Re: gdb/dwarf-frame.c

[Daniel Jacobowitz <drow@mvista.com>]

On Fri, May 09, 2003 at 02:45:29AM -0700, Roland McGrath wrote:
> (Hi Mark!  It's been too long since we hacked together.)
> [Please note that I am not on the mailing list, so keep me CC'd directly.]
> 
> I have been looking at the kettenis_i386newframe-20030419-branch gdb code.
> I've been told that the new dwarf-frame.c replaces the dwarf2cfi.c code
> that's on mainline.  I don't know the guts of either or of DWARF2 itself
> well enough to compare them.
> 
> What I have noticed is that dwarf-frame.c does not seem to handle the
> .eh_frame section, only the .debug_frame section.  The dwarf2cfi.c code
> looks at both.  As well as looking for the section, it needs to grok the
> "augmentation" values and different encodings used in .eh_frame, and I
> don't see any of that handled in the new code.  Is this an intentional
> omission and if so what is the rationale?

My understanding from Mark's earlier post is that it is an intentional
but probably temporary omission - since dwarf-frame is only a week or
two old at this point.

> I think grokking .eh_frame sections in the absence of .debug_frame is a
> nice thing in general--it might give you at least some more helpful
> backtraces than otherwise when dealing with binaries without debugging
> info.  But the particular reason it is of concern to me is that it's needed
> for unwinding PC values within the special kernel entrypoint page now being
> used in Linux on x86.  glibc now uses this entrypoint code for every system
> call, and so any thread blocked in a system call (which most threads one
> looks at are when one starts looking) will have its PC inside this code and
> need to be able to unwind that frame-pointer-less leaf frame to produce a
> useful backtrace.  This is magic kernel code for which there is never going
> to be debugging information, but for which we do have .eh_frame information
> we can get at.  I am setting about attacking how we get at it in all the
> relevant cases, but I had been working from the assumption that upon
> locating some information in .eh_frame form (including "zR" augmentation
> and pcrel pointer encoding) it would plug easily into the DWARF2 unwinding
> machinery.  If that's not so, it throws a monkey wrench into my plans.

Should any work even be necessary?  My understanding was that the
kernel code would show up in the shared library list.  Oh, I guess it
is - we usually locate .eh_frame via BFD, which means section headers
and an on-disk file.  I see.

-- 
Daniel Jacobowitz
MontaVista Software                         Debian GNU/Linux Developer