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Re: glibc 2.19 - asyn-signal safe TLS and ASan.
- From: Kostya Serebryany <kcc at google dot com>
- To: Rich Felker <dalias at aerifal dot cx>
- Cc: "Carlos O'Donell" <carlos at redhat dot com>, Paul Pluzhnikov <ppluzhnikov at google dot com>, OndÅej BÃlka <neleai at seznam dot cz>, "Joseph S. Myers" <joseph at codesourcery dot com>, Andrew Hunter <ahh at google dot com>, GNU C Library <libc-alpha at sourceware dot org>, Roland McGrath <roland at hack dot frob dot com>, address-sanitizer at googlegroups dot com
- Date: Thu, 30 Jan 2014 21:38:59 +0400
- Subject: Re: glibc 2.19 - asyn-signal safe TLS and ASan.
- Authentication-results: sourceware.org; auth=none
- References: <20140125010534 dot GA14587 at domone dot podge> <CALoOobNc8mNUHLhry-hcqT6fx9m=HukMw1Sy41Q+uhRDBznbSw at mail dot gmail dot com> <20140125030232 dot GA14213 at domone dot podge> <CAN=P9pgiMRxxYK16ZaVje1AQbC+fQ=QUDhsdww5-8RWaiJswoA at mail dot gmail dot com> <CALoOobMdh7wOHPPB6vTTLb0wXeO-yGA75MvHohrqGPAY-CcC2g at mail dot gmail dot com> <CAN=P9pjAW6bMip+hsE6mDUA2MFfdC7fNCvrgNyhG2GRQGJJNCw at mail dot gmail dot com> <CAN=P9pj+EVcrUwxa+70eEVY-_sqaU1d6s8HjsP4BkY8N+G-JVg at mail dot gmail dot com> <52E9E1E7 dot 3090505 at redhat dot com> <CAN=P9pgyfA6hTC2LcQEpALo62g8+wN0Ne-eJsMnZ7iR1m9rzOg at mail dot gmail dot com> <52E9E7CF dot 9060706 at redhat dot com> <20140130165445 dot GU24286 at brightrain dot aerifal dot cx>
On Thu, Jan 30, 2014 at 8:54 PM, Rich Felker <dalias@aerifal.cx> wrote:
> On Thu, Jan 30, 2014 at 12:49:03AM -0500, Carlos O'Donell 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.
>
> Or a signal handler happened to interrupt __tls_get_addr and something
> from the signal handler caused __libc_memalign to get called. Is this
> case handled?
Is __libc_memalign AS-safe? I guess not.
AddressSanitizer's implementation is certainly not.
So, if __libc_memalign is called in a signal handler it is a problem by itself.
That's what forced the the change in 2.19 in the first place.
>
>> > 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.
>
> I would say making assumptions about the format of the header is
> pretty ugly and might even break in the future,
can't agree more.
--kcc
> especially if the
> intent is to eventually allow overriding the AS-safe malloc
> implementation...
>
> Rich