Re: uprobes and empty functions

[Josh Stone <jistone at redhat dot com>]

On 11/03/2010 04:59 PM, Jim Keniston wrote:
> I wonder if this is the same problem reported in bugzilla #11249 (which
> never got resolved, as far as I can tell).  In comment #5 there, I
> speculated as follows:
> 
> The general protection fault on the call suggests that the destination
> address
> of the call is bad.  (I think the IP is left at the faulting instruction.)  The
> destination is computed by adding the displacement in the call instruction to
> the address of the next instruction.  If you're in a big vma (e.g.,
> libc-x.x.so), that displacement can be big.  If you move the call instruction to
> the (1-page) SSOL area, that big displacement can take you out of the SSOL vma,
> to a potentially unmapped address.  Does that warrant a GP fault?  Uprobes
> adjusts the IP after the call, so it's back in the probed vma, but by then it
> may be too late.

Interesting... the Intel manual has this pseudo-code for near relative:

  IF OperandSize = 32
    THEN
      tempEIP <- EIP + DEST; (* DEST is rel32 *)
      IF tempEIP is not within code segment limit THEN #GP(0); FI;
      IF stack not large enough for a 4-byte return address
        THEN #SS(0); FI;
      Push(EIP);
      EIP <- tempEIP;
  FI;

So there's a specified GP if tempEIP is out of range.  There's not the
same check under OperandSize=64, so maybe that's why x86_64 is working
ok for me.  There are other reasons given for GP too, but I'm not sure
whether any of those could be a side-effect of SSOL.

> Sorry, that's my only idea.  It looks interesting, but uprobes and stap
> are no longer in my bailiwick.

In #11249, you also mention that you have an idea of how to support
relative calls and jumps -- care to elaborate?  Can we just convert them
to absolute?  Or if the SSOL area is close enough, could we just give
them a new displacement from SSOL to the right target?

I understand you're no longer working in this area, so thanks for your
input so far!  Glad we could keep you interested... ;)

Josh