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

[Eric W. Biederman]

Jeremy Fitzhardinge <jeremy@xxxxxxxx> writes:

> H. Peter Anvin wrote:
>> Eric W. Biederman wrote:
>>>>
>>>> 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.
>>>
>>
>> Actually, it's not all that unusual... it's pretty common in various
>> restricted environments.  That being said, it's probably uncommon for *64-bit*
>> code.
>
> Well, it's also unusual because 1) it's vaddr 0, but paddr <high>, and 2) the
> PHDRs are not sorted by vaddr order.  2) might actually be a bug.

I just looked and gcc does not use this technique for thread local data.

My initial concern about all of this was not making symbols section relative
is relieved as this all appears to be a 64bit arch thing where that doesn't
matter.

Has anyone investigated using the technique gcc uses for thread local storage?
http://people.redhat.com/drepper/tls.pdf

In particular using the local exec model so we can say:
movq %fs:x@tpoff,%rax

To load the contents of a per cpu variable x into %rax ?

If we can use that model it should make it easier to interface with things like
the stack protector code.  Although we would still need to be very careful
about thread switches.

Eric
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