Re: livepatch: reuse module loader code to write relocations

[Josh Poimboeuf <jpoimboe@redhat.com>]

On Thu, Nov 12, 2015 at 04:27:01PM +0100, Miroslav Benes wrote:
> On Wed, 11 Nov 2015, Jessica Yu wrote:
> 
> > +++ Miroslav Benes [11/11/15 15:30 +0100]:
> > > On Mon, 9 Nov 2015, Jessica Yu wrote:
> > >
> > > So I guess we don't need klp_reloc anymore.
> > 
> > Yes, that's correct. I am noticing just now that I forgot to remove
> > the klp_reloc struct definition from livepatch.h. That change will be
> > reflected in v2...
> > 
> > > If true, we should really
> > > start thinking about proper documentation because there are going to be
> > > plenty of assumptions about a patch module and we need to have it written
> > > somewhere. Especially how the relocation sections look like.
> > 
> > Agreed. As a first step the patch module format can perhaps be
> > documented somewhere. Perhaps it's time we create
> > Documentation/livepatch/? :-)
> 
> Yes, I think so.
> 
> > > > diff --git a/kernel/livepatch/core.c b/kernel/livepatch/core.c
> > > > index 087a8c7..26c419f 100644
> > > > --- a/kernel/livepatch/core.c
> > > > +++ b/kernel/livepatch/core.c
> > > > @@ -28,6 +28,8 @@
> > > >  #include <linux/list.h>
> > > >  #include <linux/kallsyms.h>
> > > >  #include <linux/livepatch.h>
> > > > +#include <linux/elf.h>
> > > > +#include <asm/cacheflush.h>
> > > > 
> > > >  /**
> > > >   * struct klp_ops - structure for tracking registered ftrace ops structs
> > > > @@ -281,46 +283,54 @@ static int klp_find_external_symbol(struct module
> > > > *pmod, const char *name,
> > > >  }
> > > > 
> > > >  static int klp_write_object_relocations(struct module *pmod,
> > > > -					struct klp_object *obj)
> > > > +					struct klp_object *obj,
> > > > +					struct klp_patch *patch)
> > > >  {
> > > > -	int ret;
> > > > -	struct klp_reloc *reloc;
> > > > +	int relindex, num_relas;
> > > > +	int i, ret = 0;
> > > > +	unsigned long addr;
> > > > +	unsigned int bind;
> > > > +	char *symname;
> > > > +	struct klp_reloc_sec *reloc_sec;
> > > > +	struct load_info *info;
> > > > +	Elf_Rela *rela;
> > > > +	Elf_Sym *sym, *symtab;
> > > > +	Elf_Shdr *symsect;
> > > > 
> > > >  	if (WARN_ON(!klp_is_object_loaded(obj)))
> > > >  		return -EINVAL;
> > > > 
> > > > -	if (WARN_ON(!obj->relocs))
> > > > -		return -EINVAL;
> > > > -
> > > > -	for (reloc = obj->relocs; reloc->name; reloc++) {
> > > > -		if (!klp_is_module(obj)) {
> > > > -			ret = klp_verify_vmlinux_symbol(reloc->name,
> > > > -							reloc->val);
> > > > -			if (ret)
> > > > -				return ret;
> > > > -		} else {
> > > > -			/* module, reloc->val needs to be discovered */
> > > > -			if (reloc->external)
> > > > -				ret = klp_find_external_symbol(pmod,
> > > > -							       reloc->name,
> > > > -							       &reloc->val);
> > > > -			else
> > > > -				ret = klp_find_object_symbol(obj->mod->name,
> > > > -							     reloc->name,
> > > > -							     &reloc->val);
> > > > -			if (ret)
> > > > -				return ret;
> > > > -		}
> > > > -		ret = klp_write_module_reloc(pmod, reloc->type, reloc->loc,
> > > > -					     reloc->val + reloc->addend);
> > > > -		if (ret) {
> > > > -			pr_err("relocation failed for symbol '%s' at 0x%016lx
> > > > (%d)\n",
> > > > -			       reloc->name, reloc->val, ret);
> > > > -			return ret;
> > > > +	info = pmod->info;
> > > > +	symsect = info->sechdrs + info->index.sym;
> > > > +	symtab = (void *)info->hdr + symsect->sh_offset;
> > > > +
> > > > +	/* For each __klp_rela section for this object */
> > > > +	list_for_each_entry(reloc_sec, &obj->reloc_secs, list) {
> > > > +		relindex = reloc_sec->index;
> > > > +		num_relas = info->sechdrs[relindex].sh_size /
> > > > sizeof(Elf_Rela);
> > > > +		rela = (Elf_Rela *) info->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 = info->strtab + sym->st_name;
> > > > +			bind = ELF_ST_BIND(sym->st_info);
> > > > +
> > > > +			if (sym->st_shndx == SHN_LIVEPATCH) {
> > > > +				if (bind == STB_LIVEPATCH_EXT)
> > > > +					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;
> > > > +			}
> > > >  		}
> > > > +		ret = apply_relocate_add(info->sechdrs, info->strtab,
> > > > +					 info->index.sym, relindex, pmod);
> > > >  	}
> > > > 
> > > > -	return 0;
> > > > +	return ret;
> > > >  }
> > > 
> > > Looking at this... do we even need reloc_secs in klp_object? Question is
> > > whether we need more than one dynrela section for an object. If not then
> > > the binding between klp_reloc_sec and an object is the only relevant thing
> > > in the structure, be it index or objname. So we can replace the
> > > list of structures with just the index in klp_object, or get rid of it
> > > completely and rely on the name of dynrela section be something like
> > > __klp_rela_{objname}.
> > 
> > Hm, you bring up a good point. I think theoretically yes, it is
> > possible to just have one klp_reloc_sec for each object and therefore
> > a list is not required (I have not checked yet how difficult it would
> > be to implement this on the kpatch-build side of things).  However,
> > considering the final format of the patch module, I think it is
> > semantically clearer to leave it as a list, and for each object to
> > possibly have more than one __klp_rela section.
> > 
> > For example, say we are patching two functions in ext4. In my
> > resulting kpatch module I will have two __klp_rela_ext4 sections, and
> > they might look like this when we run readelf --sections:
> > 
> > [34] __klp_rela_ext4.text.ext4_attr_store RELA ...
> > [35] __klp_rela_ext4.text.ext4_attr_show RELA ...
> > 
> > Then these two klp rela sections end up as two elements in the
> > reloc_secs list for the ext4 patch object. I think this way, we can
> > better tell which rela is being applied to what function. Might be
> > easier to understand what's happening from the developer's point of
> > view.
> > 
> > > You see, we go through elf sections here which were preserved by module
> > > loader. We even have relevant sections marked with SHF_RELA_LIVEPATCH. So
> > > maybe all the stuff around klp_reloc_sec is not necessary.
> > > 
> > > Thoughts?
> > 
> > Ah, so this is where descriptive comments and documentation might have
> > been useful :-) So I think we will still need to keep the
> > klp_reloc_sec struct to help the patch module initialize. Though the
> > name and objname fields aren't used in this patchset, they are used in
> > the kpatch patch module code [1], where we iterate through each elf
> > section, find the ones marked with SHF_RELA_LIVEPATCH, set the
> > klp_reloc_sec's objname (which we find from the "name" field,
> > formatted as __klp_rela_{objname}.text..). Once we have the objname
> > set, we can then find the object to attach the reloc_sec to (i.e. add
> > it to its list of reloc_secs).
> > 
> > Hope that clears some things up.
> 
> Ok, I'll try to explain myself and it is gonna be long. I'll try to 
> describe how we deal with dynrelas in klp today, how you use it in kpatch 
> (and this is the place where my knowledge can be wrong or obsolete), what 
> you propose and what I'd like to propose.
> 
> 1. First let's look on what we have now.
> 
> We have a patch module in which there is a section with all dynrelas 
> needed to be resolved (it was like this in kpatch some time ago and maybe 
> it is different now so just have a patience, I'll get to it). In the init 
> function of the module kpatch builds all the relevant info from dynrela 
> section. It goes through it, creates an array of klp_reloc for each object 
> and includes each dynrela record with an objname to the array of 
> klp_object with that objname. Later when we need to apply relocations for 
> patched object we just go through the list (array) of its dynrelas in 
> klp_object and call our arch-specific klp_write_module_reloc().
> 
> Now this was probably changed in kpatch and you do not have one dynrela 
> section but one dynrela section for each patched function. Is that 
> correct? (and can you tell us what the reason for the change was? It might 
> be crucial because I might be missing something.). Which leads us to your 
> proposal...
> 
> 2. So we have several dynrela section for one patched object. During init 
> function in a patch module kpatch once again builds needed structures from 
> these sections. Now they are called klp_reloc_sec and contain different 
> kind of info. There is no val, loc and such, only name of the symbol, 
> objname and index to dynrela section in ELF. So when you need to write 
> relocations for the patched object you go through all relevant dynrela 
> sections (because they are stored in the klp_object), all dynrela records 
> in each section and you resolve the undefined symbols. All needed info is 
> stored in ELF. Then you just call apply_relocate_add().
> 
> 3. I propose to go one step further. I think we don't need klp_reloc_sec 
> if there is only one dynrela section for patched object (and I currently 
> cannot see why this is not possible. It is possible even with one dynrela 
> section for whole patch module, that is for all patched objects.).

I think I agree that we don't need klp_reloc_sec, and that klp rela
sections can presumably be discovered by iterating over the sections.

But I don't think it matters much whether we have one klp rela section
per object, or multiple rela sections per object.  Either way, can't we
find them when we iterate over the sections?

For example, for one rela section per object, it could be named:

__klp_rela.objname

Or for multiple rela sections per object, they could be named:

__klp_rela.objname.func1
__klp_rela.objname.func2

Either way, when iterating over the sections, we could just look for
"__klp_rela.objname".

All that said, I think I would vote for one rela section per object,
just because it seems simpler.

> 
> In my proposal there would be nothing done in init function of the patched 
> module (albeit some optimization mentioned later). When you call 
> klp_write_object_relocations you would go through all ELF section and find 
> the relevant one for the object (it is marked with SHF_RELA_LIVEPATCH and 
> objname is in the name of the section. It is the same thing you do in 2. 
> in the init function.). Then you go through all dynrela records in the 
> section, you do the same things which you do in the proposed patch above, 
> and call apply_relocate_add.
> 
> Now it would be crazy to go through all sections each time 
> klp_write_object_relocations is called (maybe it does not even matter but 

Why would that be crazy?  To me it seems perfectly logical :-)  It
doesn't seem like a very expensive operation to me.

> nevertheless). So klp_object could have an index to its ELF dynrela 
> section. This index can be retrieved in the init function the same way you 
> do all the stuff with klp_reloc_sec.
> 
> If you insisted on more than one dynrela section for a patched object we 
> could have an array of indices there. Or whatever.
> 
> It is almost the same as your proposal but in my opinion somewhat nicer. 
> We just use the info stored in ELF directly without unnecessary middle 
> layer (== klp_reloc_sec).
> 
> Does it make sense? I hope it does. Would it work?

-- 
Josh