Using objdump to force a section to load with gdb.

Using objdump to force a section to load with gdb.

[Brendan Simon]

I have a linux kernel compiled for a mpc860 target and am trying to get
it to run by downloading it into memory using a background debugger
(BDM).  The boot code gets to the point where it trys to uncompress the
kernel but fails because the image isn't loaded into memory (only .text,
.rodata and .data are loaded).  I tried using objcopy to set the "image"
section to "load" but it does not seem to work.  How can I get gdb to
load the image section (either using a gdb command or binutils) ?

Thanks,
Brendan Simon.

Here is a before/after trace using objcopy --set-section-flags.  As you
can see, the load attribute is not set.  I've tried all combinations of
attributes but I can't get it to work.  Am I doing something wrong ?

powerpc-linux-objdump --section-headers myzimage
myzimage:     file format elf32-powerpc
Sections:
Idx Name          Size      VMA       LMA       File off  Algn
  0 .text         00004324  00100000  00100000  00010000  2**2
                  CONTENTS, ALLOC, LOAD, READONLY, CODE
  1 .rodata       00000460  00104330  00104330  00014330  2**4
                  CONTENTS, ALLOC, LOAD, READONLY, DATA
  2 .data         000002f8  00105000  00105000  00015000  2**2
                  CONTENTS, ALLOC, LOAD, DATA
  3 .bss          0000bbac  00106000  00106000  00016000  2**2
                  ALLOC
  4 image         0006ccbf  00000000  00000000  00016000  2**0
                  CONTENTS, READONLY

powerpc-linux-objcopy --set-section-flags=image=load myzimage

powerpc-linux-objdump --section-headers myzimage
myzimage:     file format elf32-powerpc
Sections:
Idx Name          Size      VMA       LMA       File off  Algn
  0 .text         00004324  00100000  00100000  00010000  2**2
                  CONTENTS, ALLOC, LOAD, READONLY, CODE
  1 .rodata       00000460  00104330  00104330  00014330  2**4
                  CONTENTS, ALLOC, LOAD, READONLY, DATA
  2 .data         000002f8  00105000  00105000  00015000  2**2
                  CONTENTS, ALLOC, LOAD, DATA
  3 .bss          0000bbac  00106000  00106000  00016000  2**2
                  ALLOC
  4 image         0006ccbf  00000000  00000000  00016000  2**0
                  CONTENTS

objdump or gdb to force a section to load ?

[Brendan Simon]

I sent this a while ago and got no response.  I'm resending it just in case
it got lost.
Sorry if you have already received this before.
Brendan Simon.

Brendan Simon wrote:

> I have a linux kernel compiled for a mpc860 target and am trying to get
> it to run by downloading it into memory using a background debugger
> (BDM).  The boot code gets to the point where it trys to uncompress the
> kernel but fails because the image isn't loaded into memory (only .text,
> .rodata and .data are loaded).  I tried using objcopy to set the "image"
> section to "load" but it does not seem to work.  How can I get gdb to
> load the image section (either using a gdb command or binutils) ?
>
> Here is a before/after trace using objcopy --set-section-flags.  As you
> can see, the load attribute is not set.  I've tried all combinations of
> attributes but I can't get it to work.  Am I doing something wrong ?
>
> powerpc-linux-objdump --section-headers myzimage
> myzimage:     file format elf32-powerpc
> Sections:
> Idx Name          Size      VMA       LMA       File off  Algn
>   0 .text         00004324  00100000  00100000  00010000  2**2
>                   CONTENTS, ALLOC, LOAD, READONLY, CODE
>   1 .rodata       00000460  00104330  00104330  00014330  2**4
>                   CONTENTS, ALLOC, LOAD, READONLY, DATA
>   2 .data         000002f8  00105000  00105000  00015000  2**2
>                   CONTENTS, ALLOC, LOAD, DATA
>   3 .bss          0000bbac  00106000  00106000  00016000  2**2
>                   ALLOC
>   4 image         0006ccbf  00000000  00000000  00016000  2**0
>                   CONTENTS, READONLY
>
> powerpc-linux-objcopy --set-section-flags=image=load myzimage
>
> powerpc-linux-objdump --section-headers myzimage
> myzimage:     file format elf32-powerpc
> Sections:
> Idx Name          Size      VMA       LMA       File off  Algn
>   0 .text         00004324  00100000  00100000  00010000  2**2
>                   CONTENTS, ALLOC, LOAD, READONLY, CODE
>   1 .rodata       00000460  00104330  00104330  00014330  2**4
>                   CONTENTS, ALLOC, LOAD, READONLY, DATA
>   2 .data         000002f8  00105000  00105000  00015000  2**2
>                   CONTENTS, ALLOC, LOAD, DATA
>   3 .bss          0000bbac  00106000  00106000  00016000  2**2
>                   ALLOC
>   4 image         0006ccbf  00000000  00000000  00016000  2**0
>                   CONTENTS

