From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (qmail 8888 invoked by alias); 9 May 2003 13:41:43 -0000 Mailing-List: contact gdb-help@sources.redhat.com; run by ezmlm Precedence: bulk List-Subscribe: List-Archive: List-Post: List-Help: , Sender: gdb-owner@sources.redhat.com Received: (qmail 8878 invoked from network); 9 May 2003 13:41:42 -0000 Received: from unknown (HELO crack.them.org) (146.82.138.56) by sources.redhat.com with SMTP; 9 May 2003 13:41:42 -0000 Received: from nevyn.them.org ([66.93.61.169] ident=mail) by crack.them.org with asmtp (Exim 3.12 #1 (Debian)) id 19E88R-0003r1-00; Fri, 09 May 2003 08:42:03 -0500 Received: from drow by nevyn.them.org with local (Exim 3.36 #1 (Debian)) id 19E880-0005T9-00; Fri, 09 May 2003 09:41:36 -0400 Date: Fri, 09 May 2003 13:41:00 -0000 From: Daniel Jacobowitz To: Roland McGrath Cc: Mark Kettenis , gdb@sources.redhat.com Subject: Re: gdb/dwarf-frame.c Message-ID: <20030509134135.GA20959@nevyn.them.org> Mail-Followup-To: Roland McGrath , Mark Kettenis , gdb@sources.redhat.com References: <200305090945.h499jTH13137@magilla.sf.frob.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <200305090945.h499jTH13137@magilla.sf.frob.com> User-Agent: Mutt/1.5.1i X-SW-Source: 2003-05/txt/msg00121.txt.bz2 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