[Crash-utility] Heads up -- live system "failure" on later x86_64 Fedora kernels

If you see this when running on live 2.6.19 or later Fedora x86_64 kernels: # crash crash 4.0-3.22 Copyright (C) 2002, 2003, 2004, 2005, 2006, 2007 Red Hat, Inc. Copyright (C) 2004, 2005, 2006 IBM Corporation Copyright (C) 1999-2006 Hewlett-Packard Co Copyright (C) 2005, 2006 Fujitsu Limited Copyright (C) 2006, 2007 VA Linux Systems Japan K.K. Copyright (C) 2005 NEC Corporation Copyright (C) 1999, 2002 Silicon Graphics, Inc. Copyright (C) 1999, 2000, 2001, 2002 Mission Critical Linux, Inc. This program is free software, covered by the GNU General Public License, and you are welcome to change it and/or distribute copies of it under certain conditions. Enter "help copying" to see the conditions. This program has absolutely no warranty. Enter "help warranty" for details. GNU gdb 6.1 Copyright 2004 Free Software Foundation, Inc. GDB is free software, covered by the GNU General Public License, and you are welcome to change it and/or distribute copies of it under certain conditions. Type "show copying" to see the conditions. There is absolutely no warranty for GDB. Type "show warranty" for details. This GDB was configured as "x86_64-unknown-linux-gnu"... crash: read error: kernel virtual address: ffffffff8062a180 type: "xtime" # It's not a crash bug. And it can be worked around by entering: # crash /dev/mem The problem is with the intermingling of upstream and Red Hat kernels, where in the 2.6.18+ timeframe, Andi Kleen made the x86_64 e820 map __initdata. That's OK for upstream kernels, because the x86_64 e820 map is not used during runtime. But because Red Hat kernels impose a restriction on /dev/mem to only allow access to the first 256 pages of RAM, an x86_64 version of page_is_ram() is also necessary for Red Hat kernels. Furthermore, for live system analysis, I had to create a Red Hat "/dev/crash" driver to access live kernel memory, and it also uses page_is_ram() as a memory qualifier -- which ends up looking at reallocated garbage data as the e820 map, and fails. Interestingly enough, the bug opens the flood-gates for /dev/mem because of this: /* * devmem_is_allowed() checks to see if /dev/mem access to a certain address is * valid. The argument is a physical page number. * * * On x86-64, access has to be given to the first megabyte of ram because that area * contains bios code and data regions used by X and dosemu and similar apps. * Access has to be given to non-kernel-ram areas as well, these contain the PCI * mmio resources as well as potential bios/acpi data regions. */ int devmem_is_allowed(unsigned long pagenr) { if (pagenr <= 256) return 1; if (!page_is_ram(pagenr)) return 1; return 0; } If a page frame above 256 is requested it's only supposed to be acceptible it if it's *not* RAM -- but since page_is_ram() is failing, *anything* is accepted. And so this inadvertantly allows /dev/mem to be used by crash, as is done for non-Red Hat kernels. Conceivably there could be a scenario where the garbage data may contain something that would cause page_is_ram() to return successfully, in which case even using the crash /dev/mem command line argument could potentially cause crash to fail. But in that case, there is no workaround. In any case, the bug will be fixed in subsequent Fedora kernels. Dave