Re: [Crash-utility] gdb on KDUMP files

On Thu, 2 Oct 2014 03:18:55 +0000 Pete Delaney <pdelaney(a)silver-peak.com&gt; 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(a)redhat.com https://www.redhat.com/mailman/listinfo/crash-utility