From: Michael Matz <matz@suse.de>
To: Florian Weimer <fweimer@redhat.com>
Cc: "H.J. Lu" <hjl.tools@gmail.com>, gnu-gabi@sourceware.org
Subject: Re: [PATCH] Make _Unwind_GetIPInfo part of the ABI
Date: Fri, 01 Jan 2016 00:00:00 -0000 [thread overview]
Message-ID: <alpine.LSU.2.20.1610211521400.5714@wotan.suse.de> (raw)
In-Reply-To: <fd505b01-3f78-b0ea-6afa-3e3e713b7e54@redhat.com>
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.
next prev parent reply other threads:[~2016-10-21 13:45 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-01-01 0:00 Florian Weimer
2016-01-01 0:00 ` Michael Matz
2016-01-01 0:00 ` Florian Weimer
2016-01-01 0:00 ` Michael Matz [this message]
2016-01-01 0:00 ` Michael Matz
2016-01-01 0:00 ` Carlos O'Donell
2016-01-01 0:00 ` Florian Weimer
2016-01-01 0:00 ` H.J. Lu
2016-01-01 0:00 ` Florian Weimer
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