Re: gdb/dwarf-frame.c

Daniel Jacobowitz
2003-05-09T13:41:43Z

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