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SubjectRe: [POC][RFC][PATCH 1/2] jump_function: Addition of new feature "jump_function"
On Mon, Oct 8, 2018 at 10:30 AM Ard Biesheuvel
<ard.biesheuvel@linaro.org> wrote:
>
> On 8 October 2018 at 19:25, Andy Lutomirski <luto@amacapital.net> wrote:
> > On Mon, Oct 8, 2018 at 9:40 AM Peter Zijlstra <peterz@infradead.org> wrote:
> >>
> >> On Mon, Oct 08, 2018 at 09:29:56AM -0700, Andy Lutomirski wrote:
> >> >
> >> >
> >> > > On Oct 8, 2018, at 8:57 AM, Peter Zijlstra <peterz@infradead.org> wrote:
> >> > >
> >> > > On Mon, Oct 08, 2018 at 01:33:14AM -0700, Andy Lutomirski wrote:
> >> > >>> Can't we hijack the relocation records for these functions before they
> >> > >>> get thrown out in the (final) link pass or something?
> >> > >>
> >> > >> I could be talking out my arse here, but I thought we could do this,
> >> > >> too, then changed my mind. The relocation records give us the
> >> > >> location of the call or jump operand, but they don’t give the address
> >> > >> of the beginning of the instruction.
> >> > >
> >> > > But that's like 1 byte before the operand, right? We could even double check
> >> > > this by reading back that byte and ensuring it is in fact 0xE8 (CALL).
> >> > >
> >> > > AFAICT there is only the _1_ CALL encoding, and that is the 5 byte: E8 <PLT32>,
> >> > > so if we have the PLT32 location, we also have the instruction location. Or am
> >> > > I missing something?
> >> >
> >> > There’s also JMP and Jcc, any of which can be used for rail calls, but
> >> > those are also one byte. I suppose GCC is unlikely to emit a prefixed
> >> > form of any of these. So maybe we really can assume they’re all one
> >> > byte.
> >>
> >> Oh, I had not considered tail calls..
> >>
> >> > But there is a nasty potential special case: anything that takes the
> >> > function’s address. This includes jump tables, computed gotos, and
> >> > plain old function pointers. And I suspect that any of these could
> >> > have one of the rather large number of CALL/JMP/Jcc bytes before the
> >> > relocation by coincidence.
> >>
> >> We can have objtool verify the CALL/JMP/Jcc only condition. So if
> >> someone tries to take the address of a patchable function, it will error
> >> out.
> >
> > I think we should just ignore the sites that take the address and
> > maybe issue a warning. After all, GCC can create them all by itself.
> > We'll always have a plain wrapper function, and I think we should just
> > not patch code that takes its address. So we do, roughly:
> >
> > void default_foo(void);
> >
> > GLOBAL(foo)
> > jmp *current_foo(%rip)
> > ENDPROC(foo)
> >
> > And code that does:
> >
> > foo();
> >
> > as a call, a tail call, a conditional tail call, etc, gets discovered
> > by objtool + relocation processing or whatever and gets patched. (And
> > foo() itself gets patched, too, as a special case. But we patch foo
> > itself at some point during boot to turn it into a direct JMP. Doing
> > it this way means that the whole mechanism works from very early
> > boot.)
>
> Does that mean that architectures could opt out of doing the whole
> objtool + relocation processing thing, and instead take the hit of
> going through the trampoline for all calls?
>

I don't see why not.

--Andy

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