Re: Compressed kernels currently won't boot

[Helge Deller <deller@gmx.de>]

On 31.07.19 22:49, James Bottomley wrote:
> On Wed, 2019-07-31 at 22:19 +0200, Helge Deller wrote:
>> On 31.07.19 21:56, James Bottomley wrote:
>>> On Wed, 2019-07-31 at 21:46 +0200, Helge Deller wrote:
>>>> On 31.07.19 21:44, Sven Schnelle wrote:
>>>>> Hi James,
>>>>>
>>>>> On Wed, Jul 31, 2019 at 12:40:12PM -0700, James Bottomley
>>>>> wrote:
>>>>>
>>>>>> What about causing the compressed make to build both a
>>>>>> stripped and a non-stripped bzImage (say sbzImage and
>>>>>> bzImage).  That way you always have the stripped one
>>>>>> available for small size things like boot from tape or
>>>>>> DVD?  but in the usual case we use the bzImage with full
>>>>>> contents.
>>>>>
>>>>> In that case we would also need to build two lifimages - how
>>>>> about adding a config option option? Something like "Strip
>>>>> debug information from compressed kernel images"?
>>>>
>>>> I agree, two lifimages don't make sense. Only one vmlinuz gets
>>>> installed. Instead of the config option, I tink my latest patch
>>>> is better.
>>>
>>> It doesn't solve the problem that if a stripped compressed image is
>>>>
>>> 128kb then it overwrites the decompress area starting at 0x00100000
>>> so we can't decompress the end because we've already overwritten it
>>> before the decompressor gets to it.
>>
>> I don't get this point.
>>    hppa64-linux-gnu-objdump -h vmlinuz
>> shows:
>> Sections:
>> Idx Name          Size      VMA               LMA               File
>> off  Algn
>>     0
>> .head.text    00000084  00000000000e0000  00000000000e0000  00001000
>>   2**2
>>                     CONTENTS, ALLOC, LOAD, READONLY, CODE
>>     1
>> .opd          00000340  00000000000e0090  00000000000e0090  00001090
>>   2**3
>>                     CONTENTS, ALLOC, LOAD, DATA
>>     2
>> .dlt          00000160  00000000000e03d0  00000000000e03d0  000013d0
>>   2**3
>>                     CONTENTS, ALLOC, LOAD, DATA
>>     3 .rodata.compressed
>> 01f3c2b0  00000000000e0530  00000000000e0530  00001530  2**0
>>                     CONTENTS, ALLOC, LOAD, DATA
>>     4
>> .text         00005cc0  000000000201d000  000000000201d000  01f3e000
>>   2**7
>>                     CONTENTS, ALLOC, LOAD, READONLY, CODE
>>     5
>> .data         00000060  0000000002022cc0  0000000002022cc0  01f43cc0
>>   2**3
>>                     CONTENTS, ALLOC, LOAD, DATA
>>
>> Only .head.text gets loaded at e0000, and it is basically just a few
>> bytes which sets-up registers and jump to .text segment (at 0201d000
>> in this case).
>
> Actually, you're looking at the wrong thing, you want to look at the
> program header (the segments) not the section header.  It's the program
> header we load.  If I extract this from the current debian kernel we
> get
>
> jejb@ion:~/git/linux-build/arch/parisc/boot/compressed> readelf -l /boot/vmlinuz-4.19.0-5-parisc64-smp
>
> Elf file type is EXEC (Executable file)
> Entry point 0xe0000
> There are 4 program headers, starting at offset 64
>
> Program Headers:
>    Type           Offset             VirtAddr           PhysAddr
>                   FileSiz            MemSiz              Flags  Align
>    PHDR           0x0000000000000040 0x0000000000001040 0x0000000000000000
>                   0x00000000000000e0 0x00000000000000e0  R E    0x8
>    LOAD           0x0000000000001000 0x00000000000e0000 0x00000000000e0000
>                   0x00000000000004d8 0x00000000000004d8  RWE    0x1000
>    LOAD           0x0000000000002000 0x0000000001400000 0x0000000001400000
>                   0x00000000003dd46c 0x00000000003e1000  RWE    0x1000
>    GNU_STACK      0x0000000000000000 0x0000000000000000 0x0000000000000000
>                   0x0000000000000000 0x0000000000000000  RWE    0x10
>
>   Section to Segment mapping:
>    Segment Sections...
>     00
>     01     .head.text .opd .dlt
>     02     .text .data .rodata .eh_frame .bss
>     03
>
> The two LOAD sections corresponding to what PALO actually loads. The
> problem happens if the length of the first load section is bigger than
> 0x20000.

What exactly is the problem if the first section is bigger than 0x20000?

> Now if you look what happens after your change:
> jejb@ion:~/git/linux-build/build/parisc64/arch/parisc/boot> readelf -l bzImage

Ok - bzImage is the same as ./vmlinuz.

> Elf file type is EXEC (Executable file)
> Entry point 0xe0000
> There are 4 program headers, starting at offset 64
>
> Program Headers:
>    Type           Offset             VirtAddr           PhysAddr
>                   FileSiz            MemSiz              Flags  Align
>    PHDR           0x0000000000000040 0x0000000000001040 0x0000000000000000
>                   0x00000000000000e0 0x00000000000000e0  R E    0x8
>    LOAD           0x0000000000001000 0x00000000000e0000 0x00000000000e0000
>                   0x00000000004ae760 0x00000000004ae760  RWE    0x1000
>    LOAD           0x00000000004b0000 0x000000000118a000 0x000000000118a000
>                   0x0000000000006044 0x000000000000a000  RWE    0x1000
>    GNU_STACK  0    0x0000000000000000 0x0000000000000000 0x0000000000000000
>                   0x0000000000000000 0x0000000000000000  RWE    0x10
>
>   Section to Segment mapping:
>    Segment Sections...
>     00
>     01     .head.text .opd .dlt .rodata.compressed
>     02     .text .data .rodata .eh_frame .bss
>     03
>
> So the first section tries to load between 0x000e0000-0x0058e760 and
> that's overwritten at 0x00100000 when the decompression starts because
> 0x00100000 is our KERNEL_BINARY_TEXT_START.

The decompression decompresses the image from .rodata.compressed
to an area behind .bss.
So, "vmlinux" ends up behind .bss for further processing.
This "vmlinux" (which can have multiple ELF sections) is then started at the high address.
That address is way above the 0x00100000 or KERNEL_BINARY_TEXT_START.
It then finally moves itself (the ELF sections) to 0x00100000.

> The result for me is that
> I get the Decompressing linux ... message followed by a HPMC.

It actually does boot for me and Sven without a HPMC.
The decompression is slow (~40 seconds on my c3000 for 160MB).
I still *believe* you are facing a HPMC because of other reasons.
On which machine do you start.
How much memory?

Helge