Re: objdump or gdb to force a section to load ?

[Jonathan Larmour]

In article < 37685993.EB50E446@dgs.monash.edu.au > you write:
>I sent this a while ago and got no response.  I'm resending it just in case
>it got lost.

All of 24 hours before. Perhaps you need to be more patient.

>> I have a linux kernel compiled for a mpc860 target and am trying to get
>> it to run by downloading it into memory using a background debugger
>> (BDM).  The boot code gets to the point where it trys to uncompress the
>> kernel but fails because the image isn't loaded into memory (only .text,
>> .rodata and .data are loaded).  I tried using objcopy to set the "image"
>> section to "load" but it does not seem to work.  How can I get gdb to
>> load the image section (either using a gdb command or binutils) ?

If it isn't marked as loadable, then you probably can't. It hasn't even got
an address for it to be loaded into. If it is to be loaded then it is the
build process (using the linker script) that has to make it loadable.

Perhaps an easier approach would be to use an uncompressed kernel rather than
a zImage.

A ppc-linux list may be able to give more specific advice about why the
linker scripts are the way they are.

Jifl
-- 
Cygnus Solutions, 35 Cambridge Place, Cambridge, UK.  Tel: +44 (1223) 728762
"I used to have an open mind but || Get yer free open source RTOS's here...
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Help fight spam! http://spam.abuse.net/  These opinions are all my own fault

SOLVED: objdump or gdb to force a section to load ?

[Brendan Simon]

I think I've solved the "load" flag problem.  I can't confirm it on my target but
objdump seems to report the right things.

Jonathan Larmour wrote:

> Try changing this to:
>
>       $(OBJCOPY) $(OBJCOPY_ARGS) -R .comment \
>        --add-section=image=../coffboot/vmlinux.gz \
>        --set-section-flags=image=load,data \
>        --adjust-section-vma=image=????
>        zvmlinux.tmp $@
>
> Choose an appropriate ???? - read the objcopy man page for details of this
> arg. And when you have something working, recommend the ppc-linux folks
> making the same change to the main makefiles since I believe it should be
> harmless for normal usage (although you may want to verify).

I tried something similar before I got your email.  I thought that objcopy might be
doing something funny because the vma was zero.
objcopy --adjust-section-vma=image=0x00100000 tempImage relocImage
objcopy --set-section-flags=image=load
This still does not work.  I can however set some other attributes such as READONLY.

I tried a few combinations and found that
"--set-section-flags=image=load,alloc,readonly" does work even thought it complains
with "BFD: sta00639: warning: allocated section `image' not in segment".  I then
tried the same options on the original file (with out any section vma offsets) and it
also worked.  I guess it was just a matter of finding the right combinations of flags
and putting them in the right order.  HOPEFULLY gdb will now load the image section
but I wont be able to confirm this until I get home tonight.

Thanks for your help.
Brendan Simon.

