as livepatch correctly resolves
the symbol, and st_value is set, apply_relocate_add() doesn't really
care about the name.

s390 plt/got information (struct mod_arch_syminfo) for each symbol is
stored in an array in mod->arch.syminfo. Every symbol that is in the
module's symbol table will get an entry, but not every symbol will
have their plt/got offsets filled in. Every symbol referenced by a
PLT/GOT rela will have those offsets filled in. Since our livepatch
symbols are real symbols, with indexes and entries in the symbol
table, they will get their own s390 mod_arch_syminfo struct, and
should any PLT rela reference them, a PLT entry as well, which is
good. Tldr, names of symbols don't affect s390 plt/got handling, so we
are safe.


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