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

[Daniel Jacobowitz <drow@false.org>]

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.

-- 
Daniel Jacobowitz
CodeSourcery, LLC