Fedora kernel list - Apr 2007 - [PATCH FC6] x86_64 page_is_ram() uses __initdata; breaks /dev/crash and /dev/mem restriction

Somewhere after the 2.6.18 timeframe, Andi Kleen made the
x86_64 e820 map __initdata:

  struct e820map e820 __initdata;

This is fine for upstream x86_64 kernels, because the e820 map never
gets used during runtime.  But because we (RHEL/Fedora) have an x86_64
version of page_is_ram(), it ends up using __init data.

I became aware of this when I got an FC6/crash-utility bugzilla #237383,
filed because the crash utility started failing on later FC6 live systems.
This is because the kernel''s /dev/crash module crash.ko started failing
because it uses page_is_ram() as a memory access qualifer.

Also, because of Red Hat''s restriction on the use of /dev/mem to only
the first 256 RAM pages, the __initdata addition ends up opening the flood
gates for /dev/mem usage.  That''s 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;
  }

The function is meant to allow /dev/mem accesses above pagenr 256
only if they are *not* RAM -- but since since page_is_ram() is failing,
it inadvertantly allows access to any pagenr.

(Interestingly enough, this bug allows the user of the crash
utility to work around the /dev/crash failure by alternatively
using /dev/mem instead!  /dev/crash was only created to begin
with because of the /dev/mem restriction...

Anyway, reverting back from __initdata fixes the situation for both
/dev/crash and the /dev/mem restriction.

Dave Anderson

(patch is against 2.6.20-1.2944)

On Thu, 26 Apr 2007 15:40:00 -0400, Dave Anderson <anderson@redhat.com>
wrote:
> This is fine for upstream x86_64 kernels, because the e820 map never
> gets used during runtime.  But because we (RHEL/Fedora) have an x86_64
> version of page_is_ram(), it ends up using __init data.
> +++ linux-2.6.20.x86_64/arch/x86_64/kernel/e820.c	2007-04-26
14:38:24.000000000 -0400
> @@ -25,7 +25,7 @@
>  #include <asm/bootsetup.h>
>  #include <asm/sections.h>
>  
> -struct e820map e820 __initdata;
> +struct e820map e820;
ACK

Although, do we follow the ACK process for stable Fedora now? I thought
it was just the Chuck''s judgement ever since he joined.

On Dave''s side I looked at Rawhide just now and this does not seem to
be there either (on 1.3126), although linux-2.6-devmem.patch and Xen are.

-- Pete