Re: RFC: Add a mechanism to stop backtraces using dwarf2 frame information

Daniel Jacobowitz
2005-04-08T12:10:39Z

On Fri, Mar 04, 2005 at 10:11:23PM +0100, Mark Kettenis wrote:
> Date: Wed, 2 Mar 2005 17:15:53 -0500
> From: Daniel Jacobowitz <drow@false.org>
>
> I'd like opinions on this patch.
>
> I like it ;-).
>
> There are some times when it's just not possible to backtrace through
> hand-written assembly. This can be true of anything which will never
> return, and is often true of functions which are called or return in
> a "unique" manner - the specific case that prompted me to write this was a
> processor exception handler. In this instance there is a theoretical return
> path, but it will almost always lead out of the binary into some other
> running code, so backtracing through it isn't useful. Trying to apply
> normal unwinding just produces garbage.
>
> Wouldn't it be useful for process startup code of UNIX processes
> (crt0/crt1) and thread startup code too?

Yes, absolutely.

> I picked an idiom which GDB currently doesn't handle to mean "no backtrace
> information is available": DW_CFA_undefined in the return address column.
> Seems a plausible interpretation to me. This idiom implies that not only
> is no DWARF unwinding data available, but also that more conventional means
> of unwinding are unlikely to succeed. Obviously, if GDB has an earlier
> sniffer which recognizes the particular location, we can continue
> backtracing. This just stops us from falling back to the prologue
> analyzers.
>
> So people should take a bit more care in stacking the sniffers;
> nothing new there.
>
> What do you think of the idea? The patch? If both seem OK, I'll propose
> the idiom to the DWARF working group. It doesn't require any changes to the
> standard, but it might be nice to document it explicitly.
>
> Could you drop a note to the dwarf working group mailing list to get a
> bit more opinions about this "abuse" of the standard before checking
> this patch in?

I did that. Andrew replied that he had suggested it previously; Todd
Allen replied that it seemed like a good idea. Andrew also suggested
an undefined CFA as a barrier, but the implementation of that will look
a bit different; so, Andrew, if you would like an undefined CFA to have
the same effect, feel free to implement it :-)

I've checked in the patch now.