From: "H.J. Lu" <hjl.tools@gmail.com>
To: Michael Matz <matz@suse.de>
Cc: Alan Modra <amodra@gmail.com>, gnu-gabi@sourceware.org
Subject: Re: Specify how undefined weak symbol should be resolved in executable
Date: Fri, 01 Jan 2016 00:00:00 -0000 [thread overview]
Message-ID: <CAMe9rOoSdG1M4K0k=Y3P+C7FSbqiqMdgPk4HoG1Fdqm-EtMddA@mail.gmail.com> (raw)
In-Reply-To: <alpine.LSU.2.20.1602231424140.20277@wotan.suse.de>
On Tue, Feb 23, 2016 at 5:55 AM, Michael Matz <matz@suse.de> wrote:
> Hi,
>
> On Tue, 23 Feb 2016, H.J. Lu wrote:
>
>> On Mon, Feb 22, 2016 at 8:40 PM, Alan Modra <amodra@gmail.com> wrote:
>> >
>> > However, that might be a bad idea. Lots of C++ code uses weak symbols
>> > for functions defined in header files, and other objects with vague
>> > linkage. These result in weak definitions in eg. libstdc++.so. I'm
>> > not sure how many executables take the address of such functions and
>> > thus might become DT_TEXTREL.
>>
>> Most, if not all, of programs will have DT_TEXTREL on x86 if undefined
>> weak symbols require dynamic relocations.
>
> Hmm, that's less than ideal of course. Well, if the goal is to make PIC
> and non-PIC the same, we could also go the opposite way: make PIC behave
> like non-PIC, i.e. resolve weak symbols always at link editing time. That
> of course would remove features (can't change libs at runtime anymore, if
> they change in definedness of such a symbol).
This "feature" never worked on x86 for non-PIC input.
> Or, a third variant: change the compiler to emit PIC like code for taking
> addresses of weak symbols also in generally non-PIC code, so we could
> avoid TEXTREL.
>
> I think the ideal solution would be that last one, change link editor now
> to behave like with PIC code, and eventually fix the compiler to not have
> to generate TEXTREL.
If we change ld now, all of a sudden, x86 binaris are marked with
DT_TEXTREL. What we can do is to add a new linker command
option, -z dynamic-undef-weak, and GCC passes it to ld only when
compiler is changed to generate PIC-like code for address of all
undefined symbol in text section.
> Note that the existence of DT_TEXTREL itself isn't that bad: only those
> pages that actually contain relocations will be unshared, so for the
> example of crtbegin.o it's only one page per process. In addition
> crtbegin could of course always be PIC code, avoiding the issue.
>
> I've looked at a normal c++ program (zypper) and the only weak undef
> symbols are those from crtbegin. There are many other weak symbols, but
> they are defined in the executable itself (it's mostly template
> instantiations), so pose no problem.
>
>
I checked cc1plus and found:
[hjl@gnu-6 5.3.1]$ readelf -sW cc1plus| grep WEAK | grep UND
33: 0000000000000000 0 FUNC WEAK DEFAULT UND
pthread_cond_signal@GLIBC_2.3.2 (3)
2924: 0000000000000000 0 FUNC WEAK DEFAULT UND
pthread_cond_broadcast@GLIBC_2.3.2 (3)
4861: 0000000000000000 0 NOTYPE WEAK DEFAULT UND pthread_key_create
6330: 0000000000000000 0 NOTYPE WEAK DEFAULT UND pthread_getspecific
7205: 0000000000000000 0 FUNC WEAK DEFAULT UND
pthread_mutex_unlock@GLIBC_2.2.5 (2)
7719: 0000000000000000 0 NOTYPE WEAK DEFAULT UND pthread_key_delete
