Re: [PATCH] Make _Unwind_GetIPInfo part of the ABI

[Florian Weimer <fweimer@redhat.com>]

On 10/21/2016 03:45 PM, Michael Matz wrote:
> 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.

Oh, then we are dealing with four different things: Calls, asynchronous 
signals (like the internal SIGCANCEL signal, which is how came to this 
topic), faults, and traps.

Using your terminology, traps are like calls (IP adjustment needed).  An 
SIGCANCEL signal is like a fault because no IP adjustment is allowed. 
The GCC unwinders currently treat all the signals the same, which causes 
it to use the wrong handler region for traps.

>> 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.

Agreed.

> 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).

It's not really clear to me how the glibc unwinders tell traps from faults.

Florian