Hello Dave, For some reason the following posting did not arrive at the crash mailing list: https://www.sourceware.org/ml/gdb/2014-10/msg00061.html https://www.sourceware.org/ml/gdb/2014-10/msg00062.html Any idea why? Michael On Thu, 2 Oct 2014 10:27:00 +0200 Petr Tesarik <ptesarik@xxxxxxx> wrote: > On Thu, 2 Oct 2014 03:18:55 +0000 > Pete Delaney <pdelaney@xxxxxxxxxxxxxxx> wrote: > > > Hi: > > > > Six years ago Dave and I were discussing using gdb on KDUMP files: > >[...] > > > > Anyone know what's going on? > > Yes, sure. GDB works very differently from crash. There main conceptual > difference is that GDB only handles with VIRTUAL addresses, while the > crash utility first translates everything to PHYSICAL addresses. > Consequently, GDB ignores the PhysAddr field in ELF program headers, > and crash ignores the VirtAddr field. > > I have looked at some of my ELF dump files, and it seems to me that > VirtAddr is not filled correctly, except for kernel text and static > data (address range 0xffffffff80000000-0xffffffff9fffffff). Your linked > list is most likely allocated in the direct mapping > (0xffff880000000000-0xffffc7ffffffffff). However, I found out that the > virtual addresses for the direct mapping segments are wrong, e.g. my > dump file specifies it at 0xffff810000000000 (hypervisor area). This is > most likely a bug in the kernel code that implements /proc/vmcore. > > But that's beside the point. Why? The Linux kernel maps many physical > pages more than once into the virtual address space. It would be waste > of space if you saved it multiple times (for each virtual address that > maps to it). The crash utility can translate each virtual address to > the physical address and map it onto ELF segments using PhysAddr. > Incidentally, the PhysAddr fields are correct in my dump files... > > I'm glad you're interested in using GDB to read kernel dump files, > especially if you're willing to make it work for real. I have proposed > more than once that the crash utility be re-implemented in pure gdb. > Last time I looked (approx. 1.5 years ago) the main missing pieces were: > > 1. Use of physical addresses (described above) > 2. Support for multiple virtual address spaces (for different process > contexts) > 3. Ability to read compressed kdump files > 4. Ability to use 64-bit files on 32-bit platforms (to handle PAE) > > HTH, > Petr Tesarik > > -- > Crash-utility mailing list > Crash-utility@xxxxxxxxxx > https://www.redhat.com/mailman/listinfo/crash-utility > -- Crash-utility mailing list Crash-utility@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/crash-utility