9118: 0000000000000000 0 NOTYPE WEAK DEFAULT UND __pthread_key_create
11985: 0000000000000000 0 NOTYPE WEAK DEFAULT UND _Jv_RegisterClasses
12269: 0000000000000000 0 NOTYPE WEAK DEFAULT UND pthread_once
15201: 0000000000000000 0 FUNC WEAK DEFAULT UND
pthread_mutex_lock@GLIBC_2.2.5 (2)
15794: 0000000000000000 0 FUNC WEAK DEFAULT UND
pthread_cond_wait@GLIBC_2.3.2 (3)
18312: 0000000000000000 0 NOTYPE WEAK DEFAULT UND
_ITM_deregisterTMCloneTable
19108: 0000000000000000 0 NOTYPE WEAK DEFAULT UND pthread_setspecific
19649: 0000000000000000 0 NOTYPE WEAK DEFAULT UND __gmon_start__
19871: 0000000000000000 0 NOTYPE WEAK DEFAULT UND
_ITM_registerTMCloneTable
20107: 0000000000000000 0 FUNC WEAK DEFAULT UND
pthread_cond_destroy@GLIBC_2.3.2 (3)
18570: 0000000000000000 0 FUNC WEAK DEFAULT UND
pthread_cond_signal@@GLIBC_2.3.2
21461: 0000000000000000 0 FUNC WEAK DEFAULT UND
pthread_cond_broadcast@@GLIBC_2.3.2
23398: 0000000000000000 0 NOTYPE WEAK DEFAULT UND pthread_key_create
24867: 0000000000000000 0 NOTYPE WEAK DEFAULT UND pthread_getspecific
25742: 0000000000000000 0 FUNC WEAK DEFAULT UND
pthread_mutex_unlock@@GLIBC_2.2.5
26256: 0000000000000000 0 NOTYPE WEAK DEFAULT UND pthread_key_delete
27655: 0000000000000000 0 NOTYPE WEAK DEFAULT UND __pthread_key_create
30521: 0000000000000000 0 NOTYPE WEAK DEFAULT UND _Jv_RegisterClasses
30805: 0000000000000000 0 NOTYPE WEAK DEFAULT UND pthread_once
33737: 0000000000000000 0 FUNC WEAK DEFAULT UND
pthread_mutex_lock@@GLIBC_2.2.5
34330: 0000000000000000 0 FUNC WEAK DEFAULT UND
pthread_cond_wait@@GLIBC_2.3.2
36848: 0000000000000000 0 NOTYPE WEAK DEFAULT UND
_ITM_deregisterTMCloneTable
37644: 0000000000000000 0 NOTYPE WEAK DEFAULT UND pthread_setspecific
38185: 0000000000000000 0 NOTYPE WEAK DEFAULT UND __gmon_start__
38407: 0000000000000000 0 NOTYPE WEAK DEFAULT UND
_ITM_registerTMCloneTable
38643: 0000000000000000 0 FUNC WEAK DEFAULT UND
pthread_cond_destroy@@GLIBC_2.3.2
[hjl@gnu-6 5.3.1]$
Do you know how many of them lead to DT_TEXTREL?
--
H.J.
next prev parent reply other threads:[~2016-02-23 14:11 UTC|newest]
Thread overview: 34+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-01-01 0:00 H.J. Lu
2016-01-01 0:00 ` H.J. Lu
2016-01-01 0:00 ` H.J. Lu
2016-01-01 0:00 ` Michael Matz
2016-01-01 0:00 ` H.J. Lu
2016-01-01 0:00 ` Michael Matz
2016-01-01 0:00 ` H.J. Lu
2016-01-01 0:00 ` H.J. Lu
2016-01-01 0:00 ` H.J. Lu
2016-01-01 0:00 ` H.J. Lu
2016-01-01 0:00 ` Alan Modra
2016-01-01 0:00 ` H.J. Lu
2016-01-01 0:00 ` Michael Matz
2016-01-01 0:00 ` H.J. Lu
2016-01-01 0:00 ` Michael Matz
2016-01-01 0:00 ` H.J. Lu
2016-01-01 0:00 ` Alan Modra
2016-01-01 0:00 ` H.J. Lu
2016-01-01 0:00 ` Alan Modra
2016-01-01 0:00 ` Carlos O'Donell
2016-01-01 0:00 ` H.J. Lu
2016-01-01 0:00 ` Michael Matz
2016-01-01 0:00 ` H.J. Lu
2016-01-01 0:00 ` Michael Matz
2016-01-01 0:00 ` H.J. Lu
2016-01-01 0:00 ` Michael Matz
2016-01-01 0:00 ` H.J. Lu
2016-01-01 0:00 ` H.J. Lu
2016-01-01 0:00 ` H.J. Lu
2016-01-01 0:00 ` Michael Matz
2016-01-01 0:00 ` H.J. Lu [this message]
2016-01-01 0:00 ` Michael Matz
2016-01-01 0:00 ` Szabolcs Nagy
2016-01-01 0:00 ` Michael Matz
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