Re: [POC][RFC][PATCH 1/2] jump_function: Addition of new feature "jump_function"

[Peter Zijlstra <peterz@infradead.org>]

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.

Heck, it could initially even error out on tail calls.