Re: SOLVED: objdump or gdb to force a section to load ?

[Ian Lance Taylor]

   Date: Fri, 18 Jun 1999 00:46:42 +0000
   From: Brendan Simon <brendan@dgs.monash.edu.au>

   I tried a few combinations and found that
   "--set-section-flags=image=load,alloc,readonly" does work even thought it complains
   with "BFD: sta00639: warning: allocated section `image' not in segment".

Yeah, this is what I meant in my previous message about getting
loadable information from the segments rather than the sections.

This is an area where it is hard for BFD to hide the specific
characteristics of ELF.

Ian

Re: Using objdump to force a section to load with gdb.

[Ian Lance Taylor]

   Date: Wed, 16 Jun 1999 00:06:11 +0000
   From: Brendan Simon <brendan@dgs.monash.edu.au>

   I have a linux kernel compiled for a mpc860 target and am trying to get
   it to run by downloading it into memory using a background debugger
   (BDM).  The boot code gets to the point where it trys to uncompress the
   kernel but fails because the image isn't loaded into memory (only .text,
   .rodata and .data are loaded).  I tried using objcopy to set the "image"
   section to "load" but it does not seem to work.  How can I get gdb to
   load the image section (either using a gdb command or binutils) ?

The problem with using objcopy to set the LOAD flag is that the only
sections which are loaded are those which are stored in program
segments.  objcopy isn't prepared to create a new program segment in
order to change a section flag.  This would be difficult to fix.

If your loader reads the section headers rather than the segment
headers, then you may be able to make this work by doing something
like
    powerpc-linux-objcopy --set-section-flags=image=alloc,load myzimage
The reason is that ELF records the ALLOC flag in the section header,
and objcopy should know how to change that.

The best way to make a loadable section is to mark the section as
loadable in the first place.  See the gas documentation for how to set
the section flags when using the .section directive.

Ian

Re: Using objdump to force a section to load with gdb.

[Brendan Simon]

Ian Lance Taylor wrote:

> The problem with using objcopy to set the LOAD flag is that the only
> sections which are loaded are those which are stored in program
> segments.  objcopy isn't prepared to create a new program segment in
> order to change a section flag.  This would be difficult to fix.
>
> If your loader reads the section headers rather than the segment
> headers, then you may be able to make this work by doing something
> like
>     powerpc-linux-objcopy --set-section-flags=image=alloc,load myzimage
> The reason is that ELF records the ALLOC flag in the section header,
> and objcopy should know how to change that.
>
> The best way to make a loadable section is to mark the section as
> loadable in the first place.  See the gas documentation for how to set
> the section flags when using the .section directive.

I'm not sure if this is applicable when the section is added with "objcopy
--add-section=image=vmlinux.gz", where vmlinux.gz is an elf file that has been
compressed with gzip.  How does one add this file as a section to an existing
elf file (bootloader) as "loadable in the first place" ?

Brendan Simon.

Re: Using objdump to force a section to load with gdb.

[Ian Lance Taylor]

   Date: Fri, 18 Jun 1999 06:50:53 +0000
   From: Brendan Simon <brendan@dgs.monash.edu.au>

   > The best way to make a loadable section is to mark the section as
   > loadable in the first place.  See the gas documentation for how to set
   > the section flags when using the .section directive.

   I'm not sure if this is applicable when the section is added with "objcopy
   --add-section=image=vmlinux.gz", where vmlinux.gz is an elf file that has been
   compressed with gzip.  How does one add this file as a section to an existing
   elf file (bootloader) as "loadable in the first place" ?

You don't.  At the time that the executable has been created, the
program segments have been created.  objcopy is not currently capable
of modifying the program segment information.

Other than teaching objcopy how to modify program segments, one
approach you can use is to include the vmlinux.gz file in the link
which creates the executable in the first place.  Put it in the right
place with the linker script, and link against it using -b binary.  I
believe that is what the i386 Linux kernel build procedure does.

Ian