Re: [PATCH] Make _Unwind_GetIPInfo part of the ABI

[Michael Matz <matz@suse.de>]

Hi,

On Fri, 21 Oct 2016, Florian Weimer wrote:

> On 10/21/2016 02:58 PM, Michael Matz wrote:
> > +This function returns the same value as \code{\_Unwind\_GetIP}.  In
> > +addition, the argument \code{ip\_before\_insn} must not be not null, and
> > +\code{*ip\_before\_insn} is updated with a flag which indicates whether
> > +the returned pointer is at or after the first not yet fully executed
> > +instruction.
> 
> I think this is rather misleading.  On x86_64, the location of the IP 
> value is the same for calls and asynchronous signals: it always points 
> to the next instruction to be executed.

No, that's simply wrong.  The saved instruction pointer points _at_ the 
instruction causing the fault for faults, and _after_ the instruction for 
traps.  Traps are things like single-stepping, breakpoints or INTO.  Most 
other interrupts are faults or aborts (the latter being imprecise and 
hence can't be restarted anyway).

For calls the saved instruction pointer always points to after the call 
and hence can be handled like a trap for unwinding purposes.

> There are no partially executed instructions.

That's not 100% correct either (e.g. certain load-state instructions can 
be interrupted in the middle, though that usually just causes a double 
fault).  But in the interest of being clearer, I guess I should have 
written "not yet completed" instruction, instead of that "fully executed" 
part.

> The difference that if we unwind through a call which has not yet 
> returned, the caller is assumed to be still within the exception 
> handling region in which the call instruction is located.  This is the 
> consequence of the desired exception handling semantics of a 
> non-returned function call.

Unwinding through one call or one trap is the same.  The interesting 
instruction is the one ending right before the reported IP.

> It is not directly related to the instruction pointer value returned by 
> _Unwind_GetIPInfo.

Yes it is.  GetIPInfo always returns the instruction pointer as encoded in 
the given unwind context (like GetIP itself).  That's exactly the one 
that's also stored on the stack (well, on x86-64 at least, for other 
architectures it might be stored in a register and might be in encoded 
form), and is the one to be used to look up exception regions _except_ 
that you normally need to subtract one from it, because the IP stored in 
the context and stack points to after the insn you're interested in.  
Except for those situations where it doesn't, for which this function was 
introduced to start with, in order to be able to differ between those 
(basically the kernel needs to mark the signal frame as being the result 
of a fault or a trap, and GetIPInfo uses this to set the flag).


Ciao,
Michael.