[PATCH 1/2] x86_64: Reflect the relocatability of the kernel in the ELF header.

Vivek Goyal vgoyal at in.ibm.com
Tue May 1 02:44:44 EDT 2007

On Mon, Apr 30, 2007 at 11:54:22PM -0600, Eric W. Biederman wrote:
> Andi Kleen <ak at suse.de> writes:
> > On Mon, Apr 30, 2007 at 11:26:50PM -0600, Eric W. Biederman wrote:
> >> Vivek Goyal <vgoyal at in.ibm.com> writes:
> >> 
> >> 
> >> >> At least without a core file it is working on with gdb 6.4.
> >> >>
> >> >
> >> > This seems to be a problem with gdb 6.5. I transferred the dump to a
> >> > different machine having GNU gdb 6.4, and it works fine there.
> >> 
> >> Ok.  The difference between those two symbols didn't seem to make
> >> any sense, so a gdb bug makes sense.
> >> 
> >> Cool.  Then the patch is good. :)
> >
> > It would still make any gdb 6.5 users unhappy. If no workaround can be found
> > I guess we'll need a CONFIG of some sort?
> It is probably worth reproducing this bug with a PIE executable.
> But it looks very much like gdb got it wrong, or else there is some
> slight mismatch between our core dump and gdb.
> Given that gdb 6.5 handles the vmlinux fine when it isn't in conjunction
> with a core dump I would not say the problem is in vmlinux.
> Rather there seems to be something messed up when gdb 6.5 tries to match
> up the kernel core dump with the kernel.  The offset for the symbol
> Vivek gave was 0x7fff70e9. ???  Although that is almost 2M...
> Vivek could we see the program headers of your core file?

Hi Eric,

Following are program headers of my core file. They look sane.

Program Headers:
  Type           Offset             VirtAddr           PhysAddr
                 FileSiz            MemSiz              Flags  Align
  NOTE           0x0000000000000190 0x0000000000000000 0x0000000000000000
                 0x0000000000000b20 0x0000000000000b20         0
  LOAD           0x0000000000000cb0 0xffffffff80200000 0x0000000000200000
                 0x0000000000742000 0x0000000000742000  RWE    0
  LOAD           0x0000000000742cb0 0xffff810000000000 0x0000000000000000
                 0x00000000000a0000 0x00000000000a0000  RWE    0
  LOAD           0x00000000007e2cb0 0xffff810000100000 0x0000000000100000
                 0x0000000000f00000 0x0000000000f00000  RWE    0
  LOAD           0x00000000016e2cb0 0xffff810009000000 0x0000000009000000
                 0x00000000c6f8dc80 0x00000000c6f8dc80  RWE    0
  LOAD           0x00000000c8670930 0xffff810100000000 0x0000000100000000
                 0x0000000130000000 0x0000000130000000  RWE    0

First PT_LOAD header is mapping kernel text/data and others just mapping
physical memory in kernel linear virtual address range.

> >From what I can tell what is left to figure out is do we have
> a bug in gdb 6.5 or do we have a bug in our core file generation.

Given the fact it works well with gdb 6.4 as well as 6.1 (crash uses gdb 6.1
as backend and crash is working fine). I would think that it is gdb bug.

> Right now I'm inclined to believe that the fedora core 6? gdb 6.5 got it
> wrong.  I'm probably just burnt out with binutils problems whenever
> I try and do something interesting.  But I'm just not inclined that
> the bleeding edge tools are working properly while there kernel
> core dump mechanism would mess up with a two byte field change.
> It does make sense to root cause this if we can.  If it's a gdb
> problem it should also apply to PIE executables, and should irritate
> a few users.
> Regardless last I heard it was crash that was the primary analysis
> tool and not gdb anyway.  With gdb serving as the double check to make
> certain that the kernel core dump was in a reasonably standard format.

I would consider gdb to be equally important, especially because many
a times crash is broken with latest version of kernels (as some data
structures or some mechanism has changed) and gdb is the only one who
can open the dump.


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