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Re: glibc 2.19 - asyn-signal safe TLS and ASan.


On Thu, Jan 30, 2014 at 9:49 AM, Carlos O'Donell <carlos@redhat.com> wrote:
> On 01/30/2014 12:33 AM, Kostya Serebryany wrote:
>> On Thu, Jan 30, 2014 at 9:23 AM, Carlos O'Donell <carlos@redhat.com> wrote:
>>>
>>> On 01/29/2014 04:48 AM, Kostya Serebryany wrote:
>>>> [text only]
>>>>
>>>>> Indeed so, thanks!
>>>>> So, exporting __signal_safe_memalign&co will allow us to extend the
>>>>> existing hack to 2.19.
>>>>> If this simple change can not be done for 2.19, can *anything* be done at all?
>>>>> (Long term we'd still prefer something less hackish)
>>>>
>>>> FTR, I've implemented an even-uglier-then-before hack that deals with
>>>> dynamic TLS in both <=2.18 and 2.19.
>>>> So, we will survive the 2.19 release.
>>>> But I would appreciate if we can resolve
>>>> https://sourceware.org/bugzilla/show_bug.cgi?id=16291
>>>> before the next one (2.20).
>>>
>>> Can you please describe the hack?
>>
>> intercept __tls_get_addr and __libc_memalign.
>> if __libc_memalign is called while we are inside __tls_get_addr, we
>> know we are in <= 2.18 mode and we know what to do.
>> if __libc_memalign was not called but the DSO ID passed to
>> __tls_get_addr was not seen before by the current thread,
>> we know that we are in 2.19 mode and that the TLS block was allocated
>> by __signal_safe_memalign, which
>> has a header with the actual block size. Ugly, as I said.
>
> Not terrible at all, quite elegant actually.

As a hack -- yes, maybe.
As a real solution for 2.20+ -- I hope we can do better.
Besides, adding an interceptor to __tls_get_addr means that dynamic
TLS under sanitizers
will become even slower.

--kcc

>
> Thanks.
>
> Cheers,
> Carlos.
>


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