Re: [crash, bisected] Re: [PATCH 3/4] x86_64: Fold pda into per cpuarea

[Mike Travis]

Jeremy Fitzhardinge wrote:
> Eric W. Biederman wrote:
>> Mike Travis <travis@xxxxxxx> writes:
>>
>>  
>>> H. Peter Anvin wrote:
>>>    
>>>> Mike Travis wrote:
>>>>      
>>>>> FYI, I did try this out and it caused the bootloader to scramble the
>>>>> loaded data.  The first corruption I found was the .x86cpuvendor.init
>>>>> section contained all zeroes.
>>>>>
>>>>>         
>>>> Explain what you mean with "the bootloader" in this context.
>>>>
>>>>     -hpa
>>>>       
>>> After the code was loaded (the compressed code, it seems that my GRUB
>>> doesn't support uncompressed loading), the above section contained
>>> zeroes.  I snapped it fairly early, around secondary_startup_64, and
>>> then printed it in x86_64_start_kernel.
>>>
>>> The object file had the correct data (as displayed by objdump) so I'm
>>> assuming that the bootloading process didn't load the section correctly.
>>>
>>> Below was the linker script I used:
>>>
>>> --- linux-2.6.tip.orig/include/asm-generic/vmlinux.lds.h
>>> +++ linux-2.6.tip/include/asm-generic/vmlinux.lds.h
>>> @@ -373,9 +373,13 @@
>>>
>>>  #ifdef CONFIG_HAVE_ZERO_BASED_PER_CPU
>>>  #define
>>> PERCPU(align)                                                  \
>>> -       . =
>>> ALIGN(align);                                               \
>>> +       .data.percpu.abs =
>>> .;                                           \
>>>         percpu : { }
>>> :percpu                                            \
>>> -       __per_cpu_load =
>>> .;                                             \
>>> +       .data.percpu.rel : AT(.data.percpu.abs - LOAD_OFFSET)
>>> {         \
>>> +              
>>> BYTE(0)                                                 \
>>> +               . =
>>> ALIGN(align);                                       \
>>> +               __per_cpu_load =
>>> .;                                     \
>>> +      
>>> }                                                               \
>>>         .data.percpu 0 : AT(__per_cpu_load - LOAD_OFFSET)
>>> {             \
>>>                
>>> *(.data.percpu.first)                                   \
>>>                
>>> *(.data.percpu.shared_aligned)                          \
>>> @@ -383,8 +387,8 @@
>>>                
>>> *(.data.percpu.page_aligned)                            \
>>>                 ____per_cpu_size =
>>> .;                                   \
>>>        
>>> }                                                               \
>>> -       . = __per_cpu_load +
>>> ____per_cpu_size;                          \
>>> -       data : { } :data
>>> +       . = __per_cpu_load + ____per_cpu_size;
>>> +
>>>  #else
>>>  #define
>>> PERCPU(align)                                                  \
>>>         . =
>>> ALIGN(align);                                               \
>>>
>>> It showed all the correct address in the map and __per_cpu_load was a
>>> relative symbol (which was the objective.)
>>>
>>> Btw, our simulator, which only loads uncompressed code, had the data
>>> correct,
>>> so it *may* only be a result of the code being compressed.
>>>     
>>
>> Weird.  Grub doesn't get involved in the decompression the kernel does it
>> all itself so we should be able to track where things go bad.
>>
>> Last I looked the compressed code was formed by essentially.
>> objcopy vmlinux -O binary vmlinux.bin
>> gzip vmlinux.bin
>> And then we take on a magic header to the gzip compressed file.
>>
>> Are things only bad with the change above?
> 
> No, the original crash being discussed was a GP fault in head_64.S as it
> tries to initialize the kernel segments.  The cause was that the
> prototype GDT is all zero, even though it's an initialized variable, and
> inspection of vmlinux shows that it has the right contents.  But somehow
> it's either 1) getting zeroed on load, or 2) is loaded to the wrong place.
> 
> The zero-based PDA mechanism requires the introduction of a new ELF
> segment based at vaddr 0 which is sufficiently unusual that it wouldn't
> surprise me if its triggering some toolchain bug.
> 
> Mike: what would happen if the PDA were based at 4k rather than 0?  The
> stack canary would still be at its small offset (0x20?), but it doesn't
> need to be initialized.  I'm not sure if doing so would fix anything,
> however.
> 
>    J

I don't know that the basing at 0 or 4k would matter.  I'll post the patch
in it's current form (as an RFC?) to show what was needed to initialize the
pda and gdt page pointer.

Thanks,
Mike
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