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Re: [PATCH] Add __private_ss to s390 struct tcbhead.

On 01/13/2016 02:52 AM, Marcin KoÅcielnicki wrote:
> On 13/01/16 03:41, Carlos O'Donell wrote:
>> On 01/02/2016 11:34 AM, Marcin KoÅcielnicki wrote:
>>> Preparation for gcc -fsplit-stack support (gcc bug #68191).  The
>>> new field is basically identical to the one on x86.  Its TCB
>>> offset needs to be constant, as it'll be hardcoded in gcc.
>>> * sysdeps/s390/nptl/tls.h: Add __private_ss to struct tcbhead.
>> What happens if you run newly compiled code with split-stack
>> support on a glibc that doesn't include this space allocated in
>> tcbhead_t? You get a write beyond the tcbhead_t into some other
>> data? Depending on the thread memory layout that could be a guard
>> page or static TLS data?
>> This is the same problem we saw in POWER when adding a field in
>> tcbhead_t for fast-architecture access.
>> The only way to do this compatibly is to add a versioned symbol
>> that the compiler references to in order to prevent new binaries
>> from running on old glibc and crashing or worse silently corrupting
>> data.
>> To reiterate: How do you plan to handle compatibility for this new
>> feature?
>> If you did an analysis of nptl/allocatestack.c and showed that
>> there were still alignment bytes left, that might be one way to
>> work around this without needing a versioned reference.
> I have added this field at the end of tcbhead_t, which is part of
> struct pthread, defined here:
> . The tcbhead_t is stuffed there inside a union with void
> *__padding[24], which is much longer than current tcbhead_t. Thus
> applying this patch to glibc won't even change anything in compiler
> output - there's always been space there, and this patch merely
> reserves it for split-stack use.

Perfect. Sorry I missed that.

> The situation is different for POWER, since it has TLS_DTV_AT_TP - in
> this case, there's no padding for tcbhead_t and no extra expension
> space.



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