From: Jeff Law <law@redhat.com>
To: "H.J. Lu" <hjl.tools@gmail.com>,
Szabolcs Nagy <szabolcs.nagy@arm.com>,
gnu-gabi@sourceware.org
Cc: Ramana Radhakrishnan <ramana.gcc@googlemail.com>,
Cary Coutant <ccoutant@gmail.com>,
Joe Groff <jgroff@apple.com>,
Alan Modra <amodra@gmail.com>,
Binutils <binutils@sourceware.org>, nd <nd@arm.com>
Subject: Re: Preventing preemption of 'protected' symbols in GNU ld 2.26
Date: Fri, 15 Apr 2016 16:36:00 -0000 [thread overview]
Message-ID: <5711188D.3000500@redhat.com> (raw)
In-Reply-To: <CAMe9rOq4CH1C5QhodxHPBteMx3ryO+SOn0awip6SuqG25+PBQg@mail.gmail.com>
On 04/15/2016 10:16 AM, H.J. Lu wrote:
> On Fri, Apr 15, 2016 at 9:09 AM, Szabolcs Nagy <szabolcs.nagy@arm.com> wrote:
>> On 31/03/16 14:26, Ramana Radhakrishnan wrote:
>>> On Thu, Mar 31, 2016 at 1:52 AM, Jeff Law <law@redhat.com> wrote:
>>>> On 03/30/2016 06:40 PM, Cary Coutant wrote:
>>>>>>
>>>>>> It would help me immensely on the GCC side if things if you and Alan
>>>>>> could
>>>>>> easily summarize correct behavior and the impact if we were to just
>>>>>> revert
>>>>>> HJ's change. A testcase would be amazingly helpful too.
>>>>>
>>>>>
>>>>> It looks like it's not just the one change. There's this patch:
>>>>>
>>>>> https://gcc.gnu.org/ml/gcc-patches/2015-07/msg01871.html
>>>>>
>>>>> which took the idea that protected can still be pre-empted by a COPY
>>>>> relocation and extended it to three more targets that use COPY
>>>>> relocations.
>>>>>
>>>>> I wonder how many other patches have been based on the same
>>>>> misunderstanding?
>>
>> (sorry i missed this thread)
>>
>> this was not a misunderstanding.
>>
>> that patch is necessary for correctness (odr) in
>> the presence of copy relocations as described in
>> https://gcc.gnu.org/ml/gcc-patches/2015-09/msg02365.html
>> and
>> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=55012
>>
>> this was a long standing code gen bug in gcc and was
>> about time to fix it (it was also broken in glibc's
>> dynamic linker, but e.g. not in musl libc).
>>
>> (i don't see what is the issue with using the copy in
>> the main executable from a shared library, performance
>> is not a correctness issue, nor how it is possible to
>> avoid the copy relocs.)
>>
>
> Here is my understanding:
>
> Copy relocation and protected visibility are fundamentally incompatible.
> On on hand, copy relocation is the part of the psABI and is used to
> access global data defined in a shared object from the executable. It
> moves the definition of global data, which is defined in a share object,
> to the executable at run-time. On the other hand, protected visibility
> indicates that a symbol is defined locally in the shared object at
> run-time. Both can't be true at the same time. The current solution
> is to make protected symbol more or less like normal symbol, which
> prevents optimizing local access to protected symbol within the shared
> object.
>
> I propose to add GNU_PROPERTY_NO_COPY_ON_PROTECTED:
>
> https://github.com/hjl-tools/linux-abi/wiki/Linux-Extensions-to-gABI
>
> GNU_PROPERTY_NO_COPY_ON_PROTECTED This indicates that there
> should be no copy relocations against protected data symbols. If a relocat-
> able object contains this property, linker should treat protected data symbol
> as defined locally at run-time and copy this property to the output share
> object. Linker should add this property to the output share object if any pro-
> tected symbol is expected to be defined locally at run-time. Run-time loader
> should disallow copy relocations against protected data symbols defined in
> share objects with GNU_PROPERTY_NO_COPY_ON_PROTECTED prop-
> erty. Its PR_DATASZ should be 0.
I'd strongly suggest discussing directly with Carlos, Cary and Alan. My
worry here is this just adding another layer of stuff to deal with a
fundamentally broken concept -- protected visibility.
Jeff
next prev parent reply other threads:[~2016-04-15 16:36 UTC|newest]
Thread overview: 52+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-04-15 16:16 H.J. Lu
2016-04-15 16:36 ` Jeff Law [this message]
2016-04-15 16:45 ` H.J. Lu
2016-04-15 16:43 ` Szabolcs Nagy
2016-04-15 23:59 ` Maciej W. Rozycki
2016-04-16 1:08 ` Szabolcs Nagy
[not found] <AB592ABD-D6D7-4D2F-A0D6-45738F168DC4@apple.com>
2016-03-29 19:31 ` Fwd: " Joe Groff
2016-03-29 19:33 ` H.J. Lu
2016-03-29 19:36 ` Joe Groff
2016-03-29 19:43 ` H.J. Lu
2016-03-29 19:51 ` Joe Groff
2016-03-29 19:54 ` H.J. Lu
2016-03-29 22:05 ` H.J. Lu
2016-03-30 1:44 ` Alan Modra
2016-03-30 1:46 ` Cary Coutant
2016-03-30 4:04 ` Jeff Law
2016-03-30 7:20 ` Cary Coutant
2016-03-30 7:34 ` Cary Coutant
2016-03-30 14:44 ` Alan Modra
2016-03-31 0:45 ` Cary Coutant
2016-03-31 0:40 ` Cary Coutant
2016-03-31 0:53 ` Jeff Law
2016-03-31 13:27 ` Ramana Radhakrishnan
2016-03-31 15:05 ` H.J. Lu
2016-04-15 16:10 ` Szabolcs Nagy
2016-04-01 19:51 ` Jeff Law
2016-04-02 2:53 ` Alan Modra
-- strict thread matches above, loose matches on Subject: below --
2016-03-24 0:00 Joe Groff
2016-03-24 0:45 ` H.J. Lu
2016-03-24 0:52 ` Joe Groff
2016-03-24 1:25 ` H.J. Lu
2016-03-24 15:01 ` Cary Coutant
2016-03-24 15:07 ` H.J. Lu
2016-03-24 16:06 ` Cary Coutant
2016-03-24 16:42 ` H.J. Lu
2016-03-24 16:56 ` Cary Coutant
2016-03-24 17:05 ` H.J. Lu
2016-03-24 17:06 ` Joe Groff
2016-03-24 17:09 ` H.J. Lu
2016-03-24 18:31 ` Cary Coutant
2016-03-27 16:26 ` Rafael Espíndola
2016-03-28 12:12 ` H.J. Lu
2016-03-28 22:12 ` Cary Coutant
[not found] ` <BC969B3B-87A2-4238-90C8-DA2E166707AF@apple.com>
2016-03-28 17:03 ` Joe Groff
2016-03-28 17:17 ` H.J. Lu
2016-03-28 22:22 ` Cary Coutant
2016-03-28 22:24 ` Joe Groff
2016-03-28 22:38 ` Cary Coutant
2016-03-28 22:41 ` Joe Groff
2016-03-28 23:21 ` Alan Modra
2016-03-29 0:29 ` Cary Coutant
2016-03-29 15:44 ` H.J. Lu
2016-03-29 12:40 ` Maciej W. Rozycki
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