Re: gdb/dwarf-frame.c

Elena Zannoni
2003-05-09T14:10:14Z

Daniel Jacobowitz writes:
> 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.
>

Which reminds me that we should probably have the NPTL thread support
patches integrated in gdb as well, since we are at this (and I have
your attention). Can somebody (Daniel, Mark?) take a look at the
patches? I can guarentee that they work, since they were shipped with
RHL9's gdb.

elena

> --
> Daniel Jacobowitz
> MontaVista Software Debian GNU/Linux Developer