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

[Jeremy Fitzhardinge <jeremy@goop.org>]

Eric W. Biederman wrote:
> Jeremy Fitzhardinge <jeremy@goop.org> writes:
>
>   
>> 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.
>>     
>
> Agreed.  Given the previous description my hunch is that the bug is occurring
> during objcopy.  If vmlinux is good and the compressed kernel is bad.
>
> It should be possible to look at vmlinux.bin and see if that was generated
> properly.
>
>   
>> 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.
>>     
>
> I'm dense today.  Why are we doing a zero based pda?  That seems the most
> likely culprit of linker trouble, and we should be able to  put a smaller
> offset in the segment register to allow for everything to work as expected.
>   

The only reason we need to do a zero-based PDA is because of the 
boneheaded gcc/x86_64 ABI decision to put the stack canary at a fixed 
offset from %gs (all they had to do was define it as a weak symbol we 
could override).  If we want to support stack-protector and unify the 
handling of per-cpu variables, we need to rebase the per-cpu area at 
zero, starting with the PDA.

My own inclination would be to drop stack-protector support until gcc 
gets fixed, rather than letting it prevent us from unifying an area 
which is in need of unification...

    J