[Qemu-devel] Multiboot using -kernel and -initrd stopped working

[Qemu-devel] Multiboot using -kernel and -initrd stopped working

[Jens Nyberg]

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Hi!

I develop my own operating system and I have been using this command to
start my operating system:

$ qemu-system-x86_64 -kernel fudge -initrd initrd.tar

I just recently upgraded the qemu package for arch linux and now this does
not seem to work anymore. I get this error message:

Error while fetching elf kernel from rom

So I checked out the latest version of qemu from git today and did a build
but it had the same result.

Now I havent made any changes to my kernel so I know I havent screwed
anything up in that regard. It is a normal 32 bit x86 multiboot compliant
kernel so it should work using these commands. I run qemu on a x86_64
machine.

I'd like to help sort this issue out. I will do my best to collect any data
you might be interested in. I will try to figure out what change made this
not work but it will take some time so if anyone has an idea what commit-id
it is please let me know.

Thanks

Jens

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Re: [Qemu-devel] Multiboot using -kernel and -initrd stopped working

[Jens Nyberg]

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I have some information to go on:

The problem occurs because of a failed call to rom_copy from
hw/i386/multiboot.c

I print debugged some values:

ELF_LOW: 0x100000
ELF_HIGH: 0x14c578
MH_LOAD_ADDR: 0x100000
MH_KERNEL_SIZE: 0x4c578
MH_ENTRY_ADDR: 0x10000c
COPY SIZE: 0x4210

The return value from rom_copy (found in hw/core/loader.c) is compared with
mh_kernel_size to see they are the same. If they are not qemu will exit and
print the error message I got. As you can see in this printout
mh_kernel_size is 0x4c578 and the return value was 0x4210. Now something
goes wrong here. I suspect it doesnt take into the account that there can
be more program headers for elf so it just takes the first one.

So to confirm this I ran readelf -a fudge I get this information about my
binary:

Program Headers:
  Type           Offset   VirtAddr   PhysAddr   FileSiz MemSiz  Flg Align
  LOAD           0x001000 0x00100000 0x00100000 0x04210 0x04210 R E 0x1000
  LOAD           0x006000 0x00105000 0x00105000 0x00000 0x47578 RW  0x1000
  GNU_STACK      0x000000 0x00000000 0x00000000 0x00000 0x00000 RWE 0x4


As you can see the MemSiz field contains the value 0x4210 meaning rom_copy
only copys the first program header but not the second causing this problem.

I will try to see if I can fix this so that qemu takes into account all
program headers.

// Jens




2013/7/20 Jens Nyberg <jens.nyberg@gmail.com>

> Hi!
>
> I develop my own operating system and I have been using this command to
> start my operating system:
>
> $ qemu-system-x86_64 -kernel fudge -initrd initrd.tar
>
> I just recently upgraded the qemu package for arch linux and now this does
> not seem to work anymore. I get this error message:
>
> Error while fetching elf kernel from rom
>
> So I checked out the latest version of qemu from git today and did a build
> but it had the same result.
>
> Now I havent made any changes to my kernel so I know I havent screwed
> anything up in that regard. It is a normal 32 bit x86 multiboot compliant
> kernel so it should work using these commands. I run qemu on a x86_64
> machine.
>
> I'd like to help sort this issue out. I will do my best to collect any
> data you might be interested in. I will try to figure out what change made
> this not work but it will take some time so if anyone has an idea what
> commit-id it is please let me know.
>
> Thanks
>
> Jens
>

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Re: [Qemu-devel] Multiboot using -kernel and -initrd stopped working

[Jens Nyberg]

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Ok, another update. The problem seems to be that in rom_copy
(hw/core/loader.c:776) where it iterates over all the roms it will fail at:

if (!rom->data) {
    continue
}

For the second program header. This means that it hasnt been able to
allocate memory for that header because rom->data is supposed to be a
pointer to a memory area. Because it just continues here the calculation
for how much it has loaded will not correlate with what it is suppose to
load.

So now I need to figure out how rom->data is set and why it is empty.



2013/7/20 Jens Nyberg <jens.nyberg@gmail.com>

> I have some information to go on:
>
> The problem occurs because of a failed call to rom_copy from
> hw/i386/multiboot.c
>
> I print debugged some values:
>
> ELF_LOW: 0x100000
> ELF_HIGH: 0x14c578
> MH_LOAD_ADDR: 0x100000
> MH_KERNEL_SIZE: 0x4c578
> MH_ENTRY_ADDR: 0x10000c
> COPY SIZE: 0x4210
>
> The return value from rom_copy (found in hw/core/loader.c) is compared
> with mh_kernel_size to see they are the same. If they are not qemu will
> exit and print the error message I got. As you can see in this printout
> mh_kernel_size is 0x4c578 and the return value was 0x4210. Now something
> goes wrong here. I suspect it doesnt take into the account that there can
> be more program headers for elf so it just takes the first one.
>
> So to confirm this I ran readelf -a fudge I get this information about my
> binary:
>
> Program Headers:
>   Type           Offset   VirtAddr   PhysAddr   FileSiz MemSiz  Flg Align
>   LOAD           0x001000 0x00100000 0x00100000 0x04210 0x04210 R E 0x1000
>   LOAD           0x006000 0x00105000 0x00105000 0x00000 0x47578 RW  0x1000
>   GNU_STACK      0x000000 0x00000000 0x00000000 0x00000 0x00000 RWE 0x4
>
>
> As you can see the MemSiz field contains the value 0x4210 meaning rom_copy
> only copys the first program header but not the second causing this problem.
>
> I will try to see if I can fix this so that qemu takes into account all
> program headers.
>
> // Jens
>
>
>
>
> 2013/7/20 Jens Nyberg <jens.nyberg@gmail.com>
>
>> Hi!
>>
>> I develop my own operating system and I have been using this command to
>> start my operating system:
>>
>> $ qemu-system-x86_64 -kernel fudge -initrd initrd.tar
>>
>> I just recently upgraded the qemu package for arch linux and now this
>> does not seem to work anymore. I get this error message:
>>
>> Error while fetching elf kernel from rom
>>
>> So I checked out the latest version of qemu from git today and did a
>> build but it had the same result.
>>
>> Now I havent made any changes to my kernel so I know I havent screwed
>> anything up in that regard. It is a normal 32 bit x86 multiboot compliant
>> kernel so it should work using these commands. I run qemu on a x86_64
>> machine.
>>
>> I'd like to help sort this issue out. I will do my best to collect any
>> data you might be interested in. I will try to figure out what change made
>> this not work but it will take some time so if anyone has an idea what
>> commit-id it is please let me know.
>>
>> Thanks
>>
>> Jens
>>
>
>

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