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

[Steven Rostedt]

On Mon, 8 Oct 2018 17:57:57 +0200
Peter Zijlstra <peterz@xxxxxxxxxxxxx> 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 donât 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?

Yes, this is exactly what I was thinking of doing. All we need to do is
have objtool (or a modification of whatever we come up with), to find
the call sites of a specific function (we can have a table to look up
for), that creates a section listing all these call sites, and on boot
up, we can confirm that they are indeed calls (e8 operations).

-- Steve