Re: livepatch: reuse module loader code to write relocations
From: Jessica Yu
Date: Wed Dec 16 2015 - 14:14:37 EST
+++ Miroslav Benes [16/12/15 13:59 +0100]:
On Wed, 16 Dec 2015, Jessica Yu wrote:
+++ Jessica Yu [09/12/15 14:10 -0500]:
> +++ Josh Poimboeuf [08/12/15 12:38 -0600]:
> >
> > There was a lot of discussion for v1, so I'm not sure, but I thought we
> > ended up deciding to get rid of the klp_reloc_sec struct? Instead I
> > think the symbol table can be walked directly looking for klp rela
> > sections?
> >
> > The benefits of doing so would be to make the interface simpler -- no
> > need to "cache" the section metadata when it's already easily and
> > quickly available in the module struct elf section headers.
>
> Ah, I might have interpreted the conclusions of that last discussion
> incorrectly.
>
> In that case, I think I can just get rid of my klp_for_each_reloc_sec
> macro as well as the lreloc scaffolding code from kpatch. The only
> potentially "ugly" part of this change is that I'll have to move the
> string parsing stuff here to extract the objname of the __klp_rela
> section (which may not actually look that bad, we'll see how that
> turns out).
Yes, that was a conclusion we made. At least I thought so :)
Turns out the string parsing stuff, even with the help of lib/string.c,
doesn't
look very pretty. As I'm working on v3, I'm starting to think having
klp_write_object_relocations() loop simply through all the elf sections might
not be a good idea. Let me explain.
Hm, I still think it is worth it...
I don't like the amount of string manipulation code that would potentially
come
with this change. Even with a string as simple as ".klp.rela.objname", we'll
end up with a bunch of kstrdup's/kmalloc's and kfree's (unless we modify and
chop the section name string in place, which I don't think we should do) that
are going to be required at every iteration of the loop, all just to be able
to
call strcmp() and see if we're dealing with a klp rela section that belongs to
the object in question. This also leads to more complicated error handling.
But this shouldn't be much different from what init function of the patch
module does now, right? You have a klp_extract_objname() which I thought
could be moved right to livepatch code.
First, thanks for the comments. I think my only real gripe is that, in the
pseudo code I provided, we are not only doing the for loop for each object, but
also a bunch of kmalloc's/kstrdup's and kfree's just to be able to check if
this klp section belongs to the current object, *and* also kmallocing/kfreeing
just to provide buffers to extract parts of a symbol's name. In the patch
module code, each klp section was parsed just once and sorted by object:
https://github.com/flaming-toast/kpatch/blob/no_dynrela_redux/kmod/patch/livepatch-patch-hook.c#L213
^^^ This was before we decided to get rid of 'external', so only klp sections
had to be parsed. In v3, we are also looking to parse the symbol name (to
eliminate 'external'), so now I'm just trying to reduce all the string stuff to
the bare minimum needed to have this work.
function klp_write_object_relocations(..,struct klp_object *obj,..) {
for each section in mod->sechdrs { // iterate through all elf sections, no more obj->reloc_secs list
if not a klp rela section
continue;
-> sec_objname = extract_objname_from_section(section); // .klp.rela.[objname].sectionname
-> if (strcmp(sec_objname, obj->name)) { // this klp rela section doesn't belong to this object
-> kfree(sec_objname);
-> continue;
}
for each rela in this klp rela section {
sym = symtab + ELF_R_SYM(rela->r_info);
-> sym_objname = extract_objname_from_symbol(sym) // symname.klp.[objname]
-> if (!sym_objname)
-> goto handle_error; // calls kfree(sec_objname);
-> symname = extract_symname_from_symbol(sym); // [symname].klp.objname
-> if (!symname)
-> goto handle_error; // calls kfree(sec_objname);
ret = klp_find_object_symbol(sym_objname, symname, &addr)
if (ret)
-> goto handle_error2; // calls kfree(symname), then kfree(sec_objname)
sym->st_value = addr;
-> if (sym_objname) // free buffers before proceeding to next iteration of loop
-> kfree(sym_objname);
-> kfree(symname);
}
[ snipped rest of function ]
I'm trying to figure out a way to simplify all the lines marked with '->', and
make this code as readable and palatable as possible. I think primarily the
kfree's and multiple error-handling goto's bothered me, which is why I
suggested putting the symname.klp.[objname] part in .kpatch.strings and using
those masks I suggested. Then we wouldn't even need to set-up temporary buffers to
hold the sym_objname + symname strings. You're right that this would require
another assumption about the patch module, but I think having the names of
livepatch symbols formatted as 'symname.klp.objname' was already another
assumption, and using .kpatch.strings would just be changing this assumption.
In v1, the string parsing was done only *once* for each klp rela section in
the
patch module code, and each klp rela section is sorted into their respective
object with the reloc_secs list. Then all klp_write_object_relocations() had
to
do is iterate over object->reloc_secs. The tradeoff here is the addition of
another struct (klp_reloc_sec), but that is not nearly as bad as string
parsing, which is imo way more error prone and is unnecessary extra work.
If it is a problem (and Josh thought it was not if I recall correctly) we
can cache indices to relevant rela sections in klp_object as we discussed
in v1.
Here's some pseudocode to help visualize the potential issues:
function klp_write_object_relocations(..,struct klp_object *obj,..) {
for each section in mod->sechdrs { // iterate through all elf sections, no
more obj->reloc_secs list
if not a klp rela section
continue;
sec_objname = extract_objname_from_section(section); //
.klp.rela.[objname].sectionname
if (strcmp(sec_objname, obj->name)) { // this klp rela section doesn't
belong to this object
kfree(sec_objname);
continue;
}
for each rela in this klp rela section {
sym = symtab + ELF_R_SYM(rela->r_info);
sym_objname = extract_objname_from_symbol(sym) //
symname.klp.[objname]
This is because of 'external' stuff, right?
Correct, in the v2 discussion we decided symbol names can have the
names of their objects attached, which will eliminate the need for the
external flag and klp_find_external_symbol().
if (!sym_objname)
goto handle_error; // calls kfree(sec_objname);
symname = extract_symname_from_symbol(sym); // [symname].klp.objname
Can't we use a method from the patch? That is
symname = pmod->core_strtab + sym->st_name;
Since we are removing 'external', we decided to append the objname to the
symbol name. But then extra string parsing is needed to get the real symbol
name. So, pmod->core_strtab + sym->st_name would correspond to the full name,
formatted as 'symname.klp.objname'. We parse to get the real symname and the
objname. Then we no longer need the klp_find_external_symbol() call, it'd just
be one call to klp_find_object_symbol(sym_objname, symname, addr)
if (!symname)
goto handle_error; // calls kfree(sec_objname);
ret = klp_find_object_symbol(sym_objname, symname, &addr)
if (ret)
goto handle_error2; // calls kfree(symname), then
kfree(sec_objname)
...etc., you get the idea how messy that is getting, and we haven't even
reached the call to apply_relocate_add() yet.
So I personally think it is better to keep the old v1 way for applying the klp
reloc secs. The sections are still named ".klp.rela.objname" but the parsing
is
done once in the patch module code and used only to build the patch
scaffolding
structure.
In order to avoid adding any additional string parsing code in v3, I no longer
thing we should rename livepatch symbols to include the symbol objname (i.e.
'symbol.klp.objname'). Recall that this change is for getting rid of external
dynrelas. Instead of parsing the symbol name, we could just encode 1) the
sympos and 2) the index into the .kpatch.strings strtab (that contains the
sym_objname) in sym->st_value. We define two masks (KLP_MASK_SYMPOS,
KLP_MASK_OBJNAME) that extract these values. st_value has either a 32 bit or
64
bit size, both of which are big enough sizes for the sympos and string table
index. One perk we lose is being able to see which object a symbol belongs to
in readelf, but then again we didn't have this benefit to begin with in
v1 nor v2 (where we didn't know the symbol's object and had to use
klp_find_external_symbol anyway), so I wouldn't say it's a loss.
Just a note. This would lead to another assumption about a patch
module... .kpatch.strings. I suspect 'external' symbols are kpatch
specific (I think I did not deal with it while working on relocations for
kgraft. But I am not sure now. I haven't finished it too.), but let's try
to make it "generic".
I think 'external' is kpatch-specific (correct me if I'm wrong), as I
explained above we're trying to get rid of it.
Apologies for the super long explanations, but I think avoiding string parsing
in livepatch core code will make the relocation code much more succinct and
easier to read. Any objections or thoughts on all this?
My main argument is that the work needs to be done somewhere and there is
no reason why not do it right in the kernel and not in patch modules. The
code should be more or less the same and we'd get rid of unnecessary
layers (klp_reloc_sec) as Josh pointed out.
Right, OK. I agree that this work has to be done somewhere, I am just hesitant
about the new string handling code, as I pointed out above. If people are OK and
agree that this is the better approach, then I will keep it.
Thanks again,
Jessica